No hot water: what causes it & how to fix it

There are few plumbing problems more disruptive to daily life than having no hot water in the house. Having a bath is off the cards, and probably so too is taking shower. Doing the dishes isn’t an option (unless you have a dishwasher), and who wants to wash their hands in cold water? In this article, we’ll go through the most common reasons why the hot water isn’t working in your home, and what you can do to fix it.

No hot water – the basics

Before we look at the possible reasons why there’s no hot water, there are a few really basic things to look at first:

  • Make sure that the power supply to the relevant appliance is switched on
  • Try resetting the boiler using its reset switch, or turn the power on and off again. This may clear any redundant fault codes which prevent the boiler from operating.
  • Check that you have selected the correct program on the boiler or thermostat.
  • If your hot water depends on a clock or timer, make sure that the time is set correctly, especially at the times of year when the clocks change.
  • Make sure that the fuel supply is open – e.g. the gas main.
  • Make sure that you actually have fuel – e.g. by checking the gauge on your oil tank.
  • In older boilers, check to see that the pilot light is lit.

Health & Safety

Before you attempt to diagnose the problem, there are a few health and safety considerations:

  • The loft – If you are going into the loft, you need to be using a suitable ladder. Once in the loft space, you will need a source of light, such as a torch. Be extremely careful with older lamps – a hot bulb could be a serious fire hazard – and be careful where you tread. Keep an eye out for nails and other sharp objects protruding from the rafters and other carpentry work.
  • Water – Working with hot water and hot pipes comes with a risk of scalding. The water in your central heating circuit is dirty, and contains anti-corrosion and biocidal chemicals. It’s likely to stain objects and textiles.
  • Electricity – Some fittings, such as motorised valves and immersion heaters, are supplied with mains electricity. You should never work on such components if you don’t know what you’re doing. Always isolate a circuit before working on it, and always use a pair of electrical testers to confirm that a circuit is dead.
  • Gas – It’s illegal for an individual who is not on the Gas Safe register to work on a gas appliance.

If you have no hot water and a combi boiler

Low boiler pressure

If you have a combi boiler, one of the most common reasons for having no hot water is low boiler pressure. In order to operate, the system must be pressurised to approximately 1 – 1.5 bar when cold. Without adequate pressure, the boiler won’t operate, neither for central heating nor hot water.

If the pressure gauge indicates less than 1 bar when the system is off, then low pressure may be the culprit. Topping up the system via the filling loop should solve this problem – click here to learn how to do this. If the boiler keeps losing pressure, your system has a leak somewhere. The leak may be either on the pipework/central heating circuit, or because of a faulty boiler component. Click here for more information on this topic.

Faulty diverter valve

This is a component of a combi boiler which controls how heat is used. Combi boilers will typically prioritise the domestic hot water, so if someone opens a hot tap while your central heating is on, the diverter valve will react to that, and heat will be used to heat water for the taps instead of water for the radiators. Close the hot tap, and the boiler will go back to heating your home.

A fault diverter valve is very likely if you have a combi boiler and central heating but no hot water. This will have to be replaced by a qualified engineer.

If you have no hot water and a hot water cylinder

If you have a hot water cylinder, then water from either the mains or a cold water storage cistern in the loft supplies the cylinder with fresh water. The water is then heated by the boiler via water in the central heating circuit, which is pumped through a coil of pipework inside the cylinder. This is known as an indirect system. On a direct cylinder, there is no coil – the water is heated by two electric immersion heaters.

Before you read on, it’s worth noting that the average hot water cylinder holds around 160-230 litres of water. Think back – have people in the household been taking more baths or showers? If so, then you may have no hot water simply because of high demand. Use the boost feature on the immersion heater to get hot water back again. Don’t do this too often though, as it will have an impact on your energy bills. If there is a greater demand for hot water overall, then reprogram the timer/thermostat in order to accommodate it.

Check the supply of cold water

Is the gate valve on the cold supply pipe to the cylinder open? If it’s closed, you’ll have no hot water. That’s because hot water exits the top of the cylinder due to gravity acting on the cold water cistern in the loft.

For the same reason, there also needs to be water in the storage cistern. In some houses, the storage cistern also supplies the bathroom cold taps. If you have no hot water but the kitchen cold tap is working, then the storage cistern may be empty. This is almost certainly the case if the bathroom cold taps don’t work either. A quick visual inspection will confirm the state of the cistern. Has the ball valve got stuck? Moving the arm up and down may free the valve. If the cistern starts to fill, congratulations – you have solved the problem.

Faulty motorised valve

On an indirect system, a valve known as a motorised valve diverts water from the boiler to flow through either the hot water cylinder coil or through the radiators. A faulty motorised valve is often the reason for having no hot water, especially if you have heating but no hot water, or hot water but no heating.

Blocked primary circuit

The coil in the hot water cylinder forms part of what is known as the primary circuit. Hot water from the boiler – known as the ‘flow’ – enters the coil at the top, transfers its heat, and then exits at the bottom via the ‘return’.

A blockage in the circuit may stop the boiler from heating the water in the cylinder. This could easily be the case if the boiler starts up but stops shortly after. The boiler does this because the heat it’s producing is not being transferred to the cylinder. As a result, an internal thermostat shuts the boiler off in order to protect it from damage.

You may also find that the ‘return’ pipe adjacent to the cylinder is cool. If the ‘flow’ pipe is hot, then it’s very likely that a blockage is preventing water from flowing through the coil properly.

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Condensate trap

The condensate trap is a device inside a condensing boiler which collects water from the condensing process before it is expelled into the sewer via the condensate pipe. The purpose of the trap is to prevent toxic gases from entering the condensate pipe, and to manage the flow of condensate fluid out of the boiler.

Condensate trap overview

The condensing process

When a boiler burns a fuel, one of the end products of the combustion process is water vapour. In a non-condensing boiler, this water vapour is simply expelled out of the flue. However, the water vapour is hot: it carries heat energy. This is why non-condensing boilers generally only have an energy efficiency rating of 50-80%. For every £1 you spend on fuel, 20 to 50p’s worth of heat is simply being pumped outside.

This is where condensing boilers come in. Condensing boilers draw the heat energy out of that water vapour to the extent that it condenses and turns into a liquid – water. By recovering this heat which would otherwise be wasted, condensing boilers typically have an energy efficiency rating of around 90%.

More on this topic: Condensing vs. Non-condensing boilers – what’s the difference?

The condensate trap itself

This water, or condensate as its known, is then expelled into the sewer via the condensate pipe. But before this happens, the water is collected in the boiler’s sump. It then flows into the condensate trap.

The trap has two purposes. Firstly, the water forms a barrier between the condensate pipe and the combustion areas inside. This is important because noxious fumes should always be expelled via the flue, not the condensate pipe. Think of it in the same way as the trap in your toilet or sink, which also uses a body of water to prevent the movement of fumes – in that case, to prevent bad smells from the sewer entering your your home.

The other purpose of the trap is to control the flow of condensate exiting the boiler. This is important because in the winter months, condensate pipes have a tendency to freeze and block. The boiler will then detect that it cannot expel condensate, and will shut itself down.

Frozen condensate pipes are often a result of a constant drip-drip-drip of water. To prevent this, condensate traps have an auto-siphon. Once the water in the trap reaches a certain level, the siphon is activated and all the water is siphoned out of the trap. By releasing the condensate in bursts like this, it’s much less vulnerable to freezing, and the warmth of the water should shift any ice which has started to form.

Condensate trap types

Generally speaking, there are two different kinds of condensate trap – those with an auto-siphon, and those without. The ones without may simply be in the form of a U-bend shape of pipe.

However, regardless of type, condensate traps are always made out of plastic, like other boiler components which handle condensate. This is because condensate itself is slightly acidic and would corrode metal-based fittings.


The boiler should never be operated without the condensate trap in place, as it prevents noxious combustion gases from travelling down the condensate pipe. Should the trap have to be removed from the boiler, it must be topped up with water when it’s replaced. This will ensure the presence of the water barrier between the condensate pipe and the boiler’s combustion chambers.

Note that it is illegal for someone who is not listed on the Gas Safe register to work on a gas appliance, such as a gas boiler.

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Free Boilers & Boiler Grants: how to get a free boiler

Believe it or not, it is possible to get a completely free boiler. This is thanks to an environmental scheme from the government. If you meet the eligibility criteria, you can receive a boiler grant for up to 100% of the cost and installation of a new appliance. You don’t have to pay this money back, saving you an incredible £2,000.

How to get a free boiler

The Energy Company Obligation

Boiler grants are just one of several energy efficiency measures on offer from the ECO. The goal of the ECO is to help households on lower incomes reduce their carbon emissions and pay less for heating. As the name implies, the scheme receives financial support from large energy companies. This includes the “big 6”, such as British Gas and E.on, as well as other energy companies who are deemed large enough based on the amount of customers they have.

Other energy saving measures available from the ECO include:

  • Loft insulation
  • Cavity wall insulation
  • Solid wall insulation

ECO October 2018

The current phase of the ECO initiative comes to an end on 30th September 2018. Its replacement will run from 1st October 2018 to 31st March 2022.

The new scheme will place extra focus on the most poor households, as almost a third of those supported by the current scheme are not on low-income homes. Another change is that grants for free oil and LPG boilers won’t be available any more.

How it works

The process is relatively straightforward. After determining that you are eligible, a contractor will come round in order to review the existing installation. They will also consider other factors in order work out exactly how much financial support you are entitled to. While boiler grants can and do cover 100% of the cost, you may be required to contribute a certain percentage of the total. This depends on several factors, such as the energy efficiency of your home. On the plus side, the grant is not a loan, and you will never have to pay back the money.

Once these things have been settled, the new boiler will be installed by a qualified engineer. This shouldn’t take too long –  swapping an old combi boiler with a new one in the same place generally doesn’t take longer than a day. The engineer may also replace the heating controls with a modern programmable thermostat. Replacing an old G-rated non-condensing boiler with an A-rated condensing boiler can save you up to £340 a year. The new boiler will also be quieter, safer, and faster at heating the hot water and your home. Plus, with an annual service, you’ll probably have a manufacturer’s extended warranty of anywhere up to 10 years.

Am I eligible for a boiler grant?

The eligibility criteria are very strict. You must live in either your own home or in privately rented property and have a connection to the gas main. You also need to receive a certain combination of income-related benefits, such as Job Seeker’s Allowance and income support.

The boiler which will be replaced must also be a non-condensing boiler which is at least eight years old. Non-condensing boilers are always less energy efficient than condensing boilers. (Click here to learn more about this topic.)

What exactly are the benefit criteria?

Free boiler for pensioners and over 60s

  • You will satisfy the benefit criteria if you receive Pension Credit

Free boiler with Child Tax Credits

  • You will satisfy the benefit criteria if you receive Child Tax Credits and you live in a home whose annual income is less than £16,010 per year.

Free boiler on Income-related benefits.

If you receive either i) Income Support, ii) Income-based Jobseekers’ Allowance, or iii) Income-based Employment & Support Allowance, you will satisfy the criteria if you receive at least one of the following components. Note that full-time education excludes higher education, i.e. university or NVQ Level 4 equivalent or higher:

  • Pension Premium
  • Employment Support
  • Disability Living Allowance (Higher Rate)
  • Child under 16
  • Child under 20 (not at university or studying for NVQ Level 4 or equivalent)

Free boiler on Working Tax Credits

You will satisfy the benefit criteria if i) you receive Working Tax Credits ii) you live in a home whose annual income is less than £16,010 per year and iii) receive at least one of the following components:

  • Receiving disabled worker component or severe disability component
  • Over 60 years old
  • Child under 16
  • Child under 20 (not at university or studying for NVQ Level 4 or equivalent)

Free boiler on Universal Credit

You will satisfy the benefit criteria if i) you receive Universal Credit and ii) you earned £1,250 or less after tax in any period within the last 12 months and iii) receive at least one of the following components:

  • Child under 16
  • Child under 20 (not at university or studying for NVQ Level 4 or equivalent)
  • Receiving Disability Living Allowance (DLA) or Personal Independent Payment (PLP)
  • Limited capability for work, or limited capability for work and work-related activity
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What size boiler do I need and what do kW and kWh mean?

What size boiler do I need – come the colder months, lots of people will be asking themselves this question. There are also other considerations, such as what type of boiler is necessary. If you’re stumped for an answer, fear not – this page will explain exactly how boiler sizing works and what the associated terms mean.

What size boiler do I need?

The best boiler size, or to give its correct term, boiler output, depends on several factors. The main considerations are the number of radiators, the number of bathrooms, and how many people live in the home. Some of these factors will also affect the type of boiler that is suitable for the property. For example, a combi boiler probably won’t be suitable in a house with two or more bathrooms. This is because hot water is not stored in advance but heated on demand from the mains. If two people shower at the same time, the consequence of dividing the mains this way will be two equally disappointing showers.

Calculating the size

In order to determine 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 – 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 size must be calculated on a case-by-case basis, most properties fall into one of three ranges.

Boilers for 1 and 2 bedroom flats and houses

For these properties, a 24-27 kW combi boiler is adequate in most cases, assuming a property with one bathroom, 2-3 people, and up to 10 radiators.

Boilers for 3 and 4 bedroom houses

For medium to large properties with 3-4 bedrooms, one bathroom, or up to 15 radiators, a 28-34 kW boiler should be suitable. However, if there is more than one bathroom, you should consider a regular or system boiler which integrates a hot water cylinder. This will help ensure that there is enough hot water to keep up with demand.

Boilers for big houses and houses with 4+ bedrooms

For large properties such as those with 4 or more bedrooms, a 35-42 kW regular or system boiler is necessary, especially if the property has more than one bathroom.

It is important to remember that these ranges are a guideline rather than a rule. This guide assumes average size rooms and therefore average-sized radiators. The best boiler size should always be calculated by a qualified heating engineer.

What does kW mean?

Boiler output is measured in kW or kilowatts. A watt is a measurement of energy in relation to time, i.e., how much energy can be delivered per second. One watt of energy equals one joule of energy per second. At maximum output, a 24 kW boiler therefore delivers 24,000 joules of energy every second of operation.

So what does kWh mean?

The term kWh stands for kilowatt-hour. No doubt you will have seen this term on your energy bill. At first it may sound confusing to express a quantity of energy with a measurement that we normally associate with measuring time, but it’s actually quite straightforward. Here’s how it works:

  • There are 3600 seconds in an hour.
  • If a 24 kilowatt boiler running at full power delivers 24,000 joules of energy per second, that’s 86,400,000 joules of energy per hour. (3600 seconds x 24,000 joules).
  • 86,400,000 – eighty-six million, four hundred thousand – is a big number. Instead of having to write it out like that, it can simply be expressed as 24 kW/h.

So to continue with the same boiler:

  • If it was operating at full power for 2 hours, it would therefore deliver 48 kW/h of energy or 172,800,000 joules.
  • If it was operating at full power for half an hour, it would therefore deliver 12 kW/h of energy or 43,200,000 joules.

Bearing in mind that energy is measured in joules, if we assume that one hour of boiler use produces such a large figure, then try to imagine how many joules of energy your home uses every month – the number would be huge. It’s much easier for energy companies to express this figure as kilowatt hours. (And while we are at, if the average UK home uses 4000 kWh per month, then that figure would be 14,400,000,000 joules. That’s fourteen billion and four hundred million!)

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Boiler filling loop: the 2 different kinds and how to use them

The boiler filling loop is a boiler component which provides a temporary connection to the mains in order to fill and pressurise the central heating system with water. It also allows the system to be topped up where necessary, for example, after bleeding radiators. Regularly topping up the system usually indicates a leak, and doing so will damage the system in the long-term.

The filling loop itself consists of either a rigid section of pipe, or a braided hose. A filling loop can be either part of the boiler itself, or it can be fitted close by. Filling loops usually have a stop valve at one end and a double check valve on the other.

Filling loop overview

In a sealed central heating system with a combi or system boiler, there is no feed and expansion cistern in the loft. Instead, the system is supplied by the mains.

The requirement that the connection is temporary is because Water Regulations prohibit the backflow of water into the mains, which would risk contaminating it. The risk of contamination from a central heating system is even greater because of the corrosion and sludge which can build up inside a central heating system, and/or the presence of anti-corrosion and biocidal chemicals. A permanent connection to a central heating system is therefore not permitted, and wouldn’t be possible anyway due to the fact that cold mains water is likely to be at a higher pressure than the pressure of, for example, the central heating system of a standard combi boiler when cold.

External filling loops

A generic external filling loop simply consists of a length of braided hose with compression fittings at each end. There will be at least one stop valve which is used to control the flow of water. There will also be a double check valve to prevent the backflow of water into the mains. A double check valve is so called because it contains not one but two internal spring-loaded anti-backflow mechanisms. When installing an external filling loop, care must be taken to ensure that it is fitted the right way round. Otherwise, valve will stop the system from filling up.

Internal filling loops

Some boilers, such as the Ideal Logic Combi range, feature integrated filling loops. They may look more complicated, but still function on exactly the same principle. Since all combi boilers have a direct connection to the mains anyway (mains water is heated by the boiler for the hot taps), the filling loop may tee off from this pipe.

How to use the filling loop

If the pressure is too low or keeps dropping, repressurising your boiler and central heating system is very easy:

  1. Firstly, identify the pressure gauge on your boiler. Most combi and system boilers require a pressure of 1 – 1.5 bar in order to operate.
  2. Open the valve or valves on the filling loop. You should hear the sound of rushing water – the system is filling up. You should also see the needle on the pressure gauge rise.
  3. When the needle indicates the target pressure, close the filling loop.
  4. Use the controls on the front panel of your boiler to clear any low pressure fault codes. This may or may not be necessary, depending on the make and model of your boiler.
  5. Congratulations – you have heating and hot water again!

Why do I keep having to top up my boiler?

If you keep having to use the filling loop to top up your boiler, you probably have a leak somewhere. The leak will either be on your central heating (one of the joints or radiators), or one of the boiler’s components. Faulty expansion vessels and pressure release valves are likely suspects inside the boiler.

Either way, if your boiler pressure keeps dropping, you should get the leak fixed pronto. Regularly topping up the system with oxygen-rich mains water will encourage internal corrosion and the build-up of sludge. If left untreated, this will gradually ruin your central system from the inside out, destroying the boiler and causing your radiators to develop leaks.

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Economy 7: The guide to the Economy 7 Tariff

Economy 7 is an energy tariff. It is designed to help consumers pay less for their electricity by encouraging its use at night. This is achieved by offering two different rates based on the time of the day. Economy 7 tariffs go hand in hand with a range of specifically designed appliances, such as electric storage heaters.

What exactly is Economy 7?

As one would expect, the heaviest demand for electricity is during the day from breakfast  until dinner time. By encouraging the use of power during the night, Economy 7 tariffs reduce the peak-time demand on the national grid.

Economy 7 tariffs are widely used in properties which are not connected to the gas main. They are also used in properties where it would be difficult or impossible to install an oil tank, such as flats. Instead of wet central heating, such properties typically contain electric storage heaters. Unlike radiators, these aren’t linked together, and can be operated independently from each other. As the name suggests, they heat up ceramic bricks or clay during the night, which is then emitted throughout the day.

Heat is then emitted throughout the day, warming the home without drawing on the more expensive day rate electricity.

Economy 7 tariffs can also be utilised by specifically designed hot water cylinders. These are known as direct hot water cylinders. In a direct cylinder there is no heat exchanger – the water is heated directly by two immersion heaters. One of them is situated near the bottom, and another just beneath the crown of the cylinder. During the night, the bottom heater heats the overall body of water on the cheaper night rate. During the day, the top heater functions as a “boost” heater, operating intermittently to keep the water at the desired temperature, and heating the cylinder when the hot water is replaced with fresh cold.

When does Economy 7 go on and off?

The cheaper night rate is generally available for around seven hours, hence the name. It may be active from midnight until 0700, from 2300 to 0600, or from 0030 to 0730, for example. The exact time span, however, depends on the energy provider.

Economy 7 tariffs are only available for electricity, not gas or oil. They also require the installation of a special meter. Switching to an Economy 7 tariff or switching back to a standard tariff will require exchanging the meter. You will have to contact your energy supplier to facilitate this, and there may be a charge for it. Note that simply using electricity during the night on a non-Economy 7 tariff will not result in cheaper bills.

Economy 7 problems and tips

One of the main criticisms of this tariff is that the day rate is overpriced. While this is done to encourage night time use, it increases the chances of consumers paying too much for electricity overall. If you don’t heat your home with electric storage heaters and you tend not to use any electric appliances at night, Economy 7 is the wrong tariff for you and you will definitely end up paying too much. On the other hand, if electricity is your only heat source, Economy 7 is likely to be the best option. If you work anti-social hours and tend to use a lot of energy during the night, you may save even more money.

A disadvantage of this tariff is that since energy is stored in advance, you will have to plan ahead with your energy usage. For example, when a cold snap is coming up, you will want to make sure your heaters charge up enough during the night, otherwise they will run out towards the end of the day, and you won’t be able to have a cosy warm living room on a winter’s evening without drawing on expensive day rate electricity.

An obvious way to take advantage of the cheaper night rate with other appliances in the home is to use timer plugs. These can be digitally or manually operated. Simply set the timer, plug in, and your appliance will come on at the preset time. However, you should be aware of any considerations when it comes to leaving electric appliances unattended, and you shouldn’t leave tumble dryers unattended at all.

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Turn off the mains fast: how-to guide

Need to know how to turn off the mains? While it’s easy to take the plumbing in your home for granted, it pays to know how to shut off the water. It’s good practice if you are leaving your home for a long period of time. Plus, if you have a leak, the usefulness of knowing how to turn off the water in an emergency speaks for itself.

How to turn off the mains

In order to do this, you’ll need to find your internal stopcock or stop tap or stop valve. This is a small, brass-handled valve which controls the flow of water into your home. To turn off the mains, simply turn the handle clockwise as you would do with any other tap.

Where is the stopcock?

In most properties the mains stopcock is located near the kitchen sink or in the cupboard underneath it. However, it may also be in a pantry or in a downstairs WC. In some older properties it may even be under the floorboards. Even worse, stopcocks sometimes get concealed behind a cupboard or wooden panelling by an inept carpenter or kitchen fitter.

Some properties may have multiple stopcocks, with one valve controlling the overall flow of water in the property, and a secondary stopcock controlling the flow of water to the outlets in an extension.

Shutting off the water

You should be able to turn off the stopcock without any tools. It may take a minute or two for the water to stop flowing. Note that if you have a cold water storage cistern, turning off the mains will stop the cistern from filling, but any leak originating from the cistern or from appliances fed from the cistern will continue until the cistern is empty.

If you have a cold water storage cistern and a hot water cylinder, turning off the mains also won’t stop the flow of hot water until the cold cistern is empty. The hot cylinder will remain full even with the mains off. The same is true if your hot water cylinder is mains fed. It is the pressure of the cold water coming in at the bottom which pushes the hot water out at the top.

If your internal stopcock doesn’t work or you can’t find it

If your internal stopcock has seized up or is nowhere to be seen, the only other way to shut off the water is via the external stopcock. This is usually located underneath a small hatch in the pavement in front of your property, anywhere up to a metre below ground level to protect it from frost damge. Opening or closing an external stopcock usually requires the use of a stopcock key.

Some properties may share an external stopcock. You should consider the possibility that turning your stopcock off may also affect your neighbour’s water supply.

Generally speaking, the homeowner is usually responsible for the plumbing within the boundary of a property. Anything outside of the property is usually the responsibility of the water board. Therefore, in most cases, the external stopcock usually belongs to the water board. The homeowner may be held liable for damage to it. Its operation may require permission from the water board, even in an emergency.

See also:

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Condensing vs Non-Condensing Boilers: what’s the difference?

Condensing vs non-condensing boilers – the main difference between them is that condensing boilers are 10-25% more energy efficient. Not only can they save you several hundred pounds on your annual energy bill, they’re also better for the environment. But how do they do this? When you’re looking to buy a new condensing boiler or simply want to learn how they work, this article will point you in the right direction.

Condensing vs Non-condensing boilers: what’s the difference?

When a boiler burns fuel, one of the byproducts of the combustion process is water vapour. This water vapour – steam – contains heat. Imagine that you open a window, put a pan of water on the hob, bring it to the boil, and leave it there until all the water boils away into steam. The heat you’ve paid for has literally gone out of the window.

This principle is effectively what happens in a non-condensing boiler. The water vapour produced from the combustion process is simply expelled via the flue, heating the outdoors instead of your home or your hot water.

The condensing boiler advantage

Condensing boilers don’t let that heat go to waste. Instead, they draw so much heat out from that vapour, it condenses back into water. This recovered heat is then used to heat the water for your radiators or the hot water tank. This means that condensing boilers require less fuel to generate the same amount of heat – and fewer carbon emissions. They’re better for the environment as well as for your wallet.

The condensed water – or condensate as it is known – is collected in a trap before being discharged into the sewer via the condensate pipe. These components are usually plastic, as the slightly acidic condensate would react with metal pipework. Most boilers eject condensate in small spurts so that a constant dripping doesn’t freeze and block the pipe. Frozen condensate pipes are an extremely common reason for a winter boiler breakdown.

How energy efficient is a condensing boiler?

Very efficient – you can usually expect an energy efficiency of at least 90%. This means that for every £1 you spend on your energy bill, 90p is directly going towards heating your home and the hot water. Compare this to a non-condensing boiler, which may only have an energy efficiency of 70-80%. In older boilers, that figure may even be as low as 50-60%, which amounts to a tremendous waste of money and energy. Condensing boilers are always more energy efficient than non-condensing boilers.

UK energy efficiency regulations require that all gas boilers installed after 1st April 2005 and all oil-fuelled boilers installed after 1st April 2007 must be condensing boilers.

How much money will I save?

Of course, the answer to this question depends on the efficiency of the boiler you wish to replace. According to The Energy Saving Trust in, replacing a Band G non-condensing boiler with a Band A condensing boiler and modern central heating controls, you can save up to £340 a year on your energy bills. You’ll also dump 1,500 kg less carbon dioxide into the environment, helping to shrink your carbon footprint.

Is it still possible to install a non-condensing boiler?

Yes, but only in circumstances where a qualified heating engineer has determined that a condensing boiler is unsuitable for the property. A points system is used to determine this, and if a score of over 1000 is reached, the engineer will issue a certificate to say that a non-condensing boiler can legally be fitted. It is extremely important that the homeowner does not lose this certificate should they wish to sell the property in future, as it confirms that the presence of a non-condensing boiler is legal.

What other types of boiler are there?

The issue of condensing vs non-condensing boilers relates to energy efficiency rather than the actual type of boiler for a particular central heating system configuration.

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Boiler Types: 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 system. 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 their advantages and disadvantages.

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 at the same time.
  • 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 combi boilers is that they are entirely dependent on the flow of the mains. Combi boilers often 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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