A heat pump is roughly three times more efficient than a gas boiler and can still cost you more to run. Both halves of that sentence are true at the same time, and the gap between them is where most disappointed homeowners live.
Key takeaway: a heat pump is worth it when four things line up — your electricity price is less than about three times your gas price per unit, your house doesn't leak heat, your heat emitters can work at a low water temperature, and you'd value the cooling as well as the heating. Get all four and the decision is easy. Miss two and the honest answer is to insulate first and revisit in a few years.
What a Heat Pump Actually Does
A boiler or furnace makes heat by burning fuel. A heat pump moves heat that already exists — out of the outdoor air or the ground, and into your house. Because moving is cheaper than making, you get out more energy than you put in.
That ratio is the COP (coefficient of performance). A COP of 3 means one unit of electricity delivers three units of heat. It is not fixed: COP falls as the outdoor temperature drops and as the water temperature you ask for rises. The seasonal average is reported as SCOP in Europe and HSPF2 in North America. When a spec sheet quotes a single glorious COP, check what outdoor and flow temperatures it was measured at — the number is meaningless without them.
Check 1: The Price Ratio (Do This First, It Takes Two Minutes)
This is the calculation that decides running cost, and almost nobody does it before getting quotes.
Find the per-kWh price of electricity and of gas on your own bills, including standing charges only if you'd be dropping the gas connection entirely. Then:
A heat pump costs less to run than a gas boiler when the electricity-to-gas price ratio is lower than the heat pump's seasonal COP divided by the boiler's efficiency.
Worked through: if your electricity costs three times what your gas costs per kWh, and a realistic seasonal COP for your installation is 3.2, and your existing boiler runs at about 0.9 efficiency in practice, then the heat pump wins — 3.2 ÷ 0.9 is about 3.6, comfortably above your ratio of 3. Push the price ratio to 4 and the same installation now costs more to run than the boiler it replaced.
Nothing here is a reason to dismiss heat pumps. It is a reason to run your numbers, because electricity and gas prices vary enormously between countries and tariffs and the answer genuinely flips. If you have solar and a time-of-use tariff, the ratio may be very favourable; in a market with cheap gas and expensive electricity, it may not be.
One thing the calculation misses: a heat pump runs on electricity, and grids keep getting cleaner, while a gas boiler burns the same fuel on its last day as its first. If your reason is carbon rather than cost, the carbon case is far more robust than the money case in most grids.
Check 2: Your Heat Loss
A heat pump delivers heat gently and continuously at a low temperature; a boiler blasts it out in short bursts at a high one. That difference punishes a leaky house twice — you need a bigger unit, and the bigger unit costs more to buy and to run. So deal with the envelope first:
- Air leakage is the cheapest win and the one most people skip. Our guide to draft-proofing your home covers the order to do it in.
- Loft and attic insulation is the next best value per unit spent — see how to choose attic insulation.
- Rooms that are already too hot or too cold signal a distribution problem that a new heat source will not fix, and may make worse. Why some rooms run hotter than the rest is worth reading before you size anything.
Any competent installer will do a room-by-room heat loss calculation — Manual J in North America, an equivalent standard elsewhere. If a quote arrives without one, based on floor area or on the size of your old boiler, get another quote. Sizing from the old boiler is how houses end up with oversized units that short-cycle, wear out faster, and dehumidify badly in cooling mode.
Check 3: Your Emitters
This is the one that surprises people, and it's a physical constraint rather than a preference.
A gas boiler typically sends water to radiators at 60–70°C. A heat pump is most efficient at 35–45°C — and its efficiency drops sharply the harder you push it above that. Lower water temperature over the same radiator surface means less heat delivered into the room. So one of three things has to happen:
- Underfloor heating — the ideal match: large surface area, low temperature. Rarely worth retrofitting throughout an existing house purely for this.
- Larger radiators in some or all rooms. Often only a handful need changing, but it's real cost and it belongs in the quote rather than appearing later as a surprise.
- Air-to-air instead — ductless mini-splits that heat and cool the air directly, sidestepping the water-temperature problem. The standard approach in much of North America and Asia, and the least disruptive retrofit where there's no radiator system worth keeping. It won't heat your water, so you'll need a separate solution.
If an installer proposes a heat pump into your existing radiators with no discussion of flow temperature, they are either confident your radiators are already generously sized or they haven't looked. Ask which.
Check 4: Your Climate — and the Cooling Question
Modern cold-climate air-source units are rated to keep working well below freezing, commonly down to around −15°C and in some models considerably lower. But output and COP both fall as it gets colder, so cold-climate models are sized against the coldest days rather than the average ones, and most systems include a backup heat source for the few worst days of the year.
Two cold-weather realities worth knowing:
- Defrost cycles. In damp weather near freezing, frost forms on the outdoor coil and the unit periodically reverses to melt it. This is normal and brief. A unit that seems to be "blowing cold" for a few minutes in December is usually defrosting, not broken.
- Continuous running is correct. A heat pump run like a boiler — off all day, blasted on for two hours in the evening — performs badly. It wants steady low-temperature output. People who fight that habit change are the people who report disappointing bills.
On the other side of the ledger: an air-to-air heat pump is an air conditioner running backwards, so it cools in summer at no extra equipment cost. As summers get hotter, that's a real benefit a boiler replacement doesn't offer, and it belongs in the decision rather than being treated as a bonus.
The Trade-Offs Brochures Skip
Upfront cost is meaningfully higher than a like-for-like boiler swap, before any grant. Ground-source is higher still — better and steadier COP, because the ground stays warmer than winter air, but boreholes or trenches dominate the price.
Incentives change constantly. National and regional schemes get introduced, revised and withdrawn on political timetables, so check what is actually live in your jurisdiction this month rather than trusting an article or a salesperson's leaflet. Don't let a grant that might expire drive the timing of a decision you'd otherwise defer.
Electrical capacity. Some homes need a panel or consumer-unit upgrade. Cheap to discover now, expensive to discover on installation day.
Noise and siting. The outdoor unit is not silent, and many places regulate how close to a boundary it can sit. Walk the site with the installer and stand where your neighbour's window is.
Refrigerant matters. Older units use R-410A, which has a high global warming potential; R-32 is lower, and propane-based R-290 is very low but flammable, which constrains siting. If the environmental case is your main motivation, ask which refrigerant is in the model quoted.
Lifespan. Air-source units are commonly expected to last around fifteen to twenty years with servicing, broadly comparable to a boiler. Payback arithmetic that assumes thirty years is doing you no favours.
So — Worth It or Not?
Likely yes if your electricity-to-gas price ratio is around 3 or below, your house is reasonably insulated or you're willing to insulate first, you have underfloor heating or generously sized radiators or no wet system at all, and you want cooling in summer.
Likely not yet if your ratio is 4 or above, your house is draughty and uninsulated, and your radiators were sized for 70°C water. Spend the money on the envelope instead — it lowers your bills immediately whatever heat source you end up with, and it makes the heat pump cheaper and smaller when you do come back to it.
Genuinely marginal in between, which is most houses. Get two or three quotes that each include a room-by-room heat loss calculation, a stated design flow temperature, and a list of emitters to be changed. Quotes without those three things aren't comparable to anything.
FAQ
Do heat pumps work in cold weather? Yes — cold-climate models are rated to operate well below freezing, and they're standard in countries with long, hard winters. What changes is that output and efficiency fall as temperatures drop, which is why sizing is done against your coldest design day and why most systems keep a backup heat source for the worst few days.
Is a heat pump cheaper to run than gas? That depends on your local electricity-to-gas price ratio, not on the technology. Divide the seasonal COP by your boiler's real-world efficiency; if the answer is bigger than your price ratio, the heat pump is cheaper. Run it on your own bill.
Air-to-air or air-to-water? Air-to-water if you have a wet radiator or underfloor system worth keeping and want hot water from the same unit. Air-to-air if you don't — it's a simpler retrofit and gives you summer cooling, but you'll need a separate hot-water solution.
Next Step
Do the price-ratio calculation tonight, off your own bills. It costs nothing and it tells you more than any brochure will.
Then, when you're ready to compare what's actually on the market, see the sustainable home picks and comparisons on Just Green — the comparison pages weigh energy use, materials, and how credible each green claim really is, so you can judge the options side by side instead of one salesperson at a time.
