Switching to an air source heat pump is one of the more consequential decisions a homeowner makes about heating, and it is worth approaching with clear eyes. The technology genuinely delivers three to four units of heat for every unit of electricity consumed, which no combustion system can match. It also depends on conditions in your building that determine whether you receive that performance or a fraction of it.
This guide covers the case for switching, the conditions that determine whether it works well, what to check before committing, and where the honest limits lie.
Why the Efficiency Figures Look Impossible
An air source heat pump delivers a Coefficient of Performance of 3 to 4.5, meaning three to four and a half units of heat for every unit of electricity. A gas boiler converts fuel to heat at up to about 95%. Direct electric heating manages exactly one for one.
The explanation is that a heat pump does not generate heat. It moves heat that already exists in the outdoor air into the building, and electricity is spent on transporting that energy rather than creating it. Because it is not converting fuel, it is not bound by the ceiling that limits combustion.
This works even in cold weather, because air above absolute zero contains extractable heat. The refrigerant inside the unit is colder still, so heat flows into it from air that feels freezing to us.
The Case for Switching
Running cost
Against direct electric heating the advantage is decisive: three to four times the heat for the same electricity. Against oil it usually wins. Against gas the outcome depends on the local ratio between electricity and gas prices rather than on the equipment itself, which is why the same heat pump can be cheaper to run than a boiler in one country and more expensive in another.
Worth checking your own tariffs rather than assuming, since this is the variable that most affects the financial case and it differs substantially between markets.
Heating and cooling from one machine
This answers a question many people have and few articles address directly. Yes, air source heat pumps can cool. A reversible unit runs the same refrigeration cycle in the opposite direction, extracting heat from the building and rejecting it outside, exactly as an air conditioner does.
The distinction that matters is what delivers the cooling indoors. An air-to-air unit blows cooled air directly into the room. An air-to-water unit produces chilled water, which then needs an emitter capable of using it: fan coils handle this straightforwardly, while radiant cooling through underfloor loops is possible but requires attention to condensation, since surfaces cooled below the dew point attract moisture.
For a household that would otherwise buy an air conditioner plus a boiler, covering both seasons with one installation frequently works out cheaper overall despite the higher unit price.
Emissions that fall over time
A heat pump runs on electricity, so its emissions decline automatically as the grid decarbonises. A gas boiler installed today will emit the same in fifteen years. This is a genuine long-term difference rather than a marketing point.
No combustion at the property
No flue, no gas connection, no fuel delivery or storage, no annual combustion safety inspection, and no carbon monoxide risk. For properties off the gas grid currently running on oil or LPG, removing the fuel delivery dependency alone is a meaningful improvement.
Regulatory direction
Heating policy across most of Europe is moving consistently away from fossil fuel appliances and toward low-temperature electric systems. A heat source installed today will operate for fifteen years or more, so it is worth knowing where local rules are heading rather than only where they stand.
What Determines Whether It Actually Works Well
This is the part that decides outcomes, and it is where most disappointing conversions originate.
The emitters, above everything else
Every emitter requires water at a particular temperature to deliver its rated output. Radiators sized for a gas boiler expect 70 to 80°C. Underfloor heating needs only 30 to 45°C, because it emits from the entire floor rather than a small panel.
A heat pump becomes markedly more efficient the lower the flow temperature it must produce, and the effect is large rather than marginal.
The figures make it concrete. A unit producing 35°C water for underfloor heating achieves a COP around 4.5. The same unit producing 45°C for radiators achieves around 3.6. That is roughly 25% more heat from the same electricity, from an identical machine, determined entirely by what it is feeding. Connect a heat pump to radiators demanding 75°C and you receive a fraction of the efficiency the datasheet promised. Neither the heat pump nor the radiators are faulty. The mismatch is.
Three routes resolve this: replace the emitters with underfloor heating, oversize the radiators so they deliver the same output at 45 to 55°C, or improve insulation so demand falls and lower flow temperatures suffice. A combination of the second and third is frequently the most economical.
The building envelope
Insulation reduces heat demand permanently, which means the heat pump can be smaller, cheaper to buy, and cheaper to run. It also lowers the flow temperature the emitters need, which directly improves efficiency.
Addressing insulation before specifying equipment is the correct sequence. Reversing it leaves a system sized for a building that no longer exists, oversized and cycling inefficiently for the rest of its life.
Correct sizing
Oversizing is a real error rather than a safe margin. A unit that cannot modulate low enough cycles on and off through the mild months that make up most of the heating season, which reduces seasonal efficiency and wears the compressor.
The figure that catches people out is the minimum modulated output rather than the maximum. A unit that cannot throttle below 6.5 kW will cycle badly in a house whose mild-weather demand is 3 kW. Size against a proper heat loss calculation rather than floor area.
Domestic hot water
An air-to-water heat pump can supply hot water, but it needs a cylinder to do so. If you currently have a combi boiler producing hot water on demand, there is no cylinder in the property and space must be found for one. This is a practical constraint worth establishing early, since it occasionally decides the feasibility of a conversion in a small flat.
The Honest Limits
Being straightforward about these is more useful than presenting the technology as suitable for everything.
Upfront cost is higher. Considerably higher than replacing a boiler with another boiler, particularly where emitters or insulation also need work. Grants and incentives change the arithmetic in many markets, but the baseline difference is real.
Output falls as temperature drops. This applies to every air source heat pump and is unavoidable physics rather than a product weakness. Cold-climate projects should be sized against the local design temperature rather than the nominal rating.
The outdoor unit produces noise and needs space. It requires unobstructed airflow on all sides, and placement near a bedroom window or a neighbour’s boundary needs thought.
It responds differently from a boiler. A heat pump works best running steadily at a lower temperature rather than firing hard in short bursts. Owners accustomed to turning heating on when they feel cold sometimes find the adjustment unfamiliar, and operating it like a boiler produces both worse comfort and worse efficiency.
Poorly insulated buildings with high-temperature radiators are the difficult case. Not impossible, but a conversion that ignores this will disappoint, and it is better to know that before rather than after.
Cold Climate Performance
The objection that heat pumps fail in cold weather is largely outdated for current equipment. Older units struggled below freezing, but modern cold-climate models maintain useful output in far harsher conditions.
Legom air source heat pumps are engineered to keep operating at ambient temperatures down to -35°C, which covers the great majority of populated cold regions including Northern and Eastern Europe, and Legom exports to distribution partners in Poland and Russia among other markets.
Two design elements make this possible. The inverter compressor raises its speed to compensate as available heat in the outdoor air falls, sustaining output where a fixed-speed unit cannot. And demand-based defrosting clears frost from the air-side heat exchanger only when frost is actually detected, avoiding the frequent ineffective cycles that undermine cold-weather comfort in simpler designs.
Air-to-Air or Air-to-Water?
This distinction determines which products you should even be considering, and it is frequently confused.
| Aspect | Air-to-air | Air-to-water |
|---|---|---|
| Delivers heat into | Indoor air directly | A water circuit |
| Emitter | Wall or ceiling unit | Underfloor loops, radiators, fan coils |
| Domestic hot water | No | Yes, with a cylinder |
| Cooling | Straightforward | Via fan coils, or radiant with care |
| Suits buildings with | Ducts or no existing distribution | Existing wet heating |
They are not interchangeable. A building with radiators or underfloor loops needs air-to-water. Legom manufactures air-to-water monoblock units, and our guide to air to water heat pumps covers that category in depth.
Questions to Ask Before Committing
Four questions separate a specification based on your building from one based on a catalogue.
What is the calculated heat loss of each room? Based on a proper calculation rather than an estimate from floor area.
What flow temperature will the system need, given the existing emitters? This single number determines much of the outcome.
What efficiency will the proposed unit achieve at that flow temperature? As opposed to its headline figure under test conditions.
What would change if insulation were improved first? If the answer is a smaller and cheaper system, the sequence deserves reconsidering.
An installer who answers these clearly is engaging with your building. One who quotes equipment without them is selling a product.
Where a Hybrid Makes Sense
A hybrid arrangement pairs a heat pump with an existing boiler, switching between them according to conditions. The heat pump covers most of the heating season efficiently while the boiler handles the coldest days.
This suits retrofit projects where replacing all the emitters is impractical, and it allows the heat pump to be sized for typical rather than peak demand, reducing its cost. The trade-off is that a fossil fuel appliance remains in the building with its flue, gas connection, and maintenance obligations, so in markets phasing out gas heating it is best regarded as a transitional step rather than a permanent solution.
“The question I would ask before any conversion is what flow temperature the system will need to run at, and most homeowners have never been told. They compare heat pumps on their datasheet COP, choose the best number, connect it to radiators sized for seventy-five degrees, and then wonder why the bills are not what they expected. Nothing is faulty. The unit is doing exactly what physics allows at the temperature it was asked to produce. If your emitters can work at forty-five degrees or can be made to, a heat pump will do what it says. If they cannot, address that first and the rest follows.”
— Maggie Shen, Director of Legom
The Legom Air Source Range
Legom manufactures air-to-water monoblock heat pumps in four capacities at its facility in Jiaxing, Zhejiang Province, supplied to partners in more than 90 countries.
| Parameter | 5 kW | 6 kW | 9 kW | 16 kW |
|---|---|---|---|---|
| Heating range | 2.0–6.0 kW | 3.0–8.0 kW | 3.5–10.0 kW | 6.5–18.0 kW |
| COP at A7/W35 | 4.5 | 4.42 | 4.46 | 4.53 |
| Cooling capacity | 4 kW | 5 kW | 6.5 kW | 14 kW |
| Hot water capacity | 6 kW | 8 kW | 10 kW | 18 kW |
| ERP at 35°C | A+++ | A+++ | A+++ | A+++ |
| Noise | ≤52 dB(A) | ≤54 dB(A) | ≤55 dB(A) | ≤56 dB(A) |
All models use R32 refrigerant with full DC inverter compressors, provide reversible heating and cooling, operate down to -35°C, and carry CE and RoHS certification. Because Legom also manufactures the manifolds, floor heating pipe, thermal actuators, and room thermostats that complete a hydronic system, a full installation can be sourced from one manufacturer. OEM and ODM services are available across the range.
Frequently Asked Questions
Can air source heat pumps cool as well as heat?
Yes. A reversible unit runs the same refrigeration cycle in the opposite direction, extracting heat from the building and rejecting it outdoors, exactly as an air conditioner does. What matters is how the cooling is delivered indoors. An air-to-air unit blows cooled air directly into the room. An air-to-water unit produces chilled water, which needs an emitter capable of using it: fan coils handle this straightforwardly, while radiant cooling through underfloor loops requires attention to condensation risk since surfaces cooled below the dew point attract moisture.
Is switching to an air source heat pump worth it?
It depends less on the heat pump than on your building. Against direct electric heating the case is decisive. Against oil it usually wins. Against gas it turns on the local ratio between electricity and gas prices. What consistently determines satisfaction is whether the emitters allow the unit to run at low flow temperature. A heat pump feeding underfloor heating in a reasonably insulated building performs as promised. The same unit forced to produce 75°C for old radiators will not.
Can I use my existing radiators with a heat pump?
Physically yes, but with an important caveat. Radiators sized for a gas boiler expect 70 to 80°C water, and asking a heat pump to produce that pushes it into its least efficient range, so the system works while costing far more to run than the datasheet suggests. Oversizing the radiators so they deliver the same output at 45 to 55°C recovers most of the efficiency, and is much less disruptive than installing underfloor heating throughout. Assess this before converting rather than after.
Do air source heat pumps work in cold weather?
Modern units work well below freezing, which is a substantial improvement over older equipment. Legom units are engineered to operate down to -35°C ambient, covering the great majority of populated cold regions. Output does decline as temperature falls, which applies to every air source heat pump and is unavoidable physics rather than a product weakness, so cold-climate projects should be sized against the local design temperature rather than the nominal rating.
Will a heat pump provide my hot water?
An air-to-water unit can, but it needs a hot water cylinder to do so. If you currently have a combi boiler producing hot water on demand, there is no cylinder in the property and space must be found for one. Establish this early, since it occasionally decides feasibility in a small flat. Hot water requires higher temperatures than space heating, typically 50 to 60°C for storage, so the unit runs at lower efficiency during hot water production, which is normal rather than a fault.
How much does switching cost?
Considerably more upfront than replacing a boiler with another boiler, and how much more depends heavily on what the building already has. A property with existing wet pipework and suitable emitters costs far less to convert than one needing new distribution throughout. Grants and incentives change the arithmetic materially in many markets and are revised periodically, so check current schemes for your region before assessing payback rather than relying on general figures.
What size heat pump do I need?
Size against a calculated heat loss for the building rather than floor area, since insulation, glazing, ceiling height, orientation, and hot water demand all change the answer. Note that the minimum modulated output matters as much as the maximum: a unit that cannot throttle below 6.5 kW will cycle badly in a house whose mild-weather demand is 3 kW, and mild weather makes up most of the heating season. Oversizing is a real error rather than a safe margin.
What is a hybrid heat pump system?
An arrangement pairing a heat pump with an existing boiler, switching between them according to conditions. The heat pump covers most of the season efficiently while the boiler handles the coldest days, which allows the heat pump to be sized for typical rather than peak demand and reduces its cost. It suits retrofits where replacing all emitters is impractical. The trade-off is that a fossil fuel appliance remains in the building, making it a transitional step in markets phasing out gas heating.
Reviewed by Maggie Shen, Director at Legom, on August 24, 2026. This guide to switching to an air source heat pump was reviewed for technical accuracy, including reversible cooling operation, the relationship between emitter flow temperature and efficiency, and cold-climate operating limits.