How much noise does a heat pump make and where can the outdoor unit go? The 40 dB rule, the calculation to the property boundary and the best spot

Short answer: a modern outdoor unit of 4 to 8 kW produces 40 to 50 dB(A) at a distance of 1 metre, comparable to a fridge or a quiet cooker hood, and less in night mode. Many countries set a noise limit at the property boundary; in the Netherlands, for example, the sound may not exceed 40 dB(A) at the boundary between 19:00 and 07:00 and 45 dB(A) during the day. With a unit of 45 dB(A) at 1 metre, you meet that at about 2 metres from the boundary in a free field, at 3 to 4 metres if the unit stands against a wall, and at 5 metres or more in a corner between two walls. The best spot is a free facade or the side of the house, away from bedroom windows and from the neighbours, with 30 cm of space behind and 1 to 2 metres clear at the front. An acoustic enclosure takes off 5 to 10 dB(A) but costs efficiency if it obstructs the airflow.

What the rules say

Noise limits for outdoor units differ per country, so check the rules that apply where you live. In the Netherlands, for example, newly installed outdoor units of heat pumps and air conditioners at homes have had to meet a noise limit at the property boundary since 1 April 2021: a maximum of 40 dB(A) at night (19:00 to 07:00) and 45 dB(A) during the day. The limit also applies where no permit is needed. It applies at the boundary with a neighbouring plot, not at the boundary with a public road. The installer calculates it from the appliance's noise data and the set-up using a standard calculation tool for outdoor units; the report is part of the handover. The limit refers to the appliance in night mode, if it has one; many units automatically turn down a few dB at night.

From data sheet to property boundary

Manufacturers give two figures. The sound power level (L_w, in dB(A)) is the total amount of sound the appliance makes, independent of distance; that is the figure on the energy label, usually 55 to 65 dB(A). The sound pressure level (L_p) is what you hear at a certain distance, often given at 1 or 3 metres. From sound power to sound pressure in a free field: about 8 dB lower at 1 metre, 14 at 2 metres, 20 at 4 metres, 26 at 8 metres. Each doubling of distance saves 6 dB. A wall behind the unit adds 3 dB, a corner between two walls 6 dB.

Sound power of the appliance	Free field, distance to 40 dB(A)	Against one wall	In a corner
55 dB(A) (quiet units, night mode)	2 m	3 m	4 m
60 dB(A) (gangbaar 4 tot 6 kW)	3,5 m	5 m	7 m
65 dB(A) (8 kW and larger)	6 m	9 m	12 m

The distances are rounded and assume free radiation; a 1.8-metre fence between the unit and the property boundary takes off another 5 to 8 dB if it is close to the unit. For the models in our shop: the Remeha Elga Ace gives approximately 43 to 47 dB(A) sound pressure at 1 metre, depending on output, the Intergas Xtend approximately 45 dB(A). In a normal terraced house with the unit on the side or rear facade, both meet a 40 dB limit at 3 to 4 metres from the property boundary.

The best spot

• A free facade or the side of the house, not in a recess and not between two walls; every reflecting surface adds up.
• Away from bedroom windows, yours and the neighbours'. The sound is low-frequency and travels through an open window.
• Free airflow: at least 30 cm behind the unit, 1 to 2 metres clear in front of the air outlet, nothing above it within half a metre. A unit that draws in its own cold air again loses efficiency and runs at higher speed, so makes more noise.
• Base: on a concrete foundation or steel frame with anti-vibration mounts, not directly on a timber facade or a light bedroom wall; vibrations carry through the structure.
• Condensate and defrost water: a unit defrosts in winter and then releases litres of water; provide drainage to a gravel bed or soakaway, not across a footpath that freezes.
• Sun and wind: shade is fine, a unit does not need to be in the sun. Do avoid a spot where the wind blows straight into the air outlet.
• Pipe length: with a split system up to 15 to 20 metres of refrigerant pipe between indoors and outdoors; with a monoblock (water to the inside) short, insulated pipes that cannot freeze.

A flat roof is also possible: further from the property boundary, no reflecting walls, but bear in mind vibrations on the roof structure and access for maintenance.

When to use an enclosure

An acoustic enclosure (a housing with sound-absorbing louvres around the unit) takes off 5 to 10 dB(A) and costs €500 to €1,500. It is the solution if the only possible spot is too close to the property boundary, for example in a narrow town garden. Drawback: every enclosure obstructs the airflow a little, and a poorly designed housing makes the unit draw in warm air again, which costs efficiency and actually makes the appliance work harder. Choose an enclosure designed for your type of unit and keep to the clearances specified by the manufacturer. A good fence in the right place is often cheaper and just as effective.

Noise that is not normal

A unit that hisses and knocks for a few minutes in winter is defrosting; that is normal, see Heat pump freezes and defrosts. A unit that runs loudly all the time, rattles or hums has a different cause: a fan blade that rubs, a loose casing panel, ice on the fan, an incorrectly set heating curve that keeps it at full load, or vibrations that pass into the facade via a bracket that is too light. Compressor noise that suddenly changes is a reason to call the installer, see Heat pump or air conditioner compressor. Work on the refrigerant circuit is reserved for certified refrigerant (F-gas) technicians; you can inspect the casing and the fan yourself, with the power off.

Choice of model and noise

Monoblock units with R290 (propane), such as the Intergas Xtend Eco (€3,989), often run at a lower speed and are somewhat quieter than older split units; the Remeha Elga Ace 4 kW (€ 3.069) is compact en heeft een nachtstand. Grotere vermogens maken meer geluid; kies dus niet groter dan nodig, zie How many kW heat pump do I need?. If you use an installation service , the noise calculation should be part of the site survey; the installer decides on the location together with you and includes the report at handover.

Frequently asked questions

How loud is 40 dB(A)?

As loud as a quiet living room or a fridge at two metres. It is not inaudible, but in a bedroom with the window closed you no longer hear it.

Does the 40 dB limit also apply to my own bedroom window?

No, the limit applies at the boundary with the neighbours. For your own comfort the same rule of thumb applies: do not place it directly under a bedroom window.

Do I need a permit for an outdoor unit?

Often not, but the rules differ per country and municipality. For a listed building, a protected townscape or placement on the front facade, a permit may be required; check with your local authority.

What if the neighbours complain anyway?

Have the installer measure at the property boundary. If the unit meets the applicable night-time limit, there is no legal problem; in practice a night mode, a fence or anti-vibration mounts help. If you are above the limit, the set-up must be adjusted; that is the responsibility of the installer who fitted it.