Swap like for like and a combi, a system boiler and a regular boiler cost about the same to run, because a modern condensing boiler of any type turns roughly 90p of every pound of gas into useful heat. The running-cost gap between the three is smaller than most people expect. What really varies is how each one makes your hot water, and a badly matched setup quietly wastes money all year round. This guide explains the three types plainly, shows where the pennies actually leak, and helps you choose if yours is due for replacement.
The short answer. All three burn gas at similar efficiency, so for the same heat they cost about the same. A combi makes hot water on demand with no tank, so it has no standing heat loss, which makes it the cheapest and simplest choice for a small or medium home with one bathroom. A system or regular boiler heats a stored cylinder, which loses a little heat around the clock, but it copes with several taps at once and works with solar and off-peak electricity. Match the type to your hot water habits and the running cost looks after itself.
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The three types in one minute
Every gas boiler heats water for your radiators. The difference is what it does for the hot water at your taps. A combi heats that water instantly as it flows through, so there is no storage. A system boiler heats a stored cylinder of hot water and has most of the extra parts built in. A regular boiler, sometimes called heat-only or conventional, also heats a cylinder but relies on separate tanks up in the loft. Here is how they line up.
| Type | Hot water | Needs a cylinder? | Loft tanks? | Standing loss? |
|---|---|---|---|---|
| Combi | On demand, from the mains | No | No | Little to none |
| System | From a stored cylinder | Yes | No | Yes, from the cylinder |
| Regular | From a stored cylinder | Yes | Yes | Yes, from the cylinder |
Combi boilers: hot water straight from the mains
A combi is the single most common boiler fitted in British homes today, and for good reason. It heats water only when you turn a tap on, pulling it straight from the mains and warming it as it passes through, so there is no cylinder sitting in the airing cupboard and no cold water tank in the loft. That makes it compact, cheaper to fit and a natural fit for flats and smaller houses. Because nothing is stored, nothing sits there cooling down, so there is no standing heat loss on the hot water side.
The trade-off is flow rate. A combi can only heat water as fast as its burner allows, so running a shower and a kitchen tap at once can leave both feeble, and a big family bathroom can outrun a smaller model. There is no immersion heater to fall back on if the boiler breaks, and you cannot easily store cheap night-rate electricity or spare solar as hot water, because there is no tank to put it in. One more thing worth checking: many combis have a preheat or keep-warm mode that holds a little water hot inside the boiler so it responds faster, and that burns a small amount of gas all day. If quick hot water at the tap matters less to you than the gas it costs, the eco setting that turns preheat off is worth finding in the manual.
System boilers: a cylinder without the loft tanks
A system boiler heats a hot water cylinder, usually a pressurised unvented one, but keeps the pump and expansion vessel inside the boiler itself, so there are no tanks in the loft. Hot water is stored ready to use, which means several taps and showers can run at once at full mains pressure without fighting each other. It also opens the door to storing energy: a solar diverter can push spare panel output into the cylinder, and an immersion heater gives you a backup and a way to use off-peak electricity.
The cost is space and standing loss. The cylinder takes up a cupboard, and however well it is lagged it leaks a little heat to the room all day, which the boiler has to keep topping up. Run out of stored hot water and you wait for it to reheat. For a home with two or more bathrooms, or a household where several people shower in the same short window, that stored capacity usually earns its keep.
Regular boilers: the traditional layout
A regular boiler, the older conventional setup, also heats a cylinder but needs a cold water feed tank and a small expansion tank in the loft to work. It is what you find in a lot of older houses, and if your home already has one with sound pipework there is often no strong reason to rip it all out. Where mains pressure is poor, the gravity-fed layout can actually deliver a gentler but steadier flow than a pressurised system would.
The downsides mirror the system boiler, plus the loft tanks: more to go wrong, more to freeze in a hard winter, and the same cylinder standing loss. Older regular setups are also the ones most likely to have a tired, thinly insulated cylinder that leaks heat far faster than a modern one, which is where a chunk of avoidable cost hides.
Where the running-cost difference actually comes from
Because the burners are all condensing and land around the same efficiency, two homes heated to the same temperature in the same house will pay close to the same for the heating itself whichever boiler is fitted. The gap shows up on the hot water side, and it comes down to standing loss from the cylinder. A modern, well-insulated cylinder might lose around 1.5 to 2 kWh a day just sitting there hot. An old, poorly lagged one can lose 3 to 4 kWh a day.
Put money on that. At an example gas rate of 7.97p per kWh, the current Ofgem cap for October to December 2026, a modern cylinder losing 2 kWh a day costs about £58 a year in reheating (2 x 365 x 7.97p). A tired old one losing 4 kWh a day costs around £116 a year, and that is before you have washed a thing. A combi sidesteps that entirely, though it is not quite free of waste either: it reheats water from cold on every draw, and any keep-warm preheat adds a little standby gas. If your cylinder is heated by the immersion rather than the boiler, the sums get worse, because electricity at 26.32p per kWh is over three times the price of gas. You can price an immersion top-up for your own usage on the running cost calculator.
Figures are illustrative, at 7.97p per kWh gas and 26.32p per kWh electricity, the Ofgem cap for October to December 2026. Real cylinder losses depend on the cylinder, its jacket and the room it sits in.
Which type suits which home
There is no single best boiler, only the right one for how you live. As a rough steer:
- Small flat or house, one bathroom, decent mains pressure: a combi is usually cheapest to buy, fit and run, with no cylinder loss to worry about.
- Two or more bathrooms, or several people showering at once: a system boiler with a good cylinder handles the demand a combi would struggle with.
- Solar panels or an Economy 7 tariff: you want a cylinder, so you can store free solar or cheap night-rate electricity as hot water. A combi cannot do that. See Economy 7 and night rates.
- Older home, low mains pressure, existing gravity system: a regular boiler may be the least disruptive swap, though it is worth getting an installer to check whether an upgrade to a system boiler is feasible.
Whatever the type, size it correctly. An oversized boiler short-cycles and wastes gas; an undersized one cannot keep up. That is a job for a heating engineer with a proper heat-loss calculation, not a rule of thumb.
Getting the most from whatever you already have
You do not need a new boiler to cut the bill, and most of these cost nothing. Turn the flow temperature down so the boiler condenses properly, which is the biggest free win of the lot and explained in boiler flow temperature and how condensing boilers work. If you have a cylinder, lag it well and set it to about 60C, covered in hot water cylinder temperature. If you have a combi, decide whether the preheat is worth its standby gas. And if the boiler is old and on its way out, weigh a like-for-like replacement against a heat pump before you commit, since the running costs can be closer than you think; start with heat pumps explained. For what a boiler costs to run over a year and what drives that figure, see gas boiler running cost and the wider home heating hub.