Air Source Heat Pumps: The Complete UK Guide

What An Air Source Heat Pump Actually Does

It's a fridge running in reverse. A fridge takes warmth out of your food and dumps it into your kitchen. A heat pump takes warmth out of the outdoor air, even cold air, and dumps it into your home's central heating water. Same physics, opposite direction.

The only thing the electricity actually does is run a compressor that concentrates the heat. The heat itself is free, harvested from the air. That's why a heat pump can deliver 3 to 4 units of heat for every 1 unit of electricity it draws. A gas boiler, for comparison, gives you about 0.9 units of heat per unit of gas burned. Electricity costs about 3.3 times as much as gas per unit, though, so the running cost gap is much smaller than those numbers suggest (see Real Running Costs below).

Two practical implications:

• A heat pump is an electrically-driven central heating system. It replaces your gas, oil or LPG boiler entirely. It heats your radiators or underfloor heating and your hot water, from an outdoor unit and a hot water cylinder.
• Because it harvests heat from the air, the colder it gets outside, the harder it has to work. Efficiency falls in a cold snap, but a properly sized unit keeps heating the house; our cold weather guide has what UK trials actually found.

The Refrigeration Cycle, For The Curious

If you don't care how the magic happens, skip ahead. For everyone else, here's the four-step cycle running inside the heat pump every minute it's on:

1. Evaporate: A refrigerant liquid circulates through a coil in the outdoor unit. A fan blows air across the coil. Even at 0C, air contains enough energy to make the refrigerant boil and turn into a low-pressure gas.
2. Compress: An electric compressor squeezes the gas, raising its pressure and (because of the gas laws) its temperature.
3. Condense: The hot, compressed gas passes through a second heat exchanger that's connected to your central heating water. Heat transfers into the water; the refrigerant cools and condenses back to a liquid.
4. Expand: The liquid refrigerant passes through an expansion valve, dropping in pressure and temperature, and starts the cycle again.

The hot water from the second heat exchanger flows around your radiators and into your hot water cylinder, exactly the same way it did when you had a gas boiler.

COP, SCOP and What You Should Actually Care About

There are two efficiency numbers manufacturers quote, and only one of them is the one to focus on.

COP (Coefficient of Performance) is a snapshot. It tells you how efficient the heat pump is at one specific outdoor temperature, usually 7C, and one specific flow temperature (the temperature of the water leaving the heat pump and going to your radiators). A COP of 3.5 means 3.5 kW of heat per 1 kW of electricity at that exact moment.

SCOP (Seasonal Coefficient of Performance) is the figure that actually matters. It's the average across a typical heating season, including the cold snaps where efficiency drops and the mild stretches where it climbs. A SCOP of 3.5 means that across all the hours your heat pump runs, you get 3.5 units of heat per unit of electricity on average.

What's a good SCOP in 2026? Our rough guide:

• Under 3.5: mediocre. Either a poorly designed system, a poorly insulated home, or both. Below 2.97 it costs more to run than a 90% gas boiler at the current cap.
• 3.5 to 4.0: the realistic target for a well-designed install in a decently insulated home, paired with weather compensation.
• 4.0+: excellent, and achievable on a well-insulated home with low-temperature radiators or underfloor heating. The 307 monitored homes on HeatpumpMonitor.org with a full year of data averaged 4.01 when we checked in October 2026, though those are systems whose owners choose to publish their data.

The single biggest variable is flow temperature. The same heat pump is markedly more efficient sending water out at 35C than at 55C. This is why insulation, radiator sizing and weather compensation matter so much. They're all really about keeping your flow temperature low.

Air Source vs The Alternatives

Before you commit to an air source heat pump, it's worth seeing how it stacks up against the realistic alternatives in 2026: ground source, a hybrid (heat pump plus existing boiler), and a like-for-like new gas boiler. The short version is that air source wins for most UK homes, but the other three options each fit a specific corner case.

The Refrigerant Decision: R290 vs R32

Heat pumps need a refrigerant fluid to work. The fluid you'll see on quotes in 2026 is either R32 or R290 (propane), and the difference matters more for what happens in ten years than for what happens on install day.

*GWP = Global Warming Potential, the relative warming impact per unit with CO2 as 1. R32 and R410A are from gov.uk's published F-gas list; R290 isn't an HFC, so it isn't on that list, and its figure is the one Vaillant publishes.

One correction worth making. You may be told that from 1 January 2027 new heat pumps under 12kW must use a refrigerant with a GWP below 150, which would rule out R32. That is an EU rule. We could not find it anywhere in gov.uk's own F-gas guidance, which describes something different: the UK is phasing HFCs down by 79% by 2030 against average use between 2009 and 2012, through quota rather than through a product ban. The practical effect is real but slower: R32 stays legal and serviceable, its quota shrinks, and its price is likely to drift up. Don't let anyone use a cliff-edge deadline to justify a higher quote.

What this means for you:

• If you're installing now: ask for R290 if you can. The Vaillant aroTHERM plus and Daikin's Altherma 4 are both R290; for any other model, the refrigerant is on the datasheet. In our view it's the safer bet as the rules tighten.
• R32 is still fine. Existing R32 systems will keep running and being serviced, and refrigerant for repairs isn't going away. But the phase-down means top-ups are likely to get dearer over the system's life, and you're buying the previous generation.
• R290 is flammable (it's propane), which is why it sits sealed in the outdoor unit and why the manufacturer sets rules on where the unit can go. Your installer handles the siting; it's not something you'll notice in operation.
• R290 also reaches higher flow temperatures. Vaillant and Daikin both quote up to 75C, which helps in retrofits to older homes with existing radiator sizes, though every degree of flow temperature still costs efficiency.

Real Running Costs At Today's Prices

Let's do the maths properly. At the Ofgem price cap for 1 October to 31 December 2026: gas is 7.97p/kWh including 5% VAT, standard electricity is 26.32p/kWh with no VAT until 31 March 2027. Take a 3-bed home on gas: it uses around 9,500 to 10,300kWh of heat a year for heating and hot water. That's our estimate of the heat, from DESNZ's median and mean gas use for 3 bedrooms in England and Wales, put through a 90% boiler.

Three things stand out from those numbers.

• On a standard tariff, a heat pump is now cheaper than gas, by about £130 to £140 a year on this home. That's recent. Under the April 2026 cap the same sums gave 7.0p a kWh of heat from the heat pump against 6.4p from gas, and gas won. The July cap tipped it, and the October cap widened the gap by putting gas up 8.7% while electricity rose 0.8% and lost its VAT.
• A heat pump tariff more than doubles the saving, to around £300 to £320 a year. Switching tariff is no longer what makes the maths work, but it is still a big lever you control.
• If you are on oil, the saving is in a different league: roughly £550 to £600 a year on this home, before the extra £1,500 the Boiler Upgrade Scheme pays off-gas homes until March 2027. For LPG there's no price cap on bulk supplies and we haven't a sourced price, so put your own invoice into the calculator.

One caveat worth more than any of the above: these figures assume a SCOP of 3.5. Break-even against a 90% gas boiler is a SCOP of 2.97, so a poorly designed system running at 2.5 makes heat at 10.5p a kWh and still costs more than gas. Installation quality, not the technology, now decides whether you save.

The Heat Pump Tariff Game-Changer

Several UK suppliers now sell dedicated heat pump tariffs that price electricity differently across the day. The cheap windows line up with when a heat pump can do useful work (early morning, the middle of the day, overnight), which changes the running cost equation.

We'd take EDF's Heat Pump Tracker unless your controller can reliably keep the heating out of Cosy's peak, in which case Cosy. Octopus raised Cosy's rates on 21 October 2026, and on our example home EDF's Tracker now comes out a little cheaper unless you schedule most of your heating into the cheap hours. Cosy's 4pm to 7pm peak sits 50% above its day rate, so on Cosy you do need to time-shift; check your controller can do it. Either way, a well-scheduled heat pump at SCOP 3.5 makes heat for about 5.6p to 5.7p a kWh on these tariffs, against 8.9p from gas.

To see which tariff is cheapest for your whole household, use our heat pump tariff comparison.

System Components: What You're Actually Paying For

A complete air-to-water heat pump install includes:

• Outdoor unit: the heat pump itself. It sits on a wall bracket or ground pad and contains the compressor, fan and outdoor heat exchanger. In our worked example it's the biggest single item on the quote (see the worked example in how to compare heat pump quotes).
• Indoor unit / hydrobox: the box that connects the outdoor unit to your central heating pipework. Some systems are monobloc, with most of this in the outdoor unit; others use a separate indoor unit with the pump, expansion vessel and controls in it.
• Hot water cylinder: a well-insulated tank providing your domestic hot water. If you currently have a combi boiler, you'll need a cylinder and a place to put it (airing cupboard, loft, utility room). Heat pumps don't do instant on-demand hot water like a combi.
• Controls and weather compensation: a wall-mounted controller plus an outdoor temperature sensor. Modern brand controllers handle weather compensation natively. See the next section.
• Buffer tank or volumiser (sometimes): a small water tank that smooths out heat pump operation. Some installs need one for hydraulic reasons; many don't. See "What You Don't Need" below.

Weather Compensation: A Big Efficiency Lever

One of the most useful things you can do for your heat pump's running cost is set up weather compensation correctly, then leave it alone.

Weather compensation means the heat pump automatically adjusts its flow temperature based on how cold it is outside. On a mild day it sends cooler water to your radiators, on a freezing one hotter water. Your home stays at the same indoor temperature all year round, but the heat pump runs at the lowest flow temperature the conditions allow, which means the highest efficiency.

A heat pump left on a fixed flow temperature, set for the coldest day of the year, runs hotter than it needs to for most of the winter, and every degree costs efficiency. Let the SCOP slide to 2.5 and you're paying more for heat than the gas boiler you took out (10.5p against 8.9p a kWh).

What to ask your installer:

• Which weather compensation curve will you set up? Good installers will design and document this. Ask for it in writing.
• Will you commission with weather compensation enabled? In our view it's what you're paying for, so get a yes in writing.
• How will I adjust it later? A good control system lets you tweak the curve through an app or wall controller without needing an engineer.

Cold Weather Performance: The Truth

The "do they work in cold weather" question gets asked a lot. The short answer is yes, but with nuance.

Efficiency falls as the outdoor temperature drops, and on the coldest days some systems bring in a backup electric element to top up. A properly designed system shouldn't lean on it much; it's a safety net, not a daily occurrence.

Defrost cycles are the visible reality of cold-weather operation. When the outdoor unit's coil starts to ice over in cold, damp weather, the heat pump briefly reverses its cycle to send warm refrigerant into the outdoor coil and melt the ice. You may see steam come off the unit, and notice the fan stop for a few minutes.

What gets sold as a problem ("it goes off all the time in winter!") is actually the system doing exactly what it should.

For the full picture, including real UK trial data and what a freezing day actually costs to heat, see do heat pumps work in cold weather?

Planning Permission and the Updated MCS 020(a) Rules

In England and Wales, installing an air source heat pump usually doesn't require planning permission. It's classified as Permitted Development provided the install meets the MCS 020(a) standard, which is what your MCS-certified installer will design to.

The current rules in England, from Class G of the permitted development order as the Planning Portal sets them out:

• Noise limit: the calculated sound pressure must be 37.0 dB(A) or lower at the assessment position, which is 1 metre outside the centre of any door or window to a habitable room of a neighbouring property. MCS 020 a) states it as "the permitted development noise limit of 37 dB LAeq,5mins". The 42 dB(A) figure you may still see quoted is the older MCS 020 number, which still applies in Scotland and Northern Ireland but not in England or Wales.
• One unit on a terrace, semi or block of flats; two on a detached house.
• The 1m boundary distance rule has been removed. There is now no minimum boundary distance at all, provided the noise calculation passes.
• Size limit 1.5 cubic metres for the outdoor unit on a house, 0.6 cubic metres on a block of flats.
• Listed buildings and scheduled monuments are out entirely. Pitched roofs are out, and on a flat roof the unit must be at least 1m from the edge.
• In a conservation area or World Heritage Site, the unit can't go on a wall or roof fronting a highway, or nearer to a bounding highway than the house. Elsewhere in a conservation area it's still permitted development, so a conservation area doesn't automatically mean a planning application.
• Everywhere else, a wall-mounted unit on a highway-facing wall must be at ground floor storey level, not above it.
• You can't have both a heat pump and a wind turbine at the property under permitted development.
• Your council can remove these rights with a planning condition or an Article 4 direction, so check if you're on a newer estate or in a conservation area.

What to ask your installer: "What does the MCS 020(a) sound calculation say for my proposed install location?" A serious installer will run the calculation as part of the design and share the result. If they wave the question off, that's a red flag.

What To Look For In Your Quote

A good heat pump quote names everything specifically. If your quote uses generic phrases like "5kW heat pump", push back. You want the list below (and there's a fuller version, with red flags and a worked example, in how to compare heat pump quotes):

• Brand and exact model number. "Vaillant aroTHERM plus VWL 75/6 A 230V" not just "5kW heat pump".
• MCS certification confirmed. Look the installer up on the MCS website. Required for the BUS grant.
• Refrigerant type (R290 or R32). R290 is the future-proof choice.
• Design flow temperature. Lower is better. Below 50C is good; 45C or lower is excellent.
• Heat loss survey results. A proper installer will produce a room-by-room heat loss calculation to MCS's design standard (MIS 3005-D). Ask for it. The kW recommendation should match.
• Radiator changes. Which radiators (if any) will be upgraded? Ask which size and to what spec. A well-insulated home may need only a few radiators upgraded, if any.
• Hot water cylinder. Brand, size (litres), insulation rating. On our estimates a cylinder is £500 to £1,500 of the quote.
• Buffer tank. Are they fitting one, and why? Ask them to justify it.
• Weather compensation. Will it be commissioned active, with a designed curve specific to your home?
• Warranty period and registration. Long warranties come with conditions and the conditions are where people get caught. Vaillant's own product page says the aroTHERM plus has a two-year standard guarantee that "can be extended up to 7 years". Its footnote limits that to Platinum and Renewable Partner installers who also register the appliance with a Vaillant cylinder. Other brands run similar accredited-installer registration schemes. Ask for the exact term, the exact conditions, and who is responsible for registering it, in writing.
• Annual service price after install. Published one-off prices are £189 to £275. Some installers offer the first year free.

What You Don't Need (Common Upsells)

Things we'd push back on if they appear in your quote:

• An unnecessary buffer tank. Buffer tanks are needed in specific cases (very small system volume, multi-zone setups where zones close off frequently). Ask: is this required because of a specific design constraint, or is it optional? An unnecessary buffer adds cost.
• An oversized heat pump. A unit much bigger than the heat loss is more likely to switch on and off in short bursts in mild weather. A government-commissioned study found that short cycling (runs under about six minutes) cuts a heat pump's performance, air source more than ground source. Insist on a unit sized to your heat loss calculation, not "the next size up to be safe". Our heat pump size calculator gives you a ballpark figure to hold a quote against.
• Replacing all your radiators "for the heat pump". On our estimates a typical semi changes five to seven, not a full house's worth. If your installer is quoting more than £2,800 in radiator changes on a typical semi, the top of our estimated range for one, get a second opinion. A larger or poorly insulated home can need £1,050 to £4,800 (our radiators guide).
• "Smart" extras as line items. The heat pump's own controls already do the important part: Vaillant sells its sensoCOMFORT as a weather compensation control, and Daikin's units come with its Onecta app. Ask what the standard controller does before paying extra for an add-on that duplicates it.
• A three-phase electrical upgrade. Domestic units like the aroTHERM plus run on an ordinary single-phase 230V supply. If one appears on your quote, ask why your house needs it.

Maintenance, Service and Lifespan

Heat pumps need an annual service, similar to a gas boiler. Published one-off prices are £189 to £275 (British Gas £189, Octopus £275 for one it installed), and many warranties require it. Our heat pump service cost guide has each brand's warranty rules. The annual service typically includes:

• Cleaning the outdoor coil and clearing any debris from the fan
• Checking for refrigerant leaks
• Inspecting electrical connections and the controller
• Checking system pressure and the filter on the central heating side
• Updating firmware and controller settings if needed

Lifespan: a quality heat pump lasts 15 to 20 years. No manufacturer warranty covers anything like that: Vaillant's standard guarantee is two years and the seven-year headlines are conditional extensions, so the later years are on you. Towards end of life you're mainly buying the outdoor unit again (the pipework, cylinder and electrics may be reusable); on our estimates the unit alone is £3,000 to £7,000.

The hot water cylinder typically lasts 20 to 25 years, often longer than the heat pump itself.

Honest Pros and Cons

The genuine pros

• Much more efficient than a gas boiler. The government's Warm Homes Plan says heat pumps are "around 3 times more efficient than gas boilers", and monitored UK installs on HeatpumpMonitor.org made an average of 4.01 units of heat per unit of electricity when we checked
• £7,500 BUS grant in England and Wales, or £9,000 for a property replacing oil or LPG with no mains gas connection, until March 2027
• 0% VAT on installation until 31 March 2027
• 15 to 20 year lifespan with a correctly designed system and proper servicing, against 10 to 15 for a gas boiler
• Can be quiet enough for permitted development with the right spot. The two makers we checked that label a figure sound power (Grant, and Vaillant, whose label also mentions a 3m distance) give roughly 54 to 65 dB(A). That's roughly 36 to 47 dB(A) three metres away, against a limit of 37 dB(A) at the neighbour's window, so where you put it matters: distance and screening do most of the work (our noise guide shows how)
• Pairs naturally with solar panels for very low running costs. On our model, a 3-bed gas home that adds a heat pump, 5kWp of solar and a 10 kWh battery saves about £1,360 a year against gas and grid electricity, on a heat pump tariff, with the gas supply disconnected so you stop paying its standing charge
• Built for where new homes are going: the Future Homes Standard makes low-carbon heating the default in new English homes from 24 March 2027

The genuine cons

• Higher upfront cost than a like-for-like boiler swap, even after the grant: about £2,000 more for a typical 3-bed, on our figures
• Standard electricity still costs about 3.3 times as much per unit as gas (26.32p against 7.97p), so the saving depends entirely on the heat pump clearing a SCOP of 2.97. It's a real saving at 3.5, and it disappears at 2.5. And it will move: electricity has no VAT only until 31 March 2027, and both prices change with the price cap every three months.
• The outdoor unit needs a place to live, with clear airflow
• Requires a hot water cylinder, which combi-boiler homes don't currently have
• Old, poorly insulated homes need radiator upgrades or insulation work first to be efficient
• The install itself takes a few days to a week (British Gas says most of its installs take five days) and involves some disruption (drilling for pipework, plumbing changes)
• Efficiency does drop in cold snaps, although modern units handle it well

Bottom Line

We'd fit an air source heat pump in 2026 if four things are true: your home is even reasonably insulated, your boiler is due for replacing (or you're on oil or LPG), you can fit the outdoor unit somewhere outside, and you'll stay long enough for it to pay back. On our figures that's 6 to 7 years on a heat pump tariff when the boiler was due anyway, 14 to 15 on a standard one, on fuel alone. Count a heat pump's dearer servicing (about £70 a year) and it's about 8 on a heat pump tariff and past the heat pump's life on a standard one, so we'd go on a heat pump tariff. DESNZ Boiler Upgrade Scheme statistics put the middle half of real installs at about £11,100 to £15,600 before the grant. With the £7,500 BUS grant, the median 6 to 8kW install lands within about £2,000 of a new combi. On a standard tariff it's already cheaper to run than gas at SCOP 3.5; on a heat pump tariff it's clearly cheaper; with solar panels, it's genuinely cheap.

The brands worth asking for are Vaillant aroTHERM plus (premium, R290, guarantee extendable to 7 years on conditions), Mitsubishi Ecodan (guarantee up to 7 years, depending on your installer's tier with Mitsubishi) and Daikin (5 years free once registered, with the R290 Altherma 4 alongside the R32 Altherma 3). For a comparison of these and the rest, see our best heat pumps UK guide.

For the full cost picture see our heat pump costs guide; for the grant detail see heat pump grants in the UK; and to model your specific running cost, use the savings calculator. Then get three MCS-certified installer quotes and pick the one whose answers to "what's the design flow temperature, will weather compensation be on, and is a buffer tank really needed" feel sharpest.