Air source heat pump: siting, noise and the real costs

Most domestic air source heat pumps are monobloc units: the whole refrigerant circuit is sealed inside the outdoor box, and water pipes run into the house. Split systems put part of the equipment indoors and run refrigerant pipes between the two. Splits can suit a long distance between unit and plant room, but the refrigerant work brings in F gas qualifications for the engineer, and more joints to check.

Many newer monoblocs use propane (R290) as the refrigerant, which is not an F gas but is flammable, so manufacturers set clearance zones around the unit that affect where it can go. Ask the installer which refrigerant the proposed model uses and what that means for its position.

In England, an air source heat pump on a house or in its garden is permitted development if all the conditions are met. The installation must comply with the MCS 020a planning standard, which covers noise at the nearest neighbouring window. The outdoor unit, including its housing, must not exceed 1.5 cubic metres on a house or 0.6 cubic metres on a block of flats.

Detached houses may have two units under permitted development; other houses one. Pitched roofs are ruled out, and on a flat roof every part of the unit must be at least one metre from the roof edge. On a wall that fronts a highway, the unit must not go above ground floor level. In a conservation area or World Heritage Site it must not go on a wall or roof fronting a highway, or closer to that highway than the house itself. Listed buildings and scheduled monuments have no permitted development rights for this.

Approved Document L adds practical points: units should not sit next to bedrooms or on materials that carry vibration, and the fan and compressor should be placed to keep disturbance to neighbours low. Good installers also keep the unit clear of spots where cold air it blows out gets drawn back in, and plan where defrost water drains so it does not freeze on a path.

Source: planningportal.co.uk

An air source heat pump keeps working below freezing, but its output and efficiency fall as the outside temperature drops, and it periodically defrosts its outdoor coil. That is why the design uses a cold design day for your location, and why the unit should be sized for the full heat demand on that day rather than leaning on an electric immersion or backup heater.

The heat pump itself is only part of the bill. Radiator upgrades, a new cylinder, pipework changes, groundworks for a base, electrical work and the length of the pipe run to the plant space all add up. Larger houses need larger units, and a detached house with high heat loss can cost noticeably more than a small terrace.

Government figures for installations under the Boiler Upgrade Scheme give a useful reference point; they include the grant value, so the price you pay after the grant is lower.

• make, model, refrigerant and rated output of the unit
• the MCS 020a noise calculation for the chosen position
• base or wall brackets, anti-vibration mounts and condensate drainage
• pipe route and insulation between unit and house
• weather compensation set-up and a handover of the controls

Your request describes the property and the space outside. Installers near you see it without your name or contact details, and the first to buy it is the only one who gets it. The installer pays Offrano; you pay nothing for the introduction.

Before the installer receives your details, we email you its name, address and phone number, and you can withdraw consent from any of our emails. We cannot guarantee that the request is taken, and you are never bound to accept the quote. Check the installer's MCS certification yourself, as Offrano only confirms it is a registered business.