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Heat pumps versus gas boilers: the honest comparison
Heat pumps are either the future of home heating or an expensive mistake, depending entirely on who is talking and what they sell. We fit gas boilers and we fit air conditioning. Here is what we would tell a friend.
There is very little honest writing about heat pumps. Companies that only fit heat pumps will tell you they suit every home. Companies that only fit boilers will tell you they suit none. Both are selling.
We fit gas boilers, and we fit air conditioning, which is a heat pump. We have no reason to push you either way, and we would rather talk somebody out of the wrong system than fit it and field the phone calls next winter.
How a heat pump actually works
A heat pump does not create heat. It moves it.
There is warmth in outside air even when it feels cold to you — there is energy in air right down to absolute zero, which is around minus 273 degrees. A heat pump uses a refrigerant circuit to collect some of that warmth, compress it to raise its temperature, and release it inside your home. It is a fridge running backwards, and it is exactly what your air conditioning does in reverse.
Because it is transporting heat rather than generating it, it can deliver more energy than it consumes. That is not a violation of physics; the extra energy was already outside, and the electricity is paying for the moving, not the making.
The efficiency claim, examined
You will see heat pumps described as 300 or 400 per cent efficient. The proper term is coefficient of performance, and a COP of 3 means three units of heat for one unit of electricity.
The figure is real but conditional. COP falls as the outside temperature drops and as the flow temperature you demand rises. A heat pump producing water at 35 degrees on a mild day might achieve a COP of 4. The same unit producing 55 degree water on a freezing morning might manage 2.5.
The number that matters is the seasonal figure, averaged across a real year in a real house — SCOP. Ask for it, and ask what flow temperature it assumes. A SCOP quoted at 35 degrees tells you nothing useful if your radiators need 50.
Running costs against a gas boiler
Here is the arithmetic that decides it.
Electricity typically costs three to four times as much per unit as gas. A heat pump delivers around three units of heat per unit of electricity. A gas boiler delivers around 0.9 units of heat per unit of gas.
Multiply it through and the two come out broadly comparable — sometimes the heat pump is ahead, sometimes the boiler, and the deciding factors are your tariff and how well your house holds heat.
That is a less exciting answer than either side offers, and it is the truthful one. A heat pump in a well-insulated house with large radiators, running at a low flow temperature, will beat a gas boiler on running cost. The same heat pump in a draughty solid-walled house with small radiators, forced to run hot to keep up, will lose — and the owner will be unhappy.
The house matters more than the equipment. That sentence is the whole article.
The radiator problem
This is the practical obstacle that turns theory into cost, and it is routinely underplayed.
A gas boiler heats water to around 70 degrees. A heat pump works efficiently at around 45. A radiator emits heat in proportion to the difference between its surface and the room, so the same radiator fed with cooler water puts out considerably less heat.
To get the same warmth from cooler water, you need more radiator surface. In practice that means many radiators in the house need replacing with larger ones, or with the same size in a higher-output design. Underfloor heating is ideal because it has an enormous surface area and works beautifully at low temperatures.
A heat pump quote that does not mention radiator sizing is not a complete quote. Ask specifically which radiators will be changed and what the calculated flow temperature is. If the answer is "it will be fine with your existing ones", be sceptical — it occasionally is, and usually is not.
Insulation comes first, always
If we could give one piece of advice to somebody considering a heat pump, it would be this: spend the first portion of the budget on the fabric of the building, not on the machine.
A heat pump in a leaky house is an expensive way to heat the outdoors. A well-insulated house needs less heat, which means a smaller heat pump, lower flow temperatures, fewer radiator changes and a better COP. Every pound spent on insulation makes the heat pump cheaper to buy and cheaper to run.
In rough order of value: loft insulation to current standards, draught-proofing, cavity wall insulation where the walls are suitable, then glazing, then solid wall insulation where it is feasible.
This advice loses us work. It is still the right advice.
What makes a good candidate
- Loft insulation at a decent depth and in good condition
- Cavity walls, insulated, or a modern well-built property
- Double glazing in reasonable order
- Underfloor heating, or radiators that are generous for the rooms
- Somewhere outside for the unit, with airflow and a sensible distance from a neighbour's window
- Currently on oil, LPG or direct electric — because that is what the heat pump is being compared against
- An owner planning to stay long enough to see the investment through
Tick most of these and a heat pump is a genuinely good decision that will be cheaper to run than what you have now.
What makes a poor one
- Solid walls with no insulation and no plan to insulate
- Single glazing, or draughty original windows
- Small radiators sized for a hot boiler flow
- No sensible outdoor position
- A recently fitted, efficient gas boiler with years left in it
- A planned move within a few years
Two or three of these and we would tell you to insulate first and revisit it. Nobody should be selling a heat pump into that house without saying so.
Grants, MCS and what it means for you
Government grants for heat pump installation are available in England and Wales and take a substantial amount off the cost. There is one condition that matters above all others: the grant can only be claimed through an MCS certified installer. No certification, no grant, however good the work.
This has two consequences for you as a customer. First, always check certification before anything else. Second, be aware that certification carries cost for the installer, which is part of why certified quotes are not always the cheapest headline figure.
Where heatfully stands. We are working through MCS certification for heat pumps. Until it is in place we will give you free, straight advice on whether your house suits one, and point you to a certified installer if it does, rather than pretending otherwise. We are not going to take on work we cannot get you the grant for.
The option nobody mentions
Almost all of the heat pump conversation is about air-to-water systems that replace a boiler and feed your radiators. There is a second kind that gets far less attention and is often the better answer to a smaller problem.
Air-to-air heat pumps heat the air in a room directly. You already know them: that is air conditioning. Run in reverse, an air conditioning unit delivers the same three-to-one efficiency, heating a room for roughly a third of what an electric heater would cost.
For one or two rooms your central heating has never handled — a loft conversion, a garden office, an extension at the end of a long pipe run — this is frequently the cheapest effective solution available. It needs no radiator changes, no cylinder and no upheaval, and you get cooling in summer as a by-product.
It does not qualify for the grant, because the grant is for air-to-water. But for the right problem it solves more, for less, than anything else on the list.
Questions to ask any installer
- Are you MCS certified? Ask first. No certification means no grant.
- What is the calculated heat loss of my house? They should have a figure, room by room, not a rule of thumb.
- What flow temperature is the design based on? Lower is better. Anything above 50 degrees deserves an explanation.
- Which radiators need changing? A specific list. "None" is a claim to be tested.
- What SCOP should I expect, at my flow temperature? Not the brochure figure at 35 degrees.
- What will it cost to run compared with what I have now? Based on my heat loss and my tariff.
- Where does the outdoor unit go, and how loud is it at my neighbour's window?
- What insulation work would you recommend first? An installer who says "none needed" without looking has not looked.
An installer who welcomes these questions is one worth using. An installer who finds them irritating has told you what you need to know.
Where this leaves you
If your house is well insulated, has generous radiators or underfloor heating, and you are currently on oil, LPG or direct electric, a heat pump is very likely the right move and will save you money.
If your house is solid-walled, draughty and on mains gas with a decent boiler, it is not — not yet, and not until the fabric is dealt with.
And if the real problem is one or two rooms that have never been comfortable, an air-to-air unit will probably solve it faster and cheaper than anything else, and cool them in August too.
Common questions
Are heat pumps cheaper to run than a gas boiler?
In a well-insulated house with large radiators, usually yes. In a poorly insulated house forced to run at a high flow temperature, usually no. The building matters more than the equipment.
Do heat pumps work in winter?
Yes. There is heat in air well below freezing and modern units extract it. Efficiency falls as it gets colder, which is why correct sizing and good insulation matter so much.
Will I need new radiators?
Usually some. A heat pump runs cooler than a boiler, so radiators need more surface area to deliver the same heat. Any quote that does not address this is incomplete.
How much is the government grant?
A substantial contribution, claimable only through an MCS certified installer. Check certification before anything else.
Are heat pumps noisy?
The outdoor unit produces a low hum, similar to an air conditioning unit. Position matters more than the specification, particularly in relation to a neighbour's window.
Can I keep my gas boiler as well?
Hybrid systems exist and can work well, with the heat pump doing most of the year and the boiler covering the coldest days. They are more complex and more expensive, and grant eligibility varies.
Is air conditioning a heat pump?
Yes, an air-to-air one. It heats a room directly rather than heating water for radiators, and for one or two awkward rooms it is often the cheapest effective answer.
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