Heat pump noise; what is actually acceptable

A typical 5 to 7 kW residential air-source heat pump in normal operation produces between about 45 and 55 decibels of sound pressure measured at one metre from the unit, with the quieter low-power modes on modern R290 units sitting around 30 to 38 decibels. To translate that into something familiar: a quiet conversation reads at about 50 decibels, a domestic fridge in a kitchen at about 40, and a normal back garden at night without traffic at about 35.

The figure that matters to a neighbour is the sound pressure at their nearest assessment position, not the figure at the unit. Distance, the angle to any reflective wall and the time of day all change what they actually hear; the same machine that reads 50 decibels at the unit will often read in the high 30s at a neighbour's bedroom window six or seven metres away.

In England, an outdoor air-source heat pump installation qualifies for permitted development without planning permission only if it meets MCS 020. The headline rule is that sound pressure at the closest assessment position on a neighbour's habitable building must not exceed 42 decibels. The MCS-registered installer is required to demonstrate this through a documented calculation before commissioning the system.

A unit that fails the MCS 020 calculation does not become impossible; it requires a full planning application, which most installers will avoid by re-siting the unit, adding an acoustic enclosure or moving to a quieter model. Permitted-development rules also vary slightly across Scotland, Wales and Northern Ireland; check the local position before assuming the English rule applies.

The dominant variables, in order, are distance from the listener, the surface immediately behind the unit, and whether the listener has a line of sight to it. A heat pump bolted to a flat brick wall in a tight courtyard will reflect sound back across the yard and into adjacent windows; the same heat pump on anti-vibration mounts in a side passage, with an open run of garden between it and the neighbour, can drop ten or more perceived decibels with no change to the machine itself.

The second variable is operating mode. Most modern heat pumps modulate down at night, producing a quieter steady hum rather than the brief louder burst they make during the morning warm-up. A unit you visit in the early evening will not sound the same as the one you hear at 3 am, and both matter.

The single most useful thing is to invite the neighbour to stand in their own garden while your installer runs a comparable unit at a nearby property. That changes the conversation from an abstract worry to a five-minute experience of what the noise actually is.

If a unit does sit close to a neighbour's habitable room and the calculation is tight, ask the installer to model an acoustic enclosure as a costed option. They are not always needed; when they are, they are usually a few hundred pounds and they take the worry off the table without affecting performance.