Explained
What actually happens inside one, why it runs at 45°C instead of 70°C, what that does to your radiators and your hot water, and the houses where a gas boiler is still the better buy. Written by people with no reason to push you either way.
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A heat pump does not make heat. It collects heat that is already outside in the air, concentrates it, and pushes it into your house. That is why it can deliver more heat than the electricity it uses, which no boiler of any kind can do.
The price of that is temperature. A gas boiler hands you water at around 70°C. A heat pump works best at around 45°C, and the lower you can go the better it performs. Cooler water carries less heat, so the radiators have to be bigger to warm the same room — and that, not the heat pump itself, is usually what makes the quote a shock.
Everything else on this page follows from those two paragraphs. It also needs a cylinder, because there is no combi version, and it needs somewhere sensible outside with clear air around it. Where those things are easy, it is an excellent machine. Where they are not, we will say so — there is a whole section further down about the houses where a boiler still wins.
The machine itself
Four components in a sealed loop, and the same fluid going round and round them for fifteen years. Once this makes sense, nothing else about heat pumps is mysterious.
The detail
This is the objection everybody raises, and it comes from a reasonable instinct: cold air does not feel like it contains heat. But ‘cold’ is only relative to us. The refrigerant inside a heat pump boils at a temperature far below anything a British winter produces, so to that refrigerant, air at freezing point is comfortably warm and worth harvesting.
It does get harder as the air gets colder. There is less heat available per cubic metre, the compressor has to work through a bigger temperature gap, and efficiency falls. That is why the system is sized against the coldest conditions your area actually sees, rather than against a pleasant day.
When the outdoor coil drops below freezing, moisture in the air condenses on it and turns to frost. Frost blocks airflow, so periodically the machine reverses itself for a few minutes and sends hot refrigerant out to the coil to melt it. You will see steam and a puddle. It is not a fault; it is the machine working.
It does matter where that water goes. Defrost water running across a path that then freezes is a genuine hazard, and it is one of the things that decides where the unit can sit.
Not to blow warm air at anything. It is there to drag a very large volume of outdoor air across the coil, because each cubic metre only gives up a small amount of heat. That is why the unit is the size it is, why it needs clear space in front of it, and why boxing it in behind a fence quietly wrecks its performance — it ends up re-breathing the cold air it has just finished with.
The claim that sounds too good
A boiler converts fuel into heat and can never give out more than it took in. A heat pump is a pump: it adds a modest amount of electricity to a large amount of heat that was already outdoors and moves the pair of them indoors. That is why the output can exceed the electricity in, and it is not a trick.
The crux
Same room, same amount of heat needed, two flow temperatures. The radiators are drawn to scale against each other. If you only read one section of this page, read this one.
What follows from it
A radiator does not put out a fixed amount of heat. What it delivers depends on how much hotter the water inside it is than the room around it. Halve that difference and you do not get half the output — you get considerably less than half, because the relationship is not a straight line.
Every radiator in a catalogue is quoted at one standard condition, and every manufacturer publishes a table of correction factors for running at anything else. These are Stelrad's, and the other makers' agree closely. The room is assumed to be at 20°C.
| If the system runs at | Mean water temperature | Difference over the room | Correction factor | What one radiator gives |
|---|---|---|---|---|
| The catalogue condition | 70°C | 50 | 1.000 | its quoted output |
| 70°C flow, 60°C return | 65°C | 45 | 0.871 | 87% of quoted |
| 55°C flow, 45°C return | 50°C | 30 | 0.513 | 51% of quoted |
| 45°C flow, 39°C return | 42°C | 22 | 0.342 | 34% of quoted |
Correction factors published by Stelrad at stelrad.com/trade/stelrad-correction-factor, read 18 August 2026. The flow and return pairs are illustrative design conditions, not a promise about any particular system.
Divide the two rows that matter and you get the number this whole page turns on. 0.871 ÷ 0.342 is about two and a half. To get the same heat into the same room at 45°C that you had at 70°C, the radiator needs an output rating roughly two and a half times as high.
In practice that rarely means a radiator two and a half times the length. A double-panel double-convector gives roughly twice what a single panel of the same size does, so the usual answer is a deeper radiator, slightly longer, in the same alcove. Sometimes it fits without anybody noticing. Sometimes there is a window in the way and it does not.
If cooler water needs more surface area, then the best emitter is a very large one. An entire floor is about as large as it gets, which is why underfloor heating and heat pumps suit each other so well and why underfloor systems can run cooler still.
The reverse is also worth knowing: if you already have underfloor heating downstairs, you are further along this road than you think.
The heat pump is rarely the expensive surprise. The emitter work is. So when you compare two heat pump quotes, the thing to compare is not the brand on the box — it is the design flow temperature and the list of which radiators change.
A quote that promises a lower flow temperature and changes more radiators is usually the cheaper system to live in. A quote that keeps every radiator by designing for 55°C is cheaper on the day and dearer every winter afterwards. Neither is dishonest. They are just different bets, and you should be told which one you are being sold.
The house's side of the bargain
Seen from above. The unit wants clear air in front of the fan and a route for defrost water, and the pipework has to get back into the house. Clearance figures are set by each manufacturer and they differ, so the honest answer is that the installation manual decides, not a rule of thumb.
Four requirements
An outdoor unit is roughly the size of a large washing machine, standing on a level base or on wall brackets. It needs air to move freely into it and away from it, and it needs to not be discharging straight at a wall, a fence or its own intake.
Think about the things that change too. A hedge grows. A bin store gets built. A shed goes up next door. Any of those can turn a well-sited unit into a badly sited one a few years later, and nobody connects the two.
This is the surprise for anybody who currently has a combi. A heat pump cannot make hot water instantly the way a combi does, because it cannot get water hot enough fast enough. It heats a cylinder, gradually, and you draw from the cylinder.
So you need somewhere for it: an airing cupboard, a corner of a bedroom, a loft with a floor strong enough. In houses that had a cylinder taken out to fit a combi ten years ago, this is sometimes the entire problem — the space became a shower room and it is not coming back.
You do not need a perfectly insulated house. But insulation changes every number in the project: a leakier house needs a bigger unit, bigger radiators and a higher flow temperature, so it costs more to install and more to run, three times over.
Loft insulation and draughtproofing are the cheapest things in this entire subject and they reduce the size of everything downstream. If your money is limited, that is where the first of it should go — and outstanding insulation recommendations on your EPC can affect grant eligibility as well.
A heat pump needs its own properly rated circuit, and the incoming supply and consumer unit get checked as part of the design. Occasionally the network operator has to be involved. This is not usually a problem, but it is the kind of thing that wants finding out at survey rather than on installation day.
Two arrangements
The difference is simply where the refrigerant circuit stops. In a monobloc it never leaves the outdoor unit and water crosses the wall. In a split, refrigerant crosses the wall to a hydrobox indoors and the heat is handed to water in there.
Most domestic installations in this country are monobloc, and for good reason: the refrigerant circuit is sealed and charged in a factory, there is no refrigerant joint to make in your house, and any competent heating engineer can connect it because what they are connecting is water pipe.
The trade-off is that there is now water outside. It has to be protected from freezing, either with antifreeze in the circuit or with automatic protection that keeps it moving — and antifreeze slightly reduces how well the water carries heat, so the design has to allow for it.
A split keeps all the water indoors, which removes that problem entirely and can be neater where the pipe run is long or awkward. The cost is that refrigerant pipework now runs into the house and has to be jointed, pressure-tested and evacuated there. That is F-Gas work, not plumbing, and it wants doing by somebody who does it regularly.
Hot water
Same vessel, same litres, completely different coil. This is the part of a heat pump project that gets glossed over most often, and reusing the wrong cylinder is a disappointment you live with daily.
The store
Boiler water never touches your bath water. It runs through a coil inside the cylinder and the stored water takes heat through the coil wall. How fast that happens depends on the coil's surface area and on how much hotter the coil is than the water around it.
With 70°C boiler water heating a cylinder to 60°C, that difference is large and a small coil keeps up easily. With heat pump water it is a fraction of that, so the same coil reheats the cylinder painfully slowly — or forces the heat pump to run at a high flow temperature to compensate, which is exactly the efficiency you were buying it for. A cylinder intended for a heat pump has a much larger coil for precisely this reason.
Water stored permanently at low temperature is a legitimate concern, and heat pump systems deal with it by periodically raising the cylinder to a higher temperature to pasteurise it. Depending on the design that is done by the heat pump itself working harder, or by an immersion heater.
It is a small amount of expensive heat once a week rather than a constant cost. It should be set up at commissioning and explained to you, not left on a factory default that nobody ever looks at.
Two bathrooms used at the same time need stored litres, and a heat pump cannot bail you out mid-shower the way a big combi can. If your family all shower between seven and eight, say so — that fact changes the cylinder more than the number of bedrooms does.
Our own cylinder page covers the vented and unvented question and the litres-per-person rule, and all of it applies here too.
Living with one
The single biggest behavioural difference, and the reason a heat pump run like a boiler costs more than it should. Nothing on either axis is numbered on purpose — the point is the shape.
How running it differs
With a boiler, the sensible habit is to heat the house twice a day and let it cool in between. A boiler has power to spare, so it can drag the house back up quickly.
A heat pump does not, and does not want to. It is most efficient running gently and continuously at the lowest flow temperature that will hold the house at temperature. Making it recover a cold house at eight in the morning forces it to the top of its range, which is exactly where it is least efficient. The habit that saved you money on gas costs you money on a heat pump.
Rather than one fixed flow temperature, the controls read the outside temperature and adjust the water temperature continuously — a little warmer when it is bitter, cooler when it is mild. Since efficiency rises every degree you drop, this is doing real work every hour of the winter.
It is also the setting most often left as it came out of the box. Ask whoever commissions the system to show you the curve they set and why.
People genuinely do complain that their radiators are not hot. They are not supposed to be. A radiator at 45°C is barely warmer than a mug of tea gone cold, and it is putting out its designed heat perfectly well — there is just more of it, working more gently, for longer.
The room ends up more even than it was, because it never swings up and down. Most people prefer it within a fortnight. Some never stop missing the hot towel rail, and it is fair to say so before you buy rather than afterwards.
Closing rooms off with valves fights a heat pump rather than helping it. Shutting half the radiators forces the same output through the remainder, which pushes the flow temperature up and the efficiency down. Most well-designed heat pump systems run open, with the whole house held at a steady temperature and only genuinely unused rooms turned back.
That is close to the opposite of the advice on our TRV page, which is written for boilers. Both are right for the machine they describe.
Planning and noise
In England most installations are permitted development, which means no planning application — but permitted development is a list of conditions, not a free pass, and one of the conditions is a noise calculation.
| The condition | What it says |
|---|---|
| Where this applies | England, under Class G of the permitted development order. Scotland, Wales and Northern Ireland set their own rules. |
| The standard it must meet | The unit has to comply with the MCS Planning Standards. Those include a noise calculation carried out at a defined assessment position outside the nearest habitable room of a neighbouring property. |
| How many you can have | One, on or within the curtilage of a house that is not detached, or of a block of flats. Two, on a detached house. |
| Size of the outdoor unit | The compressor unit including any housing must not exceed 1.5 cubic metres on a house, or 0.6 cubic metres on a block of flats. |
| Roofs | Not on a pitched roof at all. On a flat roof, not within one metre of the external edge. |
| Listed buildings and monuments | Not permitted development on a listed building, or anywhere in its curtilage, or on a scheduled monument site. That means a planning application. |
| Conservation areas and World Heritage Sites | Not on a wall or roof fronting a highway, and not sited nearer to a highway than the nearest part of the house is. |
| Everywhere else | Not on a wall fronting a highway above ground floor level. |
| Wind turbines | You cannot have a heat pump under this class if there is a wind turbine on the same building or in the curtilage. |
| Cooling | The permission is for heating. A unit used solely for cooling is not covered. |
| The boundary rule people still quote | There used to be a condition requiring the unit to be at least a metre from your boundary. It was removed on 29 May 2025. If somebody told you years ago that your garden was too narrow, that advice may simply be out of date. |
Summarised from the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 14, Class G, as amended by S.I. 2025/560, read on legislation.gov.uk on 18 August 2026 and shown there as current to that date. This is a summary of the law and not advice on your property — if there is any doubt, the local planning authority is the only body whose answer counts.
It is not silent. Outdoors you get a low hum from the fan and the compressor, loudest when it is working hardest on a cold morning, and there is a period of extra noise during a defrost. Indoors there is genuinely nothing — no burner firing, no ignition clicking, no flue.
Almost every noise complaint we hear about traces back to siting rather than to the machine: a unit tucked into a corner where the sound bounces, or discharging across a fence at somebody's bedroom window. The MCS calculation exists precisely to catch that before it is installed, and it is worth asking to see the result rather than being told it will be fine.
The money side
We have not put an installed price on this page, because we would be making it up — a heat pump price is mostly emitter work and that is different in every house. What we can set out accurately is the government support, because that is published.
The headline figure is £9,000, and it is real — but it is not the number most people will get. The Boiler Upgrade Scheme pays £7,500 towards an air source heat pump for everyone who qualifies. A further £1,500 is available until March 2027, which takes the maximum to £9,000 — but only where the property is heated by oil or LPG and has no mains gas connection. Both conditions, not either. If the property has a mains gas connection the figure is £7,500, and no amount of searching will change that. We would rather you knew which of those two numbers is yours before you start planning around it.
| The question | The answer |
|---|---|
| How much is the grant? | Up to £9,000 towards an air source (air-to-water) heat pump: £7,500 as standard, plus the £1,500 off-grid addition in the row below where it applies. The same figures for ground source. £5,000 for a biomass boiler, and £2,500 for an air-to-air heat pump. One grant per property. |
| Who actually gets the full £9,000? | Only properties meeting both conditions. Until March 2027 there is an additional £1,500 towards an air or ground source heat pump if the property is heated by oil or LPG and has no mains gas connection. Both have to be true, so a home on mains gas gets £7,500 even if it currently runs on oil, and an off-grid home already on electric heating does not qualify for the addition either. |
| Where does it apply? | England and Wales. Scotland and Northern Ireland run their own schemes. |
| Who is eligible? | You have to own the property — including a business, a second home or one you let — and you have to be replacing a fossil fuel heating system. |
| Who applies for it? | Your installer, who has to be MCS certified. It comes off your bill and the money never passes through your hands. |
| Is there a deadline once I apply? | Yes. The heat pump has to be installed and commissioned within 120 days of the grant application or it stops being eligible. |
| What is excluded? | Hybrid systems that pair a gas boiler with a heat pump. Most new builds. Social housing. Any property already funded for a heat pump or biomass boiler. And you cannot use it to replace an existing low carbon system. |
| What about VAT? | Installing a heat pump in a home is zero-rated until 31 March 2027, after which the 5% reduced rate applies. Worth knowing for comparison: a replacement energy-efficient gas boiler is standard-rated, so it carries 20% VAT. |
Grant figures and eligibility from gov.uk/apply-boiler-upgrade-scheme; VAT treatment from HMRC VAT Notice 708/6. Both read on 18 August 2026. Grant levels, eligibility and VAT rates are set by government and change — check the current position before you rely on any of it.
You will find sites that tell you a heat pump costs a specific amount a year to run, or pays for itself in a specific number of years. We are not going to, because it would be a guess dressed up as a fact. The answer depends on your tariff, on the flow temperature the system is designed for and on how carefully it is commissioned — and those three vary more than the machines do.
What we can do is point you at the government's own free service, which models cost and carbon for your actual property rather than for an average one: gov.uk/check-heat-pump. It is free, it does not sell you anything, and it is a better answer than ours would be.
Side by side
Not a scorecard. Some of these rows favour the boiler and some favour the heat pump, and which ones matter is entirely about your house.
| Gas boiler | Air source heat pump | |
|---|---|---|
| Flow temperature | Around 70°C | Around 45°C, and lower is better |
| Your radiators | Sized for it already; they stay | Some or most usually need upsizing — about two and a half times the output rating for the same room |
| Hot water | A combi makes it instantly, no cylinder needed | A cylinder, always. There is no combi equivalent |
| Outside the house | A flue terminal | A unit about the size of a large washing machine, needing clear air |
| Inside the house | A box on a wall | The cylinder, plus pipework and controls |
| What it burns | Gas, in your house, with a flue | Nothing. No combustion, no flue, no gas safety check |
| Energy in versus heat out | Always less heat out than fuel in | More heat out than electricity in, because most of it came from outdoors |
| Unit cost of the fuel | Gas is the cheaper unit | Electricity costs more per unit — but far fewer units are needed |
| Grant | None | Up to £9,000 — £7,500 as standard, and £1,500 more only if the property is on oil or LPG with no mains gas |
| VAT on installation | 20% on a replacement boiler | 0% until 31 March 2027, then 5% |
| How long it takes | A day on site, and we quote it in ninety seconds | A survey, a design and several days on site |
| If it dies in January | Replaceable inside a week | Not a job to rush; you would fit a boiler and plan properly |
| Planning | None needed | Permitted development in England if it meets every Class G condition |
| Running pattern | Heat twice a day, let it cool between | Run it gently and continuously; the old habit costs you money |
Grant and VAT figures as sourced in the tables above. Everything else is our own judgement from fitting both, and reasonable people in the trade will argue about some of it.
The honest bit
This site exists to sell boilers, so take the following in that spirit — but we would rather tell you now than have the argument in your kitchen. There are houses where a heat pump is the obvious right answer, and there are houses where it is being sold to people it will disappoint.
If you are in the first column, we would like to quote you for a boiler — it takes ninety seconds and there is no salesperson attached. If you are in the second, we would rather you spoke to the right people than bought the wrong machine from us.
Questions
Yes, if it has been sized against a calculated heat loss for the house rather than against the output of whatever boiler happens to be there now, and if the radiators can deliver that heat at the flow temperature the system is designed for. Those are two big ifs, and they are where bad installations go wrong — not in the machine.
The physics is not the limit. There is usable heat in the air well below freezing, because the refrigerant inside the machine boils at a temperature far lower than anything a British winter produces. The limit is the design work, which is why a heat pump cannot be quoted from a postcode and a photograph the way a boiler swap can.
Usually not all of them, and occasionally none. Radiators get oversized by accident all the time — somebody fitted the next size up because it was in the van, or the room has been insulated since. Those ones are already fine.
But some will change, and in a house with small panels sized tightly for a 70°C boiler it can be most of them. Anyone who tells you before surveying the house that your radiators will be fine is guessing, and it is the single most common way a heat pump quote turns out to be wrong.
Honestly: it depends, and anybody who answers that with a percentage before seeing your house is guessing. A heat pump needs far fewer units of energy for the same warmth, but electricity costs more per unit than gas does. Which way that lands depends on your tariff, the flow temperature the system is designed around and how carefully it was commissioned.
The government runs a free service called Heat Pump Check at gov.uk/check-heat-pump which models cost and carbon for your actual property. We would rather point you at that than make up a number that suits us.
Technically yes — that is a hybrid system, and it is a genuinely sensible answer for some houses. There is a catch worth knowing before you plan around it: the Boiler Upgrade Scheme does not pay a grant for a hybrid setup that combines a gas boiler with a heat pump.
So a hybrid has to stand up on its own merits without the £7,500, which changes the sums considerably.
In England most domestic installations fall under permitted development, which means no application — but only if the installation meets every condition in Class G, including complying with the MCS Planning Standards. Listed buildings are excluded outright. Conservation areas, World Heritage Sites and flats have extra restrictions, and there are limits on unit size, roof positions and how many you can have.
One thing worth flagging, because it is still repeated everywhere: the old rule requiring the unit to be at least a metre from your boundary was removed on 29 May 2025. If you were told years ago that your garden was too narrow, that advice may simply be out of date. Scotland, Wales and Northern Ireland set their own rules.
It is not silent, and anyone who says it is has not stood next to one. Outdoors you get a low hum from the fan and compressor, most noticeable when it is working hardest on a cold morning and during a defrost cycle. Indoors there is nothing to hear at all — no burner, no ignition, no flue.
Noise is not left to opinion either. The MCS Planning Standards require a calculation at a defined assessment position outside the nearest neighbouring habitable room, and passing it is a condition of the installation being permitted development. Siting the unit badly is what causes complaints, not the machine.
From a cylinder, always. There is no combi equivalent of a heat pump, and that is the detail that surprises people most — if you currently have a combi and no airing cupboard, finding somewhere for a cylinder is usually the hardest part of the whole project.
You can absolutely still fill a bath, and an unvented cylinder at mains pressure will run a shower harder than most combis do. What changes is that hot water is a finite tank rather than an endless stream, so the cylinder has to be sized for how your household actually uses it.
Almost certainly not this week. A heat pump is a designed installation: a room-by-room heat loss survey, emitter sizing, cylinder siting, an electrical check, a grant application and several days on site. That is not a job anybody should be rushing through while a family is cold.
Fit a boiler now if you need heat now. It does not close the door — and if you want to keep the option open, ask for a cylinder with a coil big enough for a heat pump, and tell whoever quotes you that is why.
Heat pumps sit with Qualis Heat Pumps in our group, and ArcticNord handles air conditioning and air-to-air. It is all the same company, so you get the same answer whichever part of it you ring.
That is also why this page is willing to tell you when a boiler is the better buy. We are not losing the customer either way, so there is no reason to sell you the wrong machine.
Heat pumps sit with Qualis Heat Pumps in our group, and ArcticNord handles air conditioning and air-to-air heating. Every one of us is part of Qualis Energy Limited, so you get the same answer whichever of us you ring — including the answer that you should stay on gas for now.
Fifteen questions and a real fixed price. No survey, no callback, and nobody will ring you about a heat pump afterwards.
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