Heat-only boilers are also referred to as regular or conventional boilers. As these names suggest, this type of boiler is the oldest and most traditional variety – if you have an old heating system or are working on one for a customer, it’s likely that this is supported by a conventional boiler.

The boiler itself is just one of several components in a heat-only setup. Like system boilers, this kind of appliance relies on a separate hot water storage tank. In addition to this, conventional boilers also require loft space for a cold water cistern as well as a feed and expansion tank.


diagram of a system boiler


Heat-only boilers explained 

A heat-only boiler usually sits in the downstairs of a house along with a hot water storage tank, which is often kept in an airing cupboard. Whereas system boilers take water directly from the mains, conventional boilers receive incoming water from a cold water cylinder (or header tank) via a feed and expansion tank in the loft.


Advantages of conventional boilers

Regular boilers may be the oldest type of appliance you’ll come across in domestic settings, but they offer some important advantages:

  1. Perfect for larger households and businesses – the dedicated hot water tank that works alongside a heat-only boiler allows several taps, showers, or baths to draw hot water simultaneously. As such, they’re ideal for bigger families and commercial premises.
  2. Effective in areas with low water pressure – the cold water cistern (header tank) stores water from the mains before transferring it to a heat-only boiler, so this kind of appliance is perfect for households in areas of low water pressure.
  3. Support for a backup heater – an immersion heater can be installed in the hot water tank to allow for a backup supply. This comes in handy if you ever find your boiler is on the blink.
  4. Safe to use with older radiators – traditional-style radiators can leak when powered by more modern boilers due to the higher pressure; conventional boilers, on the other hand, are compatible with old heating systems.
  5. Compatible with solar thermal systems – regular boilers can take energy from solar thermal systems such as rooftop arrays, enabling homeowners to cut down on carbon emissions. 


Disadvantages of conventional boilers

As you might expect with a traditional type of appliance, there are some limitations to heat-only boilers:

  1. Require a lot of space – in addition to the boiler itself, this setup requires a cupboard for a hot water tank downstairs plus room for two tanks in the loft (a cold water cistern and a feed and expansion tank). With this in mind, they’re only suitable for homes with lots of space available.
  2. Complicated to install – heat-only boilers are tricky to get your head around and more challenging to fit than other varieties: as well as three separate tanks, there’s also the pipework between each of these components to think about.
  3. Finite supply of hot water – after the water stored in the hot water tank is used up, you’ll need the boiler to kick in again. This means that the amount of hot water a household can use is limited by the size of the tank.
  4. Water takes time to heat up – as with a system boiler, you’ll need to program a conventional boiler to come on ahead of schedule so that the water is heated up when you want to use it.
  5. The hot water tank needs insulating effectively – once heated, water will sit in the tank before use, which can allow it to cool down without proper insulation. However, it’s cheap and easy to insulate the cylinder well so this shouldn’t be a problem.

Download your free information pack

Find out how you could train as a domestic gas engineer by downloading a free copy of our information pack.

Combi vs conventional boilers

Conventional boilers are poles apart from combination boilers (their more modern counterparts). Given that both are in use throughout the UK, you should understand the ins and outs of these appliances as a heating engineer.

The key differences between these two varieties of boiler include:

  • Heat-only boilers require separate hot, cold, and feed and expansions tanks – combi boilers are compact and don’t need any additional cylinders.
  • A conventional boiler can support multiple users at the same time; by contrast, combination boilers only allow one shower or bath to be in use at any point, so aren’t suitable for larger households.
  • Combi boilers receive water directly from the mains supply, unlike regular boilers which are fed by a tank in the loft.
  • Combination appliances heat water as and when you need it, whereas heat-only boilers must be programmed to come on in advance.
  • There’s only one heat exchanger inside a conventional boiler compared to two independent exchangers in a combi boiler.


How does a conventional boiler work? 

We’ve discussed the various tanks that are required to support a heat-only boiler setup. Now it’s time to consider how all of these parts work together to provide a home with a hot water supply and central heating. This section provides a step-by-step explanation of the process.

First, water enters the home from the mains supply and travels to the cold water cistern in the loft. From here, the water is transferred to the adjacent cylinder, known as a feed and expansion tank. This supplies the boiler itself downstairs in the home and maintains the correct water level at all times, making adjustments when water is lost due to evaporation or leaks.

Once the water has travelled down from the feed and expansion tank into the boiler itself, it is passed over a heat exchanger (this may be powered by gas, oil, or electricity). Energy is transferred to the water and it is heated up. At this point, a pump inside the boiler moves the heated water on into the hot water tank.

The hot water remains inside this tank until it’s ready to be used. When a tap, shower, or bath is turned on, a pump inside the hot water tank then moves the heated water on to where it needs to be. The central heating system is also powered by this tank, which pumps water around the home to the radiators when the heating is switched on.


What size heat-only boiler do I need?

We compare different sizes of heat-only boiler based on their power output in kilowatts (kW). The main consideration when deciding which size to choose is the number of radiators in the house. As a rule of thumb when working with conventional boilers, you’ll need 1.5kW for every radiator and an extra 3kW for the water cylinder. This adds up to the following totals:

  • For a small house with six radiators, you’d need at least a 12kW heat-only boiler ([6 x 1.5] + 3).
  • An average-sized home with 10 radiators would require an 18kW conventional boiler ([10 x 1.5] + 3).
  • A large household with 20 radiators would need a 33kW regular boiler ([20 x 1.5] + 3).



This guide has offered a close look at heat-only boilers, outlining how they work, their pros and cons by comparison to other types, and what size you’ll need. To learn more about other kinds of boilers, check out our guides to combi boilers and system boilers.

If you’re interested in becoming a gas engineer or progressing your career further, explore our new entrant gas safe engineer courses or get in touch today for more information.

Download your free information pack

Find out how you could train as a domestic gas engineer by downloading a free copy of our information pack.

Share the post