Types of Boiler Explained: Which One Suits Your Home?

Updated July 2026 · Emergency Plumber Chelsea

How to tell which boiler you have. You can usually work it out in two minutes. If there is no hot water cylinder anywhere in the home and no water tanks in the loft, you have a combi. If there is a cylinder, often in an airing cupboard, but no cold water tank in the loft, you have a system boiler. If there is a cylinder and one or two tanks in the loft, you have a regular boiler. If the boiler sits behind a gas fire in a fireplace, it is a back boiler. The make and model on the front panel or the data plate inside the case will confirm it, and the manufacturer's manual usually says "combi" or "system" in the title.

Combi boilers. A combination boiler does two jobs in one box: it heats the radiators and it heats water instantly as it passes through, straight from the cold mains. Nothing is stored, so nothing is wasted keeping a cylinder hot, and the space an airing cupboard and loft tanks would take is freed up. Hot water arrives at mains pressure, which makes a single shower feel strong. The limit is flow. A combi can only heat so many litres a minute, so running a shower and a kitchen tap at the same time shares that flow between them. If the mains coming into the building is weak, a combi cannot make it stronger. Our guide to how a combi boiler works explains the inside of one.

System boilers. A system boiler heats water and stores it in a cylinder, usually an unvented cylinder fed directly from the mains. Because there is a store of hot water ready to go, two or three outlets can run at once without the shower going cold when someone fills a bath. The heating pump and expansion vessel are built into the boiler, so there is no loft tank or feed-and-expansion cistern. The trade-off is the space for the cylinder and the fact that once the stored hot water is used, it takes time to reheat. In larger Chelsea townhouses with several bathrooms, a system boiler with an unvented cylinder is the most common modern set-up. Our comparison of unvented and vented cylinders explains the cylinder side.

Regular or conventional boilers. A regular boiler is the traditional arrangement: a cold water storage tank in the loft feeds a hot water cylinder, and a smaller feed-and-expansion tank tops up the heating circuit. Many period houses in SW3 and SW10 were plumbed this way, and plenty still are. Regular boilers work well with older radiator systems that may not cope with the higher pressure of a sealed system, and a stored supply of water keeps some taps running during a short mains outage. The downsides are the space the tanks take, lower pressure at the hot taps on a gravity-fed supply (often solved with a shower pump), and tanks in the loft that need looking after. Our guide to cold water storage tank problems covers what goes wrong up there.

Back boilers. A back boiler sits in the fireplace behind a gas fire and heats the radiators and a hot water cylinder. They were common from the 1960s to the 1980s and are now well past their expected life. They are not installed new as a standard choice today, and parts for old units can be hard to find. If you still have one, it is worth planning a replacement before it fails in winter. The usual route is a combi or system boiler in a new position, which means a new flue and new gas and water pipework, so it is a bigger job than a straight boiler swap.

Electric boilers. An electric boiler heats water with elements instead of burning gas, so there is no flue and no gas supply needed. That makes it an option in some flats where a flue is impractical or not permitted, and it produces no combustion gases in the home. Running costs depend on electricity tariffs, which are usually higher per unit than gas, and larger homes may need a bigger electrical supply. Electric boilers come in combi and system versions like gas ones.

Condensing is not a separate type. Since 2005, new gas boilers fitted in England have had to be condensing models in all but a few exceptional cases, whichever type they are. A condensing boiler recovers heat from its flue gases, which is why it has a plastic flue and a condensate pipe running to a drain. So you choose between combi, system and regular, and the boiler you get will be condensing either way. Our explainer on what a condensing boiler is covers the details, including why the condensate pipe matters in a frost.

Heat pumps are an alternative, not a boiler. An air source heat pump heats radiators and a hot water cylinder using electricity and heat drawn from outside air. It needs outdoor space for the unit, a cylinder, and often bigger radiators, and in a conservation area the outdoor unit may need permission. It suits some Chelsea houses and not most flats. Our guide to heat pumps versus gas boilers weighs it up honestly.

Which type suits a flat? In most one-bathroom flats and mansion block apartments, a combi is the obvious choice: no cylinder cupboard, no loft (there often is no loft), and one outlet in use at a time. The exceptions are flats with two bathrooms that are used at the same time, or buildings where the incoming mains flow is poor, especially on upper floors. In those cases a system boiler with a compact unvented cylinder, or a combi paired with a water accumulator, can be the better answer. In leasehold blocks, check the lease and the managing agent's rules on flues before choosing.

Which type suits a house? For a townhouse with two or more bathrooms, a system boiler with an unvented cylinder usually gives the best mix of pressure and capacity. A large combi can serve a house with one main bathroom and a cloakroom, but it will struggle with two showers at once. A regular boiler still makes sense in a period house where the radiators and pipework are old and you want to keep a low-pressure open-vented heating system, or where you want stored water in the loft. Our comparison of combi vs system boilers goes deeper on that choice, and what size boiler you need covers the output.

Changing from one type to another. Switching a regular boiler to a combi is the most common change, because it frees up the airing cupboard and loft. It involves removing the tanks and cylinder, converting the heating to a sealed system and checking that the mains flow can support a combi. Older radiators and joints sometimes start to weep under the higher sealed-system pressure, so a good engineer checks them first.

Chelsea-specific points. Many houses here are in conservation areas, and a new flue on a visible elevation can need permission, which can rule out moving the boiler to the obvious spot. Our note on conservation area boiler flues explains the rules. Upper-floor flats in older blocks often have weaker mains flow than people expect, which counts against a combi. And a basement conversion with an extra bathroom can tip a house from combi territory into needing a cylinder.

Getting advice. If you are replacing a boiler and not sure which type fits, Emergency Plumber Chelsea will look at your hot water use, the number of bathrooms, the mains flow and where a flue can go, and recommend a type and size with a clear price before any work starts. There is no call-out fee on jobs over 3 hours. See our boiler installation service or call 020 3002 6667.

Quick tips

• No cylinder and no loft tanks means a combi; a cylinder with no loft tank means a system boiler; both means a regular boiler.
• Count the bathrooms likely to be used at the same time before choosing between a combi and a system boiler.
• Measure the mains flow rate before committing to a combi, especially in an upper-floor flat.
• If you still have a back boiler, plan its replacement in summer rather than waiting for a winter breakdown.
• Check conservation area and lease rules on flue positions before planning a boiler move.

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