Running cost
Combi boiler running costs & annual gas bills
A new A-rated combi (around 94 percent ErP) uses meaningfully less gas than a 15-year-old boiler running at 78 percent. Here is what each property type pays per year, the realistic savings from a swap, and how long the new boiler takes to pay itself back.
Average bills
Annual gas bill by property
Modelled at the Ofgem April 2026 unit rate of 6.99 p / kWh. These are heating + hot water only, not your electricity bill.
| Property | Gas use (kWh / yr) | Old boiler bill | New combi bill | Annual saving |
|---|---|---|---|---|
| 1-bed flat | 6,500 | £580 | £480 | £100 |
| 2-bed flat | 8,500 | £760 | £630 | £130 |
| 2-bed terrace | 10,500 | £940 | £780 | £160 |
| 3-bed semi | 12,500 | £1,120 | £930 | £190 |
| 3-bed detached | 14,500 | £1,300 | £1,080 | £220 |
| 4-bed detached | 17,500 | £1,570 | £1,300 | £270 |
| 5-bed detached | 21,000 | £1,880 | £1,560 | £320 |
Old boiler modelled at 78 percent efficiency, new combi at 94 percent. Real consumption varies with insulation and occupancy.
Tool
Estimate your bill
Pick your property type, the efficiency of the boiler you are replacing, and your typical heating hours. The tool returns annual cost and payback period for a typical £2,500 install.
Running cost estimator
Annual gas bill
Old boiler (78 percent efficient)
£1,120
New A-rated combi (94 percent)
£930
Yearly saving
£190
Pays back a £2,500 install in 13.2 years
The maths
Why a new combi runs cheaper
Efficiency in plain numbers
A 78 percent boiler wastes 22 p in every £1 of gas it burns. A 94 percent combi wastes only 6 p. Same heat in the radiators, less gas through the meter. The saving is structural, not seasonal.
Modulation matters
Modern combis modulate down to 20 percent of full output. Old boilers run on or off only. Modulation lets the boiler match the load, especially in milder weather, squeezing another 5 to 10 percent off your bill.
Squeeze more out
Five upgrades that cut running cost
Smart thermostat
Save £100 to £200 / yr
Nest, Hive, Tado. Geofencing and learning routines avoid heating an empty house.
TRVs on every radiator
Save 5 to 8 percent
Thermostatic radiator valves let unused rooms run cooler. £15 to £25 each fitted.
Lower flow temperature
Save 5 to 10 percent
Drop the flow to 55C. The combi spends more time in condensing mode.
Magnetic filter
Save 1 to 3 percent
Keeps sludge out of the heat exchanger. Long-term efficiency holds up.
Weather compensation
Save 5 to 8 percent
Outdoor sensor adjusts flow temp to outside air. Premium combis often have this built in.
Radiator balancing
Save 2 to 5 percent
Free, just labour. A balanced system delivers heat evenly with less boiler effort.
FAQs
Common questions
How much does a combi boiler cost to run per year?v
A new A-rated combi in a typical 3-bed semi costs around £900 to £1,000 a year on the Ofgem April 2026 unit rate. Smaller flats are nearer £500 to £600. Larger 4 to 5 bed homes can run to £1,400 or more.
Are combi boilers cheaper to run than system boilers?v
Slightly cheaper for hot water in a small household because there is no storage loss from a cylinder. For larger households with simultaneous demand, a system boiler with good cylinder insulation is comparable or sometimes cheaper because the boiler runs longer steady-state cycles.
Will a smart thermostat really save money?v
Yes, when used properly. Geofencing avoids heating an empty house, learning schedules avoid over-heating, and most save £100 to £200 a year on a typical home. The 1 to 3 year payback assumes you actually let it adapt rather than overriding it daily.
Should I run my combi at 55C or 65C flow temperature?v
55C if your radiators are large enough to keep the home warm, your combi spends more time in condensing mode, you save 5 to 10 percent on gas. If your radiators are undersized for 55C operation (the rooms feel cool), bump to 65C or upgrade individual radiators.
Keep reading