Types of boiler: The 3 different types of boiler explained

There are three different types of boiler: 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, we’ll help you understand the main differences between the three types of boiler. We’ll also go through their advantages and disadvantages.

The 3 different types of boiler

Heat-only boiler

Prior to the combi, a heat-only boiler (or regular or conventional boiler) was the most common of the types of boiler found 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.


  • They are ideal for households with a high demand for hot water. This is because the water is heated and stored in advance.
  • They tend to be low pressure, which means less wear and tear. A pump is also an option, though, for better flow rates and a better shower.
  • They can be used alongside 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.
  • Of the three different boiler types, heat-only systems are more complicated and the most expensive. You will require a cold water storage cistern and a feed and expansion cistern in the loft. The presence of cisterns and pipework in the loft 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 types of boiler 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. Of the three types of boiler, combi systems are usually the cheapest to install.
  • Combi boilers are themselves space-saving – they are usually small, compact, and wall-mountable.

Combi boiler disadvantages

  • Unlike other types of boiler with a hot water cylinder, 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

So , that’s the three main types of boiler accounted for, but what exactly is a condensing boiler? This term actually refers to a type of boiler technology which makes them more energy efficient. And the way it works is pretty simple. 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. Regardless of the different types of boiler, 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.