Where the sound comes from
An air source heat pump's outdoor unit has two moving parts that make sound: the fan, which pulls air across the coil, and the compressor, usually housed in the same box. Together they produce a steady whoosh and hum. The faster they run, the louder the unit. Ground source heat pumps have no outdoor fan and keep the compressor indoors, so outside they are almost silent; indoors the unit sounds something like a large appliance.
Two numbers that are easy to confuse
Sound power
Sound power is the total sound the unit puts out. It is a property of the machine, like the wattage of a bulb, and it is the figure on data sheets. It does not change with where the unit is placed.
Sound pressure
Sound pressure is the level at a particular spot, which is what an ear or a microphone measures. It depends on the sound power, but also on how far away you are and on the surfaces around the unit. Like a bulb that seems dimmer from across a room, the same unit is quieter at a distance.
What changes what you hear
- Distance. In open air, each doubling of distance from a small source lowers the sound pressure by about six decibels, a clearly noticeable drop.
- Walls and corners. A hard surface behind the unit reflects sound and raises the level in front of it; a corner, with two surfaces, raises it more.
- Barriers. A solid wall or fence between the unit and a window blocks the direct path and lowers the level behind it.
- Workload. On cold days the unit runs harder and louder; on mild days it may run slowly and quietly.
- Vibration. A unit fixed rigidly to a wall can pass vibration into the building; anti-vibration mounts reduce it.
Decibels in brief
Decibels are logarithmic, so the numbers do not add up the way ordinary numbers do. A change of a few decibels is noticeable; a change of ten decibels sounds roughly twice as loud or half as loud. Two identical units side by side are about three decibels louder than one. This is why small differences on a data sheet, and small changes in position, can matter.
Planning and neighbours
In England, an air source heat pump installed without a planning application has to meet the national sound standard set by MCS for that purpose, which calculates the level at the nearest neighbouring window or door from the unit's sound power, its distance and its surroundings. Other parts of the UK have their own rules. Whatever the planning position, a sensible position and a conversation with the neighbours before installation avoid most problems.
Reducing the sound
- Place the unit away from bedroom windows, yours and the neighbours'.
- Avoid corners and recesses where reflections build up.
- Put a solid barrier between the unit and the nearest windows if there is room, leaving space for air flow.
- Use anti-vibration mounts, especially on walls.
- Choose a model with a lower sound power, or one with a quiet night mode.
- Make sure the system is not oversized: a unit that suits the home runs more gently.
The government's explainer notes that you need space for the heat pump; part of choosing that space is choosing where its sound goes.
Indoors
Inside the home, an air-to-water system is usually quiet: there may be a faint hum from the circulating pump and the sound of water moving in pipes when the system starts. A ground source heat pump's indoor unit, which contains the compressor, makes more noise indoors and is usually placed in a utility room, garage or plant cupboard for that reason.