The short answer: a combi boiler suits most flats and homes with one bathroom, while a system boiler suits larger homes with two or more bathrooms where several taps and showers are used at once. A regular boiler is mainly found in older homes and is sometimes the best fit where mains water pressure is low. Below we explain how each type works, compare them side by side and look at the questions we get asked most when planning a new boiler installation.
How a combi boiler works
A combination (combi) boiler provides heating and hot water from one compact unit. It heats water instantly from the mains when you turn on a tap, so there is no hot water cylinder and no tank in the loft.
- Hot water on demand at mains pressure, with no waiting for a tank to heat up
- Saves space: no cylinder cupboard and no loft tanks
- Usually the simplest and cheapest system to install
- Hot water flow is limited, typically around 10 to 14 litres a minute depending on the model, which is roughly one good shower at a time
How a system boiler works
A system boiler heats your radiators directly and heats a separate hot water cylinder. Most modern installations pair it with an unvented cylinder, which stores hot water at mains pressure, so there is no tank in the loft.
- Plenty of hot water for several taps and showers at the same time
- Strong, mains-pressure showers when paired with an unvented cylinder
- Needs space for a cylinder, usually in an airing cupboard
- Stored hot water can run out after heavy use and takes time to reheat
- Unvented cylinders need an annual service by a G3 qualified engineer
What about regular (heat-only) boilers?
A regular, conventional or heat-only boiler works with a hot water cylinder and a cold water storage tank in the loft. It is common in older homes, including many of the 1920s and 1930s houses in areas such as Chingford, Walthamstow and Enfield. It can be the right choice where mains pressure is too low for a combi, or where you want to keep existing pipework, but water pressure at the taps is usually lower unless a pump is added.
Combi vs system vs regular: side by side
| Combi | System | Regular | |
|---|---|---|---|
| Hot water cylinder | No | Yes | Yes |
| Tank in the loft | No | No | Yes |
| Best for | Flats and 1-bathroom homes | Homes with 2+ bathrooms | Older homes, low mains pressure |
| Several showers at once | Limited | Yes | Yes |
| Space needed | Least | Cylinder cupboard | Cylinder and loft tank |
| Water pressure | Mains pressure | Mains pressure (unvented cylinder) | Lower unless pumped |
| Easy to add a heat pump or solar later | No | Yes | Yes |
Which is best for a London flat?
For most flats, a combi is the clear winner: it is compact, there is no cylinder to find room for, and one bathroom rarely needs more hot water than a combi can deliver. Some newer developments, particularly around Angel, King's Cross and Clerkenwell, were built with a system boiler or electric heating and an unvented cylinder, and in those cases it often makes sense to keep the same set-up.
Which is best for a family home with two bathrooms?
If two showers are regularly used at the same time, a system boiler with an unvented cylinder will usually give a better experience. A combi can only split its flow between outlets, so pressure drops when a second tap opens. A larger combi helps, but it cannot match a well-sized cylinder for busy mornings.
Check your mains water first
A combi or unvented cylinder can only perform as well as the water coming into your home. We test your mains pressure and flow rate during the survey before recommending either.
Switching from a system or regular boiler to a combi
Converting to a combi is one of the most popular upgrades we carry out. It frees up the airing cupboard and loft, and gives you hot water on demand. It involves removing the cylinder and tanks, capping off old pipework and often upgrading the gas pipe, so it costs more than a like-for-like swap. Our guide to new boiler costs in London explains what that means for your budget.
Thinking about a heat pump in future?
Heat pumps work with a hot water cylinder, not instant hot water. If you think you might switch to a heat pump in the next few years, keeping a system boiler and cylinder can make that change easier, because the cylinder and much of the pipework are already in place.
Not sure which boiler you need?
The right choice depends on your water pressure, how many bathrooms you have, how many people live with you and how much space you have. We look at all of this before recommending a boiler and never push a bigger system than you need. Find out more about our boiler installation service, or get an instant quote.