Heat Pump FAQ

Standard air-to-water heat pumps are rated down to about −15 °C, and models with an [EVI compressor](knowledge/glossary/evi-compressor) operate to −25 °C. Output and efficiency both fall as it gets colder, but a good unit still returns a COP near 2 at −15 °C, so it remains roughly twice as efficient as direct electric heating. Below the stated cut-off the compressor stops and the backup heat source takes over.

No — cold-weather operation is exactly what an air source heat pump is built for, and air well below freezing still holds plenty of heat that the colder refrigerant in the evaporator can absorb. What changes is capacity and [COP](knowledge/glossary/cop-coefficient-of-performance), both of which drop as the gap between outdoor air and [flow temperature](knowledge/glossary/flow-temperature) widens. Sizing against the local winter design temperature, rather than a rule of thumb, is what keeps the house warm.

Domestic air source units declare a sound power level of roughly 50–65 dB(A) under EN 12102, and anything in the low-to-mid 50s is quiet for its output. What a neighbour actually hears is sound pressure, which is lower and falls with distance. In England, MCS 020(a) requires the predicted level to be no more than 37 dB LAeq,5min at the neighbouring habitable-room assessment positions (ignoring the façade effect) for the installation to qualify as permitted development; Scotland, Wales and Northern Ireland still apply the original MCS 020 with its 42 dB(A) limit.

No — they are essentially the same machine. An outdoor air-conditioning condenser and an air source heat pump share the same compressor and fan hardware and sit in the same 50–65 dB(A) sound power band. Heat pumps are simply noticed more often because they run through quiet winter nights, whereas air conditioning runs against summer daytime background noise.

Sound pressure from a single outdoor unit falls by about 6 dB every time the distance from it doubles in open surroundings. A unit measured at 50 dB(A) at 1 m is therefore near 38 dB(A) at 4 m and around 32 dB(A) at 8 m, which is below typical suburban night-time background. Hard walls, corners and fences reflect sound and can add several decibels back, so siting matters more than the datasheet figure.

Often yes, if they were generously sized for a boiler. Radiators are catalogue-rated at ΔT 50 K under EN 442; at 45 °C flow into a 20 °C room they deliver only about 35% of that rated output, and roughly 45–50% at 50 °C flow. Compare each room's heat loss against the derated output — many rooms pass, and only the shortfalls need a larger emitter.

Any wet radiator works; what matters is surface area, not type. Double- and triple-panel convector radiators (type 22 and type 33) are the usual retrofit choice because they add output within the same wall width, and fan-assisted radiators go further where space is very tight. Underfloor heating allows the lowest [flow temperature](knowledge/glossary/flow-temperature) of all but is not a requirement for a heat pump.

As low as the emitters allow — typically 30–40 °C with underfloor heating and 40–50 °C with correctly sized radiators. Each 1 °C reduction in [flow temperature](knowledge/glossary/flow-temperature) is worth roughly 2–3% in efficiency, which is why [weather compensation](knowledge/glossary/weather-compensation) — raising the flow only as the outdoor temperature falls — is standard practice. Running at 55 °C instead of 35 °C typically costs a full COP point or more.

Repeated starting and stopping is normally caused by an oversized unit, too little water in the circuit, or emitters shut off by thermostats and TRVs so the heat has nowhere to go. Installers work to a limit of two to three cycles per hour. The fixes in order are opening up the emitter circuits, adding volume through a [buffer tank](knowledge/glossary/buffer-tank) or volumiser, and lowering the flow temperature so the compressor can modulate down.

On the return, in the large majority of installations. A volumiser is a plain insulated vessel added purely to increase system water content; putting it on the return keeps the full flow temperature going to the emitters while still providing the volume the unit needs for [defrost cycles](knowledge/glossary/defrost-cycle) and stable running. Unlike a four-port [buffer tank](knowledge/glossary/buffer-tank), it provides no hydraulic separation — where zones, several pumps or a second heat source must be decoupled, the buffer is the right component.

Once a year is the norm, and most manufacturers make a documented annual service a warranty condition. A domestic heat pump's refrigerant charge sits well below the 5 tonnes CO₂-equivalent threshold that triggers mandatory F-gas leak checking, so the annual visit covers airflow, condensate drainage, water-side pressure, filters and controls rather than the sealed circuit. Any work on the refrigerant circuit itself must be done by an F-gas certified engineer.

Keep the outdoor coil clear of leaves, snow and vegetation, maintain at least 300 mm of free space around the unit, check that the condensate drain runs away freely, and glance at the system pressure gauge monthly. Cleaning the magnetic filter or strainer is also within reach if the installer has shown you the isolation valves. Anything involving the refrigerant circuit or electrical connections is engineer-only work.

Storage at 60 °C with distribution above 50 °C is the reference used across Europe, following HSE ACOP L8 practice, and holding the whole cylinder at 60 °C for one hour is accepted as a pasteurisation cycle. [Legionella](knowledge/glossary/legionella) multiplies between roughly 20 and 45 °C and begins to die off above 50 °C. Because 60 °C water scalds within seconds, outlets must be protected by a [thermostatic mixing valve](knowledge/glossary/thermostatic-mixing-valve).

Yes — on almost every controller the pasteurisation cycle's day, start time and target temperature are user-adjustable. Move it to a mild afternoon or into a cheap electricity window rather than the coldest hours of the night, because the cycle usually calls on the immersion element and is the most expensive hour the system runs. Do not disable it altogether on a stored-water system.

There are three normal causes: the weekly anti-legionella cycle overriding the setpoint, controller hysteresis that overshoots by a few degrees before switching off, and a cylinder sensor mounted low in the tank while the top layer stratifies hotter. Check the pasteurisation schedule first. A persistent overshoot of more than about 5 °C outside that cycle points to a sensor position or control fault.

In frosting conditions — roughly +5 to −5 °C with damp air — a [defrost cycle](knowledge/glossary/defrost-cycle) every 30 to 90 minutes lasting 2 to 10 minutes is normal. Defrosting every 20 minutes, or cycles running much longer than 10 minutes, points to restricted airflow, a blocked condensate drain, ice banking up under the unit, or a faulty coil sensor. Visible steam and dripping water during a defrost are expected, not a fault.

No. The unit is designed for outdoor exposure, and enclosing it starves the fan of air, cuts capacity and can void the warranty. Manufacturers typically require at least 300 mm clear at the sides and rear and about 1 m in front of the fan discharge. Acoustic screens are acceptable only where they preserve those clearances and leave the air path open.

It can, but it is a poor choice under a bedroom or living-room window and most installers avoid it. Keep the manufacturer's clearances — commonly 300 mm to the sides and rear and 1 m in front — mount the unit 200–300 mm above ground level so meltwater drains and snow cannot bank up against the coil, and avoid tucking it into a corner where two hard walls reflect the noise back out.

Outdoors, a domestic monobloc occupies roughly 1 m² of ground or wall area plus its clearances. Indoors you need room for a hot water cylinder — a 200–300 L unvented cylinder is typically 550–650 mm in diameter and 1.2–2 m tall — plus a small wall area for the controller and any [buffer tank](knowledge/glossary/buffer-tank). A [monoblock heat pump](knowledge/glossary/monoblock-heat-pump) keeps the whole refrigerant circuit outside, so no indoor refrigerant unit is required.

It depends on the country, and the constraint is usually noise and siting rather than the heat pump itself. In England an air source heat pump can qualify as permitted development if it meets the MCS 020(a) sound calculation — 37 dB LAeq,5min at the neighbour's assessment positions — and the other permitted development conditions, while listed buildings and conservation areas carry extra restrictions. As of 2026, confirm the current rules with your national planning authority before ordering.

That is usually correct operation rather than a fault. A heat pump is most efficient when it modulates continuously at low output and matches the building's heat loss, unlike a boiler that fires hard and stops. Long steady runs at a low [flow temperature](knowledge/glossary/flow-temperature) cost less than short hot bursts. Running flat out while the house stays cold is a different symptom and points to undersizing or a control fault.

Only if the unit is a reversible model and the emitters were designed for cooling. Air-to-water systems deliver cooling through fan coil units or underfloor pipework, and both need dew-point control so the flow temperature never falls below the room's dew point and condenses moisture on floors and pipework. Underfloor cooling provides gentle background cooling; fan coils are needed where there is a real cooling load.

An air-to-air heat pump moves heat into room air through indoor fan units, like a reversible air conditioner: it heats and cools quickly but cannot produce domestic hot water. An [air-to-water heat pump](knowledge/glossary/air-to-water-heat-pump) heats water for radiators, underfloor heating and a hot water cylinder, which is why it is the standard choice for whole-house heating across Europe.

Ground source units draw from soil that stays around 8–12 °C all year, so they hold a higher seasonal efficiency than air source in deep winter and run more quietly, with no outdoor fan. They also cost substantially more to install because of the boreholes or ground loops. For most European homes a modern air source unit — with an [EVI compressor](knowledge/glossary/evi-compressor) where winters are severe — delivers the better cost per kWh saved.

Yes, when installed to the standard. [R290](knowledge/glossary/r290-propane) is a flammable A3 refrigerant, so units are built as outdoor monoblocs with the entire charge — usually under 1.5 kg — sealed outside the building, and IEC 60335-2-40 and EN 378 set both the charge limits and the exclusion zone that must be kept clear of drains, openings and ignition sources. Its [GWP](knowledge/glossary/gwp-global-warming-potential) of 3 is why it is replacing R410A ahead of the EU F-gas phase-down.

It is a North American HVAC rule of thumb, not a European design standard: it holds that a heat pump struggles to maintain an indoor-to-outdoor difference greater than about 20 °F (11 °C) without supplementary heat, and it dates from older air-to-air units with resistance backup. Modern inverter-driven units are sized against the local winter design temperature instead. In European practice the number that matters is the design [flow temperature](knowledge/glossary/flow-temperature), not the indoor-outdoor gap.

Yes. A competent quotation states the room-by-room heat loss, the design outdoor temperature used, the design flow temperature, and the resulting heat pump output. Without a design flow temperature you cannot check whether the emitters are large enough or predict running cost, because efficiency depends directly on it. Ask to see the [heat load calculation](knowledge/glossary/heat-load-calculation) before signing anything.

No. An EPC is a standardised energy rating built on assumed occupancy and default fabric values, not a measurement of your building's heat loss. Sizing requires a room-by-room [heat load calculation](knowledge/glossary/heat-load-calculation) to EN 12831 using the actual construction, glazing, ventilation and local design temperature. EPC-derived shortcuts are the single most common cause of oversizing, which then shows up as short cycling.

It can be. A room-by-room heat loss calculation sometimes shows an existing radiator is far larger than the room now needs, especially after insulation work, and a heavily oversized emitter can make balancing harder. But surplus radiator area is exactly what lets a heat pump run at a low flow temperature, so ask to see the calculation for that specific room before agreeing to anything smaller.

Usually yes, if you change brand. Heat pump controllers communicate with their own outdoor units over proprietary protocols, and the weather compensation curve, defrost logic and hot water scheduling all live inside the manufacturer's controller. Third-party thermostats can generally only add a simple demand signal or an [SG Ready](knowledge/glossary/sg-ready) input on top of it.