Should I replace my conventional boiler with a combi?

If your existing conventional boiler has broken down or is near the end of its service life, or if you are extending your property or buying a new home, you might be considering changing from an existing conventional boiler to a combi.

Unsurprisingly, combi boilers have high appeal because they: take up less space; heat water on demand; require less maintenance; and are highly efficient. In fact, given the popularity of combi boilers we are sometimes even asked, ‘can you still buy conventional boilers?’. Of course, the answer is ‘yes’ as conventional boilers are still the better choice in certain situations; especially in large households as they can handle high heating and hot water demands very effectively.

To help you to make an informed decision about which solution is best for you, here is a guide to how both types of boiler work, together with the pros and cons of each.

What is a conventional boiler?

Also known as regular, heat-only and open-vent boilers, conventional boilers provide central heating and stored hot water and are very common in traditional heating systems throughout the UK. Requiring three tanks, this system typically has:

  • A cold-water cistern typically in the loft which is filled with water from the mains supply and feeds the storage cylinder
  • A small feed and expansion cistern to maintain the appropriate level of water in the central heating system
  • An insulated cylinder to store hot water (often located in an airing cupboard)


  • They are the go-to choice for larger homes as they enable your household to run multiple appliances and use hot water taps simultaneously without a significant drop in pressure
  • If your boiler breaks down, you will still have a hot water supply
  • They are a cost-effective solution if your property previously had an older heating and hot water system and existing pipework is still in place


  • More space for is needed for conventional boiler tanks and cylinder
  • They tend to be more expensive to install and are not quite as efficient as combi boilers as some heat is lost from the water in the storage cylinder
  • Once you have used all the hot water in the storage cylinder you will need to wait for the boiler to fire up again and produce new hot water as per your programme settings

What is a combi boiler?

A combi boiler is a single unit that provides a ‘combination’ of both hot water and space heating and it is the most popular type of boiler in the UK. It heats water directly from the mains when you turn on a tap ensuring hot water is available ‘on demand’ ie. only when it’s needed.


  • Changing from a conventional boiler to combi means you can remove old cold water storage tanks from your loft and your hot water storage cylinder from the airing cupboard
  • You will have instant hot water
  • They are highly efficient and you only heat the water you need, thereby reducing your heating bills as well as your carbon footprint
  • Installation requires less pipework which makes installation quick, easy and affordable
  • They ideal for properties with limited space due to their compact size ie. they are small enough to fit in a kitchen cupboard
  • Reduced maintenance costs


  • Not suitable if you have two or more bathrooms in use at the same time • Not suitable if you have poor water pressure or flow rates
  • Not suitable if you have old radiators and pipework


In summary, changing from a conventional boiler to combi will greatly benefit your household, and the environment too. Energy efficient, compact and quick to install, they provide hot water on demand and eliminate the need for storage tanks or cylinders. They are the ideal solution for small to medium size properties with average hot water demands. Just be aware that they are not the best choice if your home has weak mains pressure, as this means the water out of your showers and taps will be weak too. Remember also that if the pipework in the old heating system is in poor condition, a modern combi boiler might not be compatible and therefore installation costs will be higher than expected.

Conventional boilers, on the other hand, are still the ideal solution for large households with a high demand for hot water where water is being used in several locations at the same time. On the downside, access to hot water is not instant as heated water is stored in a cylinder. This form of storage also means the possibility of some heat loss but good insulation around the cylinder will keep the water hot for as long as possible.

Choosing a new boiler

Looking for a new or replacement boiler? Our guide will help you understand the best option for you.

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