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. The maths is dramatically in the heat pump's favour even before you look at fuel prices.

Two practical implications:

• A heat pump is an electrically-driven central heating system. It replaces your gas/oil/LPG boiler entirely. It heats your radiators, your underfloor heating, and your hot water cylinder, all from one box outside your house.
• Because it harvests heat from the air, the colder it gets outside, the harder it has to work. Performance falls in cold snaps but not as much as you'd expect: modern units run efficiently down to -15°C and are designed to keep heating the home down to -25°C with backup support.

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 0°C, 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. The gas now sits at 55-80°C depending on system design.
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 7°C, 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 COP across a typical UK heating season, including the cold snaps where efficiency drops and the mild stretches where it climbs. SCOP of 3.5 over a year 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?

• 3.0 or below: mediocre. Either a poorly designed system, a poorly insulated home, or both. Avoid.
• 3.5-4.0: the realistic target for a well-designed install in a decently insulated home. Most quality brands hit this when paired with weather compensation.
• 4.0+: excellent. Achievable with a top-tier brand (Vaillant Arotherm Plus, Daikin Altherma 3) on a well-insulated home with low-temperature radiators or underfloor heating.

The single biggest variable is flow temperature. A heat pump running at 35°C flow can hit SCOP 5.0+. Same heat pump running at 55°C flow drops to SCOP 2.5-3.0. 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. All three figures are from gov.uk's published F-gas list.

One correction worth making, because a lot of pages (and some sales patter) say otherwise. You'll 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: Great Britain is phasing HFCs down by 79% by 2030 against average 2009–2012 use, 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 drifts up. Don't let anyone use a cliff-edge deadline to justify a higher quote.

What this means for you:

• If you're installing in 2026 or early 2027: ask for R290 if you can. The Vaillant Arotherm Plus and the Mitsubishi PUZ-WZ are both R290. They're slightly more expensive than R32 equivalents but you're buying ahead of the regulation curve. Servicing R290 systems will get easier over time as more F-gas-qualified engineers train on it.
• R32 is still fine. Existing R32 systems will keep running and being serviced for decades, and refrigerant supply for repairs isn't going away. But the quota phase-down means top-ups get dearer over the system's life, and you're buying the previous generation.
• R290 is mildly flammable (it's literally cooking gas), which is why it sits in a sealed outdoor unit and shouldn't be installed against a wall containing a vent or window. Your installer will handle the siting; it's not something you'll notice in operation.
• R290 also tends to deliver higher flow temperatures (up to 75°C from some models) without efficiency falling off a cliff. That makes it the better choice for retrofits to older homes with existing radiator sizes.

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. A typical 3-bed UK semi uses around 11,000 kWh/year for heating and hot water.

Three things stand out from those numbers.

1. On a standard tariff, a heat pump is now cheaper than gas, by about £147 a year on this home. That is new. Under the April 2026 cap the same sums made gas the cheaper option; the October cap reversed it by putting gas up 8.7% while electricity rose 0.8% and lost its VAT.
2. A heat pump tariff more than doubles the saving, to around £345 a year. Switching tariff is no longer what makes the maths work, but it is still the single biggest lever you control.
3. If you are on oil, the saving is in a different league: roughly £590 to £790 a year on this home, before the extra £1,500 the Boiler Upgrade Scheme pays off-gas homes until March 2027. LPG lands in similar territory, but there's no price cap on bulk LPG so check it against your own invoice rather than ours.

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 still costs more than gas. Installation quality, not the technology, now decides whether you save.

The Heat Pump Tariff Game-Changer

From 2024 onwards, several UK suppliers have launched dedicated heat pump tariffs that price electricity differently across the day. The cheap windows align with when heat pumps run hardest (early morning, late afternoon, overnight), which transforms the running cost equation.

Cosy Octopus is the most popular choice and the one most modern heat pump installers will recommend pairing with. The three cheap windows are explicitly designed around when heat pumps work best: pre-heating the home in the early morning before you wake up, topping up at midday, and a final boost late evening. The 16:00-19:00 peak rate sits 50% above the day rate and is genuinely punitive, so you do need to time-shift, but most modern controllers automate this once configured.

The savings calculator above lets you model your bill on a heat pump tariff vs standard electricity. Spoiler: on Cosy Octopus, most well-designed heat pump systems undercut gas.

System Components: What You're Actually Paying For

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

• Outdoor unit: the heat pump itself. Roughly 600-1000mm wide, 800-1200mm tall, sits on a wall bracket or ground pad. Contains the compressor, fan, and outdoor heat exchanger. Most of the system cost.
• Indoor unit / hydrobox: the box that connects the outdoor unit to your central heating pipework. Some brands (Vaillant, Mitsubishi) integrate most of this into the outdoor unit; others (Daikin, Samsung) use a separate indoor hydrobox with the pump, expansion vessel and controls in it.
• Hot water cylinder: a well-insulated thermal store, typically 180-300 litres, providing your domestic hot water. If you currently have a combi boiler, you'll need a cylinder fitting and a place to put it (airing cupboard, loft, utility room). This is non-negotiable: heat pumps don't do instant on-demand hot water like a combi.
• Controls and weather compensation: a wall-mounted controller (often touch-screen) 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 (50-200L) 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: The Single Biggest Efficiency Lever

The single most impactful thing 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 10°C day, it'll send 35°C water to your radiators. On a freezing -3°C day, it'll send 50°C water. Your home stays at the same indoor temperature (say 21°C) all year round, but the heat pump runs at the lowest possible flow temperature for the conditions, which means the highest possible efficiency.

A heat pump on weather compensation typically achieves SCOP 3.5-4.5. The same heat pump running on a fixed flow temperature, sized for the coldest day of the year, drops to SCOP 2.5-3.0. That's 25-40% more electricity for the same heat, and at SCOP 2.5 you're paying more than the gas boiler you took out.

What to ask your installer:

• Which weather compensation curve will you set up? Good installers will design and document this. Bad ones will leave it on factory default and walk away.
• Will you commission with weather compensation enabled? Some installers leave fixed-flow as the default to avoid customer complaints in the first cold snap. Push back: weather comp is what you're paying for.
• 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. Vaillant's sensoCOMFORT and Daikin's onecta app are both decent at this.

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.

Modern UK heat pumps are designed to deliver full heat output down to about -7°C to -10°C outdoor temperature. Below that, the COP drops to 2.0-2.5 and many systems activate a backup electric heating element to top up the heat. Properly designed systems in well-insulated homes rarely need the backup element in practice; 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 (typically below 5°C in damp UK weather), the heat pump briefly reverses its cycle to send hot refrigerant into the outdoor coil and melt the ice. You'll hear it click, see steam come off the unit, and notice the fan stop for 2-5 minutes. This happens every 1-3 hours when conditions are right and is completely normal.

What gets sold as a problem ("it goes off all the time in winter!") is actually the system doing exactly what it should. Defrost cycles take maybe 1-2% of total seasonal output and aren't worth worrying about.

For the full picture, including real UK trial data at -3°C and -5°C 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, straight from Class G of the permitted development order and MCS 020 a):

• 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 L_Aeq,5mins". The 42 dB(A) figure you'll see quoted everywhere 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 1.5m³ for the outdoor unit on a house, 0.6m³ on a block of flats in England (1m³ in Wales).
• 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, which is more useful than the blanket "conservation areas need permission" you'll read elsewhere.
• 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.

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 approval confirmed. Look it up by name on the MCS list. Required for the BUS grant.
• Refrigerant type (R290 or R32). For a 2026/2027 install, R290 is the future-proof choice.
• Design flow temperature. Lower is better. Below 50°C is good; below 45°C is excellent. Above 55°C and you're losing 20-30% efficiency.
• Heat loss survey results. A proper installer will produce a Heat Loss Calculation (often using MCS 3005 methodology). Ask for it. The kW recommendation should match.
• Radiator changes. Which radiators (if any) will be upgraded? Ask which size and to what spec. Most well-insulated homes need 0-3 radiator upgrades.
• Hot water cylinder. Brand, size (litres), insulation rating. A 250L unvented cylinder with thick foam insulation costs £1,200-£1,800.
• Buffer tank. Are they fitting one, and why? Often unnecessary; 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 2-year standard guarantee that "can be extended up to 7 years", and 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. Typically £150-£300/year. 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) but lots of installers fit them by default to "smooth operation". A well-designed system on weather compensation usually doesn't need one. Ask: is this required because of a specific design constraint, or is it optional? An unnecessary buffer adds £600-£1,200 and a small efficiency penalty.
• An oversized heat pump. Some installers default to oversizing because it gives them margin against a cold snap, and because larger units are more profitable. An oversized heat pump short-cycles in mild weather, killing efficiency and reducing component lifespan. Insist on a unit sized to your heat loss calc, not "the next size up to be safe".
• Replacing all your radiators "for the heat pump". Most homes need 0-3 radiators upgraded, not a full house's worth. If your installer is quoting £3,000+ in radiator changes, get a second opinion.
• "Smart" extras as line items. Modern brand controllers (Vaillant sensoCOMFORT, Daikin onecta, Mitsubishi MELCloud) include weather compensation, app control, scheduling, and tariff integration as standard. You shouldn't be paying extra for these as add-ons.
• Three-phase electrical upgrade for a single-phase home. Heat pumps under 11kW run perfectly on single-phase 230V. Three-phase upgrades cost £2,000-£10,000 and aren't needed for normal residential systems.

Maintenance, Service and Lifespan

Heat pumps need an annual service, similar to a gas boiler. Typical cost: £150-£300/year, depending on the brand and your service plan. The annual service typically includes:

• Cleaning the outdoor coil and clearing any debris from the fan
• Checking refrigerant pressure (the law also requires a leak test for systems above a certain refrigerant charge)
• Inspecting electrical connections and the controller
• Checking system pressure on the central heating side
• Updating firmware and controller settings if needed

Lifespan: a quality heat pump lasts 20 to 25 years. The compressor is the part most likely to fail. Note that no manufacturer warranty covers anything like that: five years is the usual standard offer and the seven-year headlines are conditional extensions, so the last fifteen years are on you. Towards end of life, expect a £3,000-£5,000 unit replacement (pipework, cylinder and electrics will be reusable; it's the outdoor unit and controller you're buying again).

The hot water cylinder typically lasts 15 to 25 years. A few coil replacements are possible but cylinder swaps are cheap (£800-£1,500 fitted).

Honest Pros and Cons

The genuine pros

• Roughly 3 to 4 times more efficient than a gas boiler in real-world use. The government's Warm Homes Plan puts it at "around 3 times", and monitored UK installs on heatpumpmonitor.org average a SCOP of about 3.87
• £7,500 BUS grant in England and Wales, or £9,000 for an off-gas-grid property replacing oil or LPG, available until 31 March 2027 (Ofgem)
• 0% VAT on installation until 31 March 2027
• 20 to 25 year lifespan with a correctly designed system and proper servicing, against 10 to 15 for a gas boiler
• Quiet: mainstream units publish sound power levels of roughly 45 to 56 dB(A), which works out at about 37 to 48 dB(A) of sound pressure three metres away in the open
• Pairs naturally with solar panels for very low running costs. DESNZ's own Warm Homes Plan modelling puts a heat pump plus solar plus battery at up to £550 a year off the bill against a gas boiler, on 2024 prices
• Future-proof against further gas levy increases and against the Future Homes and Buildings Standards, which from 24 March 2027 make a gas boiler impractical in a new English home

The genuine cons

• Higher upfront cost than a like-for-like boiler swap, even after grants
• 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. That ratio is also cap-dependent: the electricity figure is VAT-free only until 31 March 2027.
• Outdoor unit needs a place to live, with airflow on at least two sides
• 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 2 to 4 days and involves some disruption (drilling for pipework, plumbing changes)
• Performance does drop in cold snaps, although modern units handle it well

Bottom Line

If your home is even reasonably insulated, you plan to stay 5+ years, and you can fit a 1m × 1m unit somewhere outside, an air source heat pump in 2026 is the right call. With the £7,500 BUS grant, the install lands at gas-boiler-replacement money. With a heat pump tariff, the running cost is competitive. With solar panels, the running cost is genuinely cheap.

The brands worth asking for are Vaillant aroTHERM plus (premium, R290, warranty extendable to 7 years on conditions), Mitsubishi Ecodan (longest UK track record, widest installer network, growing R290 line), and Daikin Altherma 3 (excellent controls and app, mostly R32 still). 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, scroll back up to 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.