The 3 different boiler types explained

There are three different boiler types: combi, system, and heat-only or conventional. The most common type is the combi boiler. When selecting a new appliance, the best boiler type usually depends on the size of the property, how many bathrooms it has, and the existing plumbing configuration. Choosing a new boiler is hardly an every day task, but in this guide to boiler types, we’ll help you understand the main differences. We’ll also go through the advantages and disadvantages of the different boiler types.

Different boiler types explained

Heat-only boiler

Prior to the combi boiler, a heat-only boiler (or regular or conventional boiler) was the most common boiler type in UK homes. Heat-only boilers are usually found in family homes and require a cold water storage cistern and a feed and expansion cistern in the loft. They also require a hot water cylinder. They are so called because they simply heat water, with no direct role in heating water for the taps. Hot water from the boiler is either pumped through the radiators, or through a coil of copper pipe inside the hot water cylinder.


  • As the hot water is heated and stored in advance, they cope well in households with a high demand for hot water.
  • They tend to be low pressure, with less wear and tear. A pump is also an option, though, for better flow rates and a better shower.
  • They can be used in compliment with solar panels to reduce heating bills. Energy from the sun can be used to offset the amount of energy needed by the boiler.


  • You can run out of hot water, requiring the boiler to heat up the hot water cylinder from scratch.
  • You will require a cold water storage cistern and a feed and expansion cistern in the loft. This makes a heat-only system more complicated and more expensive. It may also make a future loft conversion trickier.
  • A bulky hot water cylinder is also necessary, taking up living space in the home.
  • Energy is wasted by heating up and storing hot water in advance.

System boiler

System boilers also require a hot water cylinder, but they do not need a feed and expansion tank. This is because they draw their water supply for the central heating directly from the mains, and they contain a built-in expansion vessel. The pump for the central heating is also integrated into the boiler itself.

System boiler advantages

  • Their self-contained design allows for an easier installation, and there is no need for a feed and expansion cistern in the loft.
  • The use of a cylinder can accommodate multiple demands for hot water
  • If the mains flow rate allows, they can be used with an unvented hot water cylinder, delivering hot water at mains pressure. This removes the need for a cold water storage cistern, freeing up even more loft space.

System boiler disadvantages

  • Once again, hot water must be heated in advance. If you run out, you must wait for the hot water cylinder to heat up again.
  • The hot water cylinder takes up living space in the home.
  • Storing hot water in advance is less energy efficient than heating it on demand.
  • Unvented cylinders are more expensive and cost more to install.

Combi boiler

As mentioned, the combination or combi boiler is the most common of the three boiler types found in British homes. Most use gas as their fuel, although oil-fed combi boilers do exist.

Combi boiler advantages

  • Combi boilers heat the water for the central heating and the water for the hot taps. They heat the domestic water instantaneously on demand, providing hot water at mains pressure.
  • Consequently, they do not “run out” of hot water. There is no need for a bulky hot water cylinder and no waiting for a cylinder of water to heat up again.
  • Depending on the mains pressure and the type of shower, there is no need for a shower pump.
  • There is no need for the complication and bulk of cisterns in the loft, such as a cold water storage cistern and a feed and expansion cistern. This makes them ideal for properties with a prospective loft conversion, or properties without any loft space, such as flats. The absence of cisterns also makes them cheaper to install.
  • Combi boilers are themselves space-saving – they are usually small, compact, and wall-mountable.

Combi boiler disadvantages

  • The main disadvantage of a combi boiler is that they are entirely dependent on the flow of the mains, and will struggle to provide water to more than one hot water outlet at a time. This makes them unsuited to larger properties and properties with more than one bathroom.

Condensing boiler

A condensing boiler is not actually a type of boiler. It actually refers to a type of boiler technology that makes them more energy efficient. One of the by-products of burning a fuel is water vapour. This water vapour contains heat energy. A non-condensing boiler would simply expel this water vapour out of the flue, wasting the heat energy it contains.

In a condensing boiler, the boiler draws so much heat out of the water vapour, it condenses back into a liquid – water. The boiler then discharges this water into the sewer via the condensate pipe.

For this reason, condensing boilers are always more energy efficient than non-condensing boilers. UK energy efficiency regulations stipulate that all gas boilers installed after 1st April 2005 and all oil boilers installed after 1st April 2007 must be condensing boilers. Non-condensing boilers may only be installed in exceptional circumstances.

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Direct & indirect hot water cylinders: what’s the difference?

Direct cylinders, indirect cylinders, unvented cylinders…the list goes on. There are many different of types of hot water cylinder available, but what is the difference between direct and indirect hot water cylinders, and what do these terms actually mean?

The difference between direct and indirect hot water cylinders explained

The answer to this question relates primarily to how the water in the cylinder is heated. In an indirect cylinder, the water is heated by the boiler. In a direct cylinder, the water is heated by an electric element

Indirect cylinders

In a household with a cold water storage cistern and a feed and expansion cistern in the loft, and a gas or oil fed heat-only/regular boiler, the hot water cylinder contains a coil of copper pipe, which functions as a heat exchanger. A thermostat is strapped to the side of the cylinder.

When requested by the thermostat, the boiler fires up and hot water is pumped through the coil, heating the rest of the water inside the cylinder to around 55-60 degrees celsius. The two bodies of water – the water from the boiler and the domestic hot water – never mix. (If they do, this is the result of a broken coil.)

Note that the heat exchanger is not always a coil. In plumbing systems with a primatic hot water cylinder, the cold water storage cistern supplies the water for both the hot water cylinder and the central heating. The heat exchanger in a primatic cylinder is shaped in such a way which allows an air bubble to form. It is this air lock which prevents the two bodies of water from merging. Due to their fundamental design, primatic cylinders are always indirect.

Most indirect cylinders are fitted with one electric immersion heater so that even if the boiler isn’t working, the household still won’t be without hot water.

Direct cylinders

In a direct hot water cylinder, there is no heat exchanger – the water is heated directly by two electric immersion heaters only. Direct hot water cylinders go hand in hand with Economy 7 electricity tariffs. The lower immersion heater heats the bulk of the water on the cheaper night tariff, and on the standard rate, the top heater keeps the water up to temperature as it is drawn off throughout the day.

Fortic cylinders

Fortic cylinders are hot water cylinders which have their own integrated cold water storage cistern, instead of requiring a separate cistern that is usually in the loft. Fortic cylinders can be both direct and indirect.


Don’t confuse direct/indirect hot water cylinders with direct/indirect plumbing systems. The latter is a reference to a whole household’s plumbing system, namely as to whether the water outlets are supplied entirely by the mains, or from a cold water storage cistern. See this page for more information on that topic.

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Electric shower stops working: why does it stop & how can it be fixed?

Does your electric shower suddenly stop working? Does it suddenly cut out and leave you with no way to rinse yourself off? This situation is perhaps more irritating than a shower which doesn’t work at all. In this article, we’ll explain why your electric shower stops working, and what you can do about it.

Electric showers are basically instantaneous water heaters. They have a single supply of cold water which is heated on demand via an electric element. Over time, calcium deposits form on the element, just like in your kettle – particularly if you live in an area with hard water.

Eventually, there is so much limescale on the element that the heat doesn’t transfer to the water properly. A sensor inside the shower detects that the element is getting too hot – and consequently turns the shower off. This is a safety mechanism which prevents the shower from overheating. You may also find that you cannot turn it back on for a few minutes. Once again, this is the shower’s safety mechanism preventing the shower from operating until the excess heat has dissipated.

In some cases, you may also detect a burning smell. Even if the safety cut-out is working, that’s not to say the excess heat isn’t damaging the shower somehow. If you do smell burning, it’s probably a bad idea to use the shower again.

Electric shower stops working – how can it be fixed?

There are several things you can try in order to get your shower going again. Take the shower head off, and run the shower on cold on its highest water flow setting for a couple of minutes. Doing this 3-4 times per year may may help to dislodge any limescale inside.  You should also make sure that the shower head is thoroughly free from limescale.

Unfortunately, this won’t be a permanent fix – the shower will still eventually scale up again. In hard water areas, electric showers may only last 1 – 2 years. The only guaranteed way to solve this issue is to replace the electric shower with a shower which doesn’t heat the water itself – i.e., a mixer shower which is supplied by a combi boiler or from a hot water cylinder.

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The boiler is the key component of a central heating system. Its main purpose is to transfer heat energy to water. The heat energy is produced by the combustion of a fuel, such as natural gas, oil, or a solid fuel such as coal or wood. Hot water is then distributed to the radiators in the household, which then heat the home. As well as for central heating, boilers are also used to heat the water for the taps – either directly on demand, or indirectly, usually by heating a body of water in a hot water cylinder.

There are several different types of boiler available. The most common type found in UK households is the combination or combi boiler. These are so called because they directly provide hot water for both the central heating and the hot taps. Other types of boiler include heat-only boilers and system boilers. Depending on the type, boilers can be supplied with water directly from the mains, or they may draw their water supply from a feed and expansion cistern in the loft. Despite the name, household boilers do not actually bring water to boiling point as part of their general operation.

Boiler history

The history of boilers in British homes goes hand in hand with the history of central heating. Early installations featured a back boiler, which consisted of a device fitted to the rear of an open fireplace. Cold water from the feed and expansion tank would enter the boiler at the bottom, before rising to the top of the circuit via gravity. This hot water would flow through the radiators and through a coil of pipe in the hot water cylinder, heating the hot water for the taps. Even so, only 30% of households in Britain had central heating. The main way of keeping warm was an open fireplace in the living room, and electric heaters, blankets and hot water bottles everywhere else.

Fifty years later, of course, and things are drastically different. As of 2017, 95% of British households now have a central heating system. Not only has the average household temperature risen by four degrees from 18 to 22, but it’s fair to say that boilers have radically changed how we live our lives. Richard Sober, a lecturer in interior design at the University of Teesside, explained to the BBC that families tend not to huddle around the fireplace any more. When it gets cold, we turn the heating up, instead of putting on more clothes and eventually going to bed when it’s cold. We don’t need to change our lifestyle or sleeping pattern. People can stay up all night – in any room, instead of congregating around the fireplace. Dr. Sober remarks that it can be argued that “…central heating played a part in the disintegration of the family.”

Boiler fuel types

Natural gas

Boilers are available in several different fuel types. By far the most common is natural gas – this accounts for the heating fuel in approximately 85% of homes in the UK. According to the Energy Networks Association, gas offers up to 90% energy efficiency, and the UK gas grid is almost entirely underground, protecting it from cold weather. Gas itself offers the homeowner a number of advantages – it is piped directly into the home and one cannot run out of it, unlike other fuel types. However, getting a household connected to the gas grid can be very expensive, and any work on a gas appliance can only legally be carried out by a Gas Safe-registered engineer.


Oil is the second most common type of boiler fuel in the UK, and is used in approximately 1.5 million of the 4 million British households which aren’t connected to the gas network. The oil is usually stored on the property itself. This generally requires a large unsightly plastic tank, which can take up a fair amount of room in the garden. It is possible for the tank to go underground, although this makes installing it more complicated and more expensive.

Oil-fuelled boilers are larger than the average boiler in the UK (the gas-fuelled combi). They are usually free-standing and are approximately the size of a large washing machine. In other words, they take up more space because they cannot be mounted on a wall. However, they do have their own advantages. Some oil-fuelled boilers can actually be installed outside, freeing up space in the home and making the sounds of a boiler much less of an annoyance. Another significant advantage is that there is no risk of a potentially deadly carbon monoxide leak inside the home. If the boiler is installed in a garage or outbuilding, then any ambient heat from the boiler may actually useful, helping to keep mould away.

The main disadvantage surrounding oil-fuelled boilers is the cost – they can be several hundred pounds more expensive than equivalent gas boilers. The annual cost of the oil may also be a few hundred pounds more expensive than the same amount of gas. Like most things, it is better to buy heating oil in bulk in order to obtain better prices. In small villages and parishes, it is quite common to find an oil buying club, where residents all pitch in together to get the best possible price per litre.

There are actually two different types of heating oil: 28sec and 35sec. The measurement of seconds is actually a measurement of viscosity or thickness, i.e., how many seconds it takes for ball bearing to sink through a column of the oil. 28sec is raw kerosene and is the most popular type of heating oil. It burns much more cleanly than 35sec and creates less smoke and soot. 28sec is also used in oil-fuelled cookers, such as Agas and Rayburns. It is easily possible to distinguish one oil from the other visually: 28sec oil is yellow, 35sec oil is red.


Electric boilers do exist, although they are few and far between. They should not be confused with storage heaters, like those typically found in flats. These are individually controlled and do not constitute ‘central’ heating.

However, some electric boilers do work on a storage principle and offer wet central heating. A classic example is the old range of GEC Nightstor boilers. These used cheap Economy 7 night electricity to heat up a core which consisted of dozens of feolite bricks. Air heated by these bricks would then be fanned onto a heat exchanger, heating the water for the radiators.

Electric boilers have a number of advantages – the main one being that they are extremely energy efficient. Since the combustion of a fuel is not required, absolutely no heat energy is expelled out of the building via a flue. Every penny of electricity is used as heat for the home. The absence of a flue also means that electric boilers are generally cheaper to install, and their simplicity means cheaper maintenance and servicing costs. However, the main disadvantage is that, as a fuel, electricity costs more than gas. Even if the boiler is more energy efficient, it may not necessarily be cheaper to run.

Types of boiler

There are several different types of boiler available. Getting the right boiler depends on several factors, including the size of the home and the source of fuel.

Condensing boiler

This term is frequently mentioned, and is more akin to a boiler attribute rather than a boiler type. A condensing boiler is a boiler with extra energy efficiency.

How do condensing boilers work?

One of the products of the combustion process is water vapour. Instead of simply expelling this hot vapour out of the flue and wasting the latent heat energy it carries, a condensing boiler draws so much heat out of it, that it cools down and turns into water. This water – condensate, as it is known – is collected in a trap inside the boiler before then being discharged into the sewer via the condensate pipe.

Due to environmental regulations, all boilers installed in the UK after April 2005 must be condensing boilers.

Combi boiler

As mentioned, the most popular type of boiler in the UK is the combination or combi boiler. These are so called because they combine both domestic hot water services and a central heating system in one unit. Combi boilers are supplied directly from the mains. Hot water for the taps is heated on demand, and water for the radiators is provided by a temporary connection to the mains. An internal expansion vessel accommodates the expansion of the central heating water when it is heated.

As a result, there is no need for a cold water storage cistern and a feed and expansion cistern in the loft – an ideal solution for a loft conversion, or in households where there is little to no loft space. There is also no need for a bulky hot water cylinder, exchanging an airing cupboard for extra living space.

However, the main disadvantage of combi boilers that they are unsuitable for larger properties, such as those with two bathrooms or indeed any household where there may be simultaneous demands for hot water at different outlets.

System boiler

Like combi boilers, system boilers integrate all of the central heating components – such as the pump and the expansion vessel – into one system, hence the name. They do, however require a separate hot water cylinder, as the hot water for the taps is not heated on demand. The hot water cylinder can be a standard open vented cylinder supplied from a cold water storage cistern, or it can be an unvented cylinder fed directly from the mains.

The main advantage of a system boiler is that it has the main advantage of a combi boiler while also compensating for its disadvantage. There is no need for tanks and pipework in the loft, and having a hot water cylinder means water means there is plenty of hot water immediately available, even if requested by multiple outlets at once. Of course, this does mean that extra space is required for the hot water cylinder.

Other disadvantages of a system boiler include the energy wastage from heating the water and then storing it – the hot water cylinder must be insulated to protect it from heat loss. A heating system with a system boiler will also be more expensive due to the extra expense of installing the cylinder.

Conventional boiler

Conventional boilers, also known as regular boilers or heat-only boilers, are typically found on older, family-size properties. As the name heat-only implies, such boilers do nothing other than heat water – there are no additional central heating components fitted. The central heating pump is typically located in the airing cupboard, and the expansion of the water is accommodated by a feed and expansion cistern in the loft. Hot water for the taps is stored in a hot water cylinder, which is fed by a cold water storage cistern.

Conventional boilers are the tried and tested solution for homes in which there may be a high demand for water which would outstrip the capacity of the mains alone. The presence of a feed and expansion cistern also means there is no need to top up the boiler – the system does this itself automatically. Plumbing systems with a conventional boiler are the most expensive and the most complicated, due to the cisterns, the extra pipework, and the hot water cylinder. However, they can be complimented with solar energy systems, helping to cut fuel bills.

How do boilers work?

While boilers may operate in slightly different ways, they all operate on the same principle with roughly the same key components. A fuel is burned in a combustion chamber. The heat from this process is transferred to water in a heat exchanger. However, what happens next depends on the type of boiler.

In a system boiler and a conventional boiler, the boiler heats water for what is known as the primary circuit. The primary circuit is essentially the central heating system – this hot water is pumped through the radiators, emitting its heat and warming the home.

As for the water for the taps in the hot water cylinder, this is heated indirectly by the boiler. As well as through the radiators, water in the primary circuit also flows through a coil of copper pipe inside the hot water cylinder. Heat is then transferred from the boiler to the water in the cylinder, ready for the taps. The two bodies of water, the primary circuit and the domestic hot water (DHW), should never come into contact with each other. (In some cylinders, there is no coil – the water is heated by two electric immersion heaters, one of them usually being on an Economy 7 tariff.)

A motorised valve, usually located on the pipework close to the hot water cylinder, controls whether or not water flows through the radiators or through the cylinder coil.

Boiler sizes

Boiler size or to be more accurate, boiler output, is generally measured in kW or kilowatts. Watts are a measurement of power because they are a measurement of how much energy is transferred per second – one watt equals one joule of energy per second. A kilowatt – one thousand watts – therefore equates to one thousand joules of energy per second. A 24 kW boiler therefore delivers 24,000 joules of energy every second. (Note that energy “used” is a bit of a misnomer – energy cannot be used, but simply converted from one form into another.)

Another important measurement is the kilowatt-hour or kW/h. This is not a reference to power, but refers to the total sum of energy delivered, expressed as kilowatt hours. There are 3600 seconds in one hour, and so per the above example, if the boiler delivers 24,000 joules of energy per second, then it delivers 86,400,000 joules of energy per hour. (3,600 x 24,000 = 86,400,000.) Instead of writing 86,400,000 joules, this figure can simply be expressed as 24 kW/h.

  • The same 24 kW boiler operating for 2 hours would therefore deliver 48 kW/h of energy or 172,800,000 joules.
  • The same 24 kW boiler operating for half an hour would therefore deliver 12 kW/h of energy or 43,200,000 joules.


Boiler and radiator output was previously – and sometimes still is – given in BTUs. This stands for British Thermal Unit. As implied by the name, this measurement was ubiquitous in Britain and is also used in many other countries. It is also used in other engineering industries, even to this day.

However, the use of BTU within the field of plumbing and heating is slightly different. This is because a BTU is a measurement of energy (like kW/h) rather than power – one BTU does not refer to the rate of energy consumption, i.e. one joule per second, but refers to the amount of heat energy needed to raise the temperature of one pound (0.454 kg) of water by one degree Fahrenheit (1055.06 joules.) Consequently, it cannot be converted directly into watts, and must instead be expressed as a quantity over time – BTU per hour or BTU/h. To clarify:

  • BTU/h and kW are measurements of power – the rate at which energy can be used.
  • BTU and kW/h are measurements of energy – the amount of energy available or expended.

1 BTU/h is equivalent to 0.000293 kW or 0.293 watts, and 1 kW is equivalent to 3,413 BTU/h.

  • In order to convert BTU/h to kW, multiply by 0.000293
  • In order to convert kW to BTU/h, multiply by 3412

These terms may sound confusing, but all they are is two different ways of quantifying the same thing: the rate at which heat can be produced (power), and how much of it can be produced over time (energy).

Heat input

This refers to the power required. For example, a 24 kW boiler may take 27 kW of heat input. What happens to that 3 kW for every 24 kW of heat produced? Energy can’t be destroyed or consumed, rather simply converted from one form into another, so where did it go? The answer is simple – it is lost through the flue, or from heating up the boiler casing and internal components, and so on.

By dividing the heat output by the heat input, we can calculate the energy efficiency of the boiler. 24 divided by 27 equals 0.88888888888. Thus, we can round this figure up to say that in the above example, the boiler has an energy efficiency of 89%.

What size boiler do I need?

In order to decide the appropriate size of boiler for a household, a competent heating engineer will need to calculate the appropriate heat output needed in every room in the home. There are numerous factors which affect how much energy is required to heat a room to the suitable temperature. These include – but are not limited to – the dimensions of the room, whether or not the windows are double glazed, whether or not the room faces north, and if the room is upstairs or on the ground floor. (A room on the first floor will benefit from heat rising from the room below it). The purpose of the room must also be taken into consideration – bedrooms may be one or two degrees cooler than the living room; bathrooms may be one or two degrees higher. If the necessary heat output is not calculated correctly and a boiler with insufficient power is selected, the home won’t be warm enough and won’t have enough hot water to keep up with demand. A boiler with excessive power will waste energy, resulting in unnecessarily expensive heating bills and damage to the environment.

While the necessary boiler capacity must be calculated on a case-by-case basis, most properties fall into one of three ranges. For small to medium-sized properties such as flats and terraced houses with 1-2 bedrooms, one bathroom or up to 10 radiators, a 24-27 kW boiler should be suitable. For medium to large properties with 3-4 bedrooms, one bathroom, or up to 15 radiators, a 28-34 kW boiler should be suitable. For large houses with 4+ bedrooms, two bathrooms and up to 20 radiators, a 35-42 kW will be necessary.

However, these ranges are a guideline rather than a rule, and assume average-sized rooms and therefore average-sized radiators. A 3 bedroom property with a one bathroom may still require 35 kW or 38 kW boiler if the room calculations deem it necessary.

Flow rates

As well as boiler power, the size of the property will also dictate the type of boiler required. Combi boilers should always be avoided on properties with more than one bathroom. This is because they can only provide a single stream of hot water, heated on demand from the mains. There is no reserve of hot water in a tank or cylinder. This may not be a problem in households occupied by a single person, but dividing a single mains supply between two outlets will result in two equally disappointing showers.

It is therefore important to take flow rates into consideration, which are measured in litres per minute. For example, a 24 kW boiler will typically deliver 9.5 – 10 litres of hot water per minute. While it may be possible to connect a shower to a combi boiler, the required flow rate for some mixer showers and digital showers may exceed the maximum flow rate of the boiler. In such cases, a hot water cylinder would be necessary – and consequently a system boiler or a heat-only boiler.

Arguably the most important flow rate to consider is the mains. Upgrading a 24 kW to a 34 kW boiler with a flow rate of 15 litres per minute still won’t fill the bath any quicker or be suitable for a certain type of mixer shower, if the mains flow rate is e.g. 9.5 litres per minute.

How much does a new boiler cost?

This not only depends on the type of boiler, the fuel type, and the capacity of boiler required, but also the cost of any additional labour required and who is actually completing the job. Independent contractors tend to be substantially cheaper than the big energy firms.

Boilers themselves can range from £500 to up to £5,000, without including installation costs. Generally speaking, the more powerful the boiler, the more expensive it will be. As a rough guideline, replacing a gas combi boiler in the same location as the previous one is likely to cost around £2,000, including parts and labour. This job can usually be completed within a day. Installing the boiler in a new location will probably cost about £500-£600 extra, and take twice as much time.

Replacing a regular or heat-only boiler in the same location will cost slightly less: around £1,750. Installing a the boiler in a new location will probably tip the balance over the £2,000 mark.

Replacing a gas heat-only boiler for a combi boiler will cost £2,500 – £3,000. The extra expense is due to the additional work required: the removal of the hot water cylinder and the cisterns in the loft, and the installation of a condensate pipe. Such a project will take 2-3 days to complete.

Where installing central heating, systems using heat-only boilers and system boilers will always be more expensive than those using combi boilers. This is due to the requirement of a hot water cylinder and its associated fittings, such as the thermostat and motorised valves. Loft cisterns may also be required.

Other costs to consider when replacing or installing a new boiler include pipework changes – the gas supply pipe may need widening – and structural work on a property, e.g. an old flue may need to be bricked up. A powerflush may also be required.


Over time, water in the central heating reacts with the metal components of the system, causing the formation of rust and other oxides. The most common oxide is magnetite, and it tends to form a black sludge-like substance which can solidify. These unwanted substances will not only reduce the efficiency of the central heating, but can cause serious damage to the boiler and other parts of the system if left unchecked.

That’s where powerflushing comes in. Powerflushing works by pumping water through the central heating system, dislodging the sludge and debris. The sludge is then collected in the powerflush unit before being disposed of.

A powerflush is often advisable when installing and commissioning a new boiler in order to protect the appliance from debris for as long as possible. In fact, some boiler manufacturers may actually require that a powerflush is carried out, in order for any guarantees or extended warranties to be valid.

Signs that the central heating may benefit from powerflushing include:

  • Grey, metallic-coloured water when bleeding the radiators
  • Radiators do not get warm at the bottom
  • Unusual sounds from the boiler or the pump
  • The central heating is slow to warm up in general

A powerflush generally costs between £250 and £500, and takes a full day to complete, assuming no extra unanticipated work is required during the process. Once it has been completed, a chemical inhibitor is put in the central heating circuit to prevent rust and oxides from forming.

Boiler energy efficiency

Calculating the energy efficiency of a boiler in terms of heat input vs heat output has already been discussed in the above section. The last few years have seen several different scales used to grade boiler energy efficiency.

The main system used in Britain is the SEDBUK rating – this stands for Seasonal Efficiency of Domestic Boilers in the UK. There are actually two different classes of SEDBUK rating.

  • SEDBUK 2005 – This system used the letters A-G to rate boiler efficiency, with G being the worst performing, and A being the highest. An ‘A’ rated appliance would have an energy efficiency of at least 90%.
  • SEDBUK 2009 – In order to prevent confusion with European energy efficiency ratings, this newer system does away with the A-G rating and simply expresses the figure as a percentage.

In September 2015, the European Union introduced the ErP (Energy-related products) rating. This system uses a letter grading system from A++ to G. Out of these three systems, the SEDBUK 2009 scale therefore gives the most precise figure as to a boiler’s efficiency.

As well as the requirement that all boilers installed after April 2005 must be condensing boilers, as of April 2018 under the new Boiler Plus legislation, all gas boilers must have an energy efficiency of no less than 92%. All gas and oil boilers must have electronic time and temperature controls, and all combi boilers must have at least one of a range of different energy saving features.

Common boiler problems

It is easy to take hot water and central heating for granted, but their absences are sorely felt. Needless to say, boiler breakdowns and call outs for emergency plumbers are extremely common during the winter months.

Low boiler pressure

On a system with a combi boiler, the system is supplied with water via a temporary connection to the mains. The system must have a water pressure of at least 1 bar, otherwise the boiler will not operate at all. Low combi boiler pressure is easy to verify by checking the gauge on the front of the boiler.

Low pressure does not necessarily indicate a fault with the boiler itself. Bleeding the radiators may cause enough of a drop in pressure to create this problem, and it is easily remedied by topping up the system via the filling loop. However, if this does not solve the issue, and the boiler must be frequently topped up, then the only other possible explanation is a leak. The leak may be on a central heating fitting (radiator, pipe, joint), or it may be one of the boiler’s components. The usual suspects are a faulty pressure release valve (PRV) or a broken expansion vessel. Click here to read more about this fault.

Frozen condensate pipe

A frozen condensate pipe is the quintessential winter boiler breakdown. As previously mentioned, condensing boilers draw so much heat out from steam in the flue gases, it condenses and turns into water. This slightly acidic water is then expelled into the sewer via the condensate pipe. The condensate pipe is made out of plastic and often runs outside the house towards the drain, much like the kitchen sink drainpipe.

A 24 kW boiler produces around 1.5 – 2 litres of condensate for every hour of operation. The constant dripping of water down this pipe is vulnerable to freezing in a cold snap and blocking the pipe, especially if it is exposed to an icy wind. Some boilers try to mitigate this problem by only releasing water in spurts, however this is not guaranteed to prevent it.

If the condensate pipe is frozen, pouring hot water on it, or wrapping a hot water bottle on it will shift the blockage and get the boiler up and running again. To prevent this problem from happening, the pipe should be insulated with water-proof insulation, and can be widened or re-fitted so that the water cannot settle inside.


While boilers do not operate in silence, a faulty, noisy boiler may produce banging, gurgling or rumbling sounds. This problem is known as “kettling”, and it may be due to a number of reasons – a frozen condensate pipe can be one of them. However, the most common reason is a build of up limescale within the heat exchanger. Mechanical causes are also possible, causing the boiler to overheat.

Pilot light keeps going out

Older boilers feature a pilot light, a small flame which must be constantly lit in order for the boiler to operate normally. When heat from the boiler is demanded, the pilot light is responsible for igniting the burner.

The most common reason for a pilot light which won’t stay lit is a faulty thermocouple. If the pilot light is extinguished, this component turns off the boiler’s internal gas supply as a safety precaution.


In the UK, any work on a gas boiler or indeed any gas appliance may only legally be carried out by an engineer who is registered with Gas Safe. Previously known as CORGI registration, this is the Health & Safety Executive’s official registration body for gas engineers.

The equivalent organisation for oil heating is OFTEC, or the Oil Firing Technical Association. Unlike Gas Safe, it is not a legal requirement to be registered with OFTEC in order to work on oil-fuelled appliances. Registering with an organisation such as Gas Safe or OFTEC as a ‘competent individual’ means that the engineer can self-certify their work, and does not need to arrange an expensive Building Control Notice and inspection from the local authority to certify that the work is safe and satisfies Building Regulations. For peace of mind, you should always use a Gas Safe or OFTEC-registered engineer.


Most boilers come with a 1-2 year guarantee as standard. Many boiler manufacturers, however, offer an engineer accreditation scheme. As an example, this means that a consumer who pays for a new Worcester-Bosch can enjoy an extended warranty of anywhere from 5-10 years, as long as the plumber is certified by Worcester-Bosch. If the boiler develops a fault, then the manufacturer will send one of its own engineers to fix the problem free of charge. For boiler manufacturers, this strategy encourages brand loyalty among engineers, and the benefits for consumers speak for themselves. As an added bonus for consumers, these extended warranties make boiler breakdown insurance – typically offered by energy firms – a completely unnecessary expense.

However, there are a number of conditions which must be met in order for the manufacturer to honour the warranty, and the manufacturer will not hesitate to wiggle out of the agreement if it believes that its terms and conditions have not been met by either the consumer or the engineer who installed the boiler. Common requirements include:

  • The boiler must be installed in accordance with the manufacturer’s instructions.
  • The engineer must register the boiler with the manufacturer within 30 days of installation. Once this is done, the homeowner or landlord will usually receive a letter confirming this.
  • The system must be powerflushed when the boiler is installed and commissioned.
  • The homeowner or landlord must have the boiler serviced annually by one of the manufacturer’s accredited installers.

Boiler grants

The UK government offers a boiler grant scheme under the Energy Company Obligation’s ‘Affordable Warmth scheme. The ECO was introduced in January 2013. Its purpose is to reduce fuel poverty by helping people on lower incomes to heat their home efficiently, and to reduce the country’s carbon emissions. The ECO also offers grants for wall cavity and loft insulation.

Only homeowners and landlords may apply for the scheme; private tenants must ask their landlord to apply on their behalf. The scheme is not open to everyone – only those receiving certain state benefits are eligible. If the applicant does meet the eligibility criteria, then the boiler will be replaced like-for-like, e.g. a heat-only boiler with a heat-only boiler, a combi for a combi, and so on. Upgrading to an A-rated, modern condensing combi boiler from e.g. a G-rated, non-condensing appliance will typically cut fuel bills by £200 – £300 per year.

Due to government cuts, the ECO may not be able to cover 100% of the costs, and may instead subsidise a boiler with the applicant expected to contribute the rest. The current ECO scheme will come to an end on 30 September 2018, and a new one will subsequently introduced under the same name.

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Isolate the cold water cistern: how-to guide

Need to isolate the cold water cistern? Whether you need to repair or replace the ball valve, or carry out further work on the low pressure side of your plumbing system, this article will explain several different ways to carry out this simple task.

How to isolate the cold water cistern

Isolation valve/service valve

The cold water storage cistern (and the feed and expansion cistern) are fed from the rising main. Water byelaws require that supply pipe to the ball valve is fitted with an isolation valve or service valve.

Before isolating the cistern, take a look at the arrangement of the feed pipe, especially if the feed and expansion cistern is close to the cold water cistern. If the isolation valve is before a branch or a tee, it may actually isolate both cisterns.

Isolation or service valves are simple to operate, and are opened or closed using a flat-headed screwdriver. When the slot on the screw is in line with the pipe, the valve is open. Turning the screw a quarter turn so that the blade is perpendicular to the pipe will close the valve.

Alternatively, you may find the isolation valve in the form of a stopcock. This will have a brass handle; do not confuse it with a gate valve. Gate valves have red, wheel-shaped handles, and are installed on the cistern’s outlet pipes.

If there is no isolation valve on the supply pipe

If there isn’t a service valve on the supply pipe, another way to isolate the cistern is to turn off the mains stopcock. This is typically located under or near the kitchen sink. Bear in mind that turning off the mains stopcock will not only stop the cold water cistern from filling, it will shut off the water supply in the entire household.

Tie up the ball valve

If there is no isolation valve on the supply pipe, and it is impractical to switch off the mains, then another way of isolating the cold water cistern is to tie up the ball valve. This can be achieved by laying a baton across the cistern and tying the arm of the valve to it with string or garden twine. However, this method obviously cannot be used if the purpose of isolating the cistern is to repair or replace the ball valve.


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Outside tap

An outside tap or garden tap is a tap fitted to the outside of a building which delivers water for use outside of the property. Outside taps are designed to be easily connectable to hoses and provide a convenient water source for a pressure washer, or for the use of a hose to wash the car or water the garden. Having an outside tap also means there is no need to impede the use of other water outlets in the home – for example, by fitting a garden hose to the kitchen tap.

Outside tap overview

Outside taps are usually made out of brass. This helps to prevent them from corroding from their exposure to the elements all year round.

A typical outside tap features a brass handle with a single spindle, and a male ¾ inch BSP thread for a 15 mm nozzle which allows for the connection of hoses. The nozzle usually features a simple layering to allow for the secure connection of a hose via a jubilee clip. However, this nozzle can be replaced with nozzles which are designed to fit directly into Hoselock-type hose connectors.

Outside tap plumbing

As for the supply side, a typical outside tap usually has a male ¾ inch BSP thread at the rear. This usually connects to a 90 degree tap connector which may also feature the backing plate for securing the tap to the wall. The tap is then connected to the supply pipe via a solder or compression fitting.

Alternatively, the backing plate and tap connector may have their own integrated length of copper pipe for running through the wall, ranging from 350 – 400 mm. This is commonly found on outdoor tap kits and can be trimmed to the desired length with some pipe cutters.

Outside taps should always be supplied by the mains and never from a cold water storage cistern, as a storage cistern would not offer satisfactory pressure.

There are several possible ways to connect an outside tap to the mains, such as teeing off from a property’s underground service pipe. However, the most common configuration is that the tap is fitted on the external wall of a kitchen or downstairs toilet, and the supply for it is tee’d off from the nearby pipework inside that room.

To prevent the contamination of the mains via the possible back flow of e.g. dirty water in a hose, water regulations require that the supply pipe is fitted with a double check valve. However, many outdoor taps available on the market at the moment come with their own integrated double check valve.

An outdoor tap must also be fitted with an isolation valve. This will allow for servicing or repairs to carried out without having to turn off the entire water supply to a property. In periods of freezing weather, an outdoor tap should always be isolated and the tap itself left open. This will prevent any water in the pipework from freezing and then bursting the pipe when it thaws.

Outdoor tap kits and fittings

While individual taps are available from most DIY stores and plumbing merchants, it is also possible to buy outdoor tap kits. These come with everything necessary for installing an outdoor tap in accordance with water byelaws.

An outdoor tap kit typically consists of:

  • The tap itself
  • The wall mounting. Depending on the kit, this may be in the form of an elbow tap connector, or it may be integrated with a length of pipe that runs through the wall.
  • A braided hose connector. This is used to connect the tap to the tee point on the mains inside the property.
  • An isolation valve/service valve
  • A double check valve. This is necessary if the outdoor tap does not have its own integrated check valve.
  • Screws and wall plugs

Some outdoor tap kits may also come with a spool of PTFE tap. This helps to ensure a watertight fitting on compression joints.

It is also possible to buy small thermal insulation jackets or “cosies”. These can be slipped over an outside tap and tied in place to prevent the tap from freezing in the winter.

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Frozen condensate pipe: how to fix

No heating or hot water? If your boiler’s suddenly stopped working during a snap of frosty weather, there’s a good chance you’ve got a frozen condensate pipe. This common problem is ironically the last thing you need during the winter months! Fortunately, there are a couple of easy solutions to get your boiler up and running again.

What exactly is the condensate pipe?

The condensate pipe is a plastic pipe which allows a condensing boiler to discharge waste water from the condensing process into the sewer.

During the combustion process inside the boiler, water vapour is produced. Instead of expelling this hot vapour out of the flue, condensing boilers draw out so much heat from it, that it cools down and condenses into a liquid. This liquid – moderately acidic water, is then expelled from the boiler via the condensate pipe.

While in some households the condensate pipe may discharge into the trap of a sink, it may run independently through the wall and into a drain. It may also connect directly to the soil stack. Residual water in the pipe during the winter may freeze, resulting in a frozen condensate pipe. The boiler’s sensors will detect that the condensate pipe is blocked, and trigger a fault code, preventing the boiler from operating.

How to unfreeze/thaw the condensate pipe

  1. Do not cut the condensate pipe. The condensate pipe is generally considered to be part of the flue of a boiler, and consequently, any work on it may only legally be carried out by a Gas Safe-registered engineer.
  2. Identify where the pipe may have frozen. Horizontal lengths of pipe with little to no fall are a likely suspect – they allow the water in the pipe to settle.
  3. Prepare some hot water. You will probably have to use your kettle or the microwave for this. Do not use boiling water.
  4. Using a suitable container such as a jug or a watering can, pour hot water over the pipe where you think it has frozen. Pipe elbows and horizontal pieces of pipe are good areas to start with. Alternatively, you can use a hot water bottle. Place it over the pipe and leave it for a few minutes.

If ice in the condensate pipe was indeed the culprit, it should melt away and any water which was behind it should flow away too. After resetting the boiler or clearing any fault codes, the boiler should now operate normally.


You can prevent a frozen condensate pipe in future by insulating the pipe with lagging. The lagging must be waterproof, as otherwise it will soak up water like a sponge and give the pipe a nice ice blanket, which will only make the problem worse.

Other ways to prevent a frozen condensate pipe include rerouting the pipe to give it the appropriate level of fall or rerouting it through the house so that is less likely to freeze. Alternatively, external runs of pipe can be widened from 22 mm to 32 mm. For these modifications, you should consult a Gas Safe-registered engineer.

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Direct & Indirect plumbing systems: what’s the difference?

There are many different types of plumbing system to be found in UK households. Each system comes with its own set of advantages and disadvantages. In this article, we’ll discuss the difference between direct and indirect plumbing systems, and what those differences mean.

Direct & Indirect plumbing systems explained

The fundamental difference difference between direct and indirect plumbing systems relates to how cold water is distributed throughout the home.

In an indirect system, the mains enters the property and branches off to supply the cold kitchen tap. It then rises up to the loft to feed the cold water storage cistern. The cold cistern supplies cold water to everywhere else in the household, such as the cold bathroom taps, and the hot water cylinder.

In a direct system, all of the cold outlets are supplied by the mains. The cold water storage cistern supplies only the hot water cylinder. Alternatively, there may not even be a cold water storage cistern, if the property has a combi boiler or an unvented hot water cylinder. Both of these appliances are also fed directly from the mains.

Advantages and disadvantages

As one would expect, each of these systems comes with its own set of advantages and disadvantages. The main advantage of an indirect system is that even in the event of an interruption to the mains, the household will still have a reserve of water. Residents will still be able to flush the toilet for a limited period of time, for example. Indirect systems also operate at a lower pressure than mains water. Consequently, they are much quieter, and less susceptible to water hammer and wear and tear.

The main disadvantage of indirect systems is the risk of stagnation and contamination that comes with keeping a large quantity of water open to the atmosphere. While current water regulations require cisterns to be fitted with a set of components to ensure the water remains potable, i.e. drinkable, the concept of brushing one’s teeth in water which has come from a tank in the loft may not be very appealing.

This rings particularly true in the past, when the approach to the quality of water in the cistern was much more relaxed, and it was not uncommon to find drowned insects, rodents, and even birds at the bottom of a cistern. Indirect systems are ultimately why people in British homes are usually advised not to drink from the bathroom taps. On the other hand, a household with a direct system will offer drinking water from every cold tap in the property.

The other main advantage of direct systems is that their higher pressure means much greater flow rates

The other disadvantage of indirect systems is that they require space in the loft for a bulky water tank. Long, narrow “coffin” tanks can be fitted, but in the case of a loft conversion, a change of plumbing system may be needed.

Hot water cylinders

So there you have it – the difference between direct and indirect plumbing systems explained. Note that direct and indirect may also refer to types of hot water cylinder. In this context, direct and indirect refers not to how the cylinder is supplied, but how the water is heated. For more information, check the hot water cylinder page.

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Expansion pipe

The expansion pipe or vent pipe is a pipe associated with a cold water storage cistern or a feed and expansion cistern. Its purpose is to accommodate the expansion of water when it is heated, and to allow any excess water to vent harmlessly into the cistern. It also serves as an escape route for any air bubbles or gases which have formed.

Expansion pipe – cold water storage cistern

On a hot water cylinder, the hot outlet at the top of the cylinder is tee’d off. One pipe from this tee takes delivers hot water to all of the outlets in the home. The other rises up to the loft, up above the cold water storage cistern and bends down into it, without touching the water. On a cold water storage cistern fitted with a Byelaw 30 or Byelaw 60 kit, the lid of the cistern should contain a rubber grommet in order to admit the expansion pipe.

Air bubbles which naturally form inside the hot water cylinder when the water is heated can rise up and out of the cylinder, and up and out of the plumbing system through the expansion pipe. This prevents the formation of air in the system, which may lead to an airlock.

The expansion pipe on a cold water storage cistern is usually a length of 22 mm copper pipe. It should never be fitted with any valves, as this would open the possibility for hot water to be contained without anywhere for it go when it expands. This could be dangerous.

The expansion pipe should also never dip into the water in the cistern. This would create a gravity circuit between the hot water cylinder and the cold water storage cistern. Since hot water is less dense than cold water, hot water in the cylinder would flow up through the expansion pipe and into the cold cistern. The water would circulate in this way until the entire contents of the cold cistern has been replenished with hot water from the hot cylinder.

Expansion pipe – feed and expansion tank

On the feed and expansion cistern for the central heating, the vent pipe is a 22 mm copper pipe which rises above the cistern and bends down into it. The top of the bend should be at least 40 cm (15.75 inches) higher than the surface of the water in the cistern. This helps to ensure that water isn’t inadvertently pumped into the cistern when the boiler is on. This problem is known as pumping over, and will enrich the water in the cistern with oxygen. If left untreated, the oxygen-rich water will damage the system by promoting internal corrosion.

In order to prevent pumping over, the vent pipe should originate on the flow from the boiler behind the pump.

In the event of negative pressure in the vent pipe, air may get sucked into the system. This problem may occur when the feed pipe and vent pipe originate too far apart from each other on the system.

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Turn off the hot water: how-to guide

Need to turn off the hot water? Whether you have a hot water cylinder or a combi boiler, this is normally a very straightforward task. In this article, we’ll discuss how to turn off the hot water in your home.

How to turn off the hot water

The first step for this task is to understand how your hot water system works. Here’s a brief list of the most common configurations:

  • Combi boiler. This is the most common arrangement in UK households. With a combi boiler, there are no cisterns in the loft. Water for both the hot taps and the radiators comes directly from the mains.
  • Hot water cylinder with a cold water storage cistern. This is a very common arrangement. The cylinder is supplied with water from a cold water storage cistern in the loft. The cistern also accommodates for the expansion of the water when it is heated. This type of configuration is known as an open vented cylinder. On some hot water cylinders, the cold storage cistern may not actually be in the loft, and is instead an integral part of the cylinder. Such cylinders are known as Fortic cylinders.
  • Hot water cylinder without a cold water storage cistern. This is becoming more common in UK homes since they were permitted by the water regulations in the mid eighties. An unvented cylinder does not need a cistern in the loft because it is fed directly from the mains.

If you have a combi boiler

If you have a combi boiler, you can turn off the hot water by turning off the mains stopcock. You can usually find the mains stopcock in the vicinity of the kitchen sink. It may be in the cupboard under the sink, or it may be on an adjacent pipe. Turning off mains stopcock will also shut off the water to every outlet in your home.

If you have a hot water cylinder with a cold water storage cistern

Turn off the gate valve on the cold supply pipe to the cylinder. This valve should be easily recognisible as a valve with a red, wheel-shaped handle on a pipe that runs from the ceiling of the airing cupboard to the bottom of the cylinder.

Hot water comes out of the top of the cylinder due to gravity acting on the cold water in the cistern in the loft. Thus, stopping the cold water going in at the bottom prevents the hot water from being pushed out at the top.

This obviously means that even if you turn off the cold supply, the hot water cylinder will still be full of water. This is a safety precaution which prevents the immersion heaters or the boiler from trying to heat an empty cylinder. If you need to drain the hot water cylinder, take a look at the how-to guide for this task.

If there is no gate valve on the supply pipe

Can’t find a gate valve on the supply pipe in the airing cupboard? It may be in the loft on the cold pipe coming from the cold storage cistern.

If there isn’t a gate valve at all between the storage cistern and the hot water cylinder, another solution is to insert a rubber tank bung into the outlet at the bottom of the cistern.

If you have a Fortic cylinder

On a Fortic cylinder, there probably won’t be a gate valve on the cold supply pipe, because the cold supply pipe is probably concealed under the cylinder’s foam insulation. If there is a gate valve on the hot outlet pipe, it’ll be on the pipe coming out of the very top of the hot water compartment. (Should there be two pipes lower than this in line with each other, then these are the flow and return from the boiler, which you can ignore.)

If you have a hot water cylinder without a cold water storage cistern

If you have a pressurised cylinder, you’ll need to turn off the stop valve or isolation valve on the cold feed pipe. You can turn off the mains stopcock if you can’t find this. As previously mentioned, the mains stopcock is usually located near the kitchen sink, perhaps in the cupboard underneath it. Note that closing the mains stopcock will also shut off the water to every tap and outlet in your home.

Getting the hot water back on should simply be a case of opening the valve which you closed. If you have to turn off the mains stopcock, don’t forget to fill up some saucepans with water so you can still make a cup of tea!

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