Heat Pump Noise Ratings Explained
An installer has quoted you a decibel figure for a heat pump outdoor unit. Here is what that number actually means, and how to compare it properly against another quotation.
Start by understanding what the noise figures mean
Our main guide to Heat Pumps covers where an outdoor unit can go and the planning position for domestic heat pumps. This article stays on a narrower question, one it deliberately does not repeat, which is how to actually read and compare the sound figures manufacturers publish for a specific unit.
Sound power against sound pressure
Manufacturers commonly publish two different kinds of sound figure, and quotations do not always make clear which one is being quoted.
Sound power describes the total amount of sound energy a unit produces at its source, and it does not change with distance. Sound pressure describes how loud that sound actually is at a particular point, and it falls away the further you get from the unit, so a sound pressure figure is only meaningful alongside the distance it was measured at. A quotation that states a single decibel figure without saying which of these it is, or without stating a measurement distance for a sound pressure figure, is not giving you enough information to judge how the unit will actually sound at your own boundary or window.
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What to ask for when comparing quotations
Rather than comparing bare decibel numbers between two quotations, ask each installer for the manufacturer's published sound figure for the specific model being quoted, whether that figure is sound power or sound pressure, and, if it is a sound pressure figure, the distance it was measured at. Also ask whether the figure quoted reflects the unit running at full output or at a lower, more typical operating level, since sound output from a heat pump is not constant and tends to be higher when the unit is working hardest, such as during very cold weather.
| Measure | What it describes | How to use it when comparing quotations |
|---|---|---|
| Sound power | Total sound energy produced by the unit at its source | Comparable between two units directly, but does not tell you how loud it will be at your own boundary or window on its own |
| Sound pressure | How loud the unit is at a specific measured point | Only comparable between two quotations if the measurement distance is the same for both |
Distance and siting still matter
Even an accurate, like for like sound figure only describes the unit itself. How noticeable it is in practice also depends on where it is actually positioned relative to your own and any neighbouring windows, boundaries and outdoor living space, which our main guide covers in its section on positioning your outdoor unit. A quiet unit sited close to a bedroom window can still be more noticeable than a slightly louder unit sited further away, so the sound figure and the siting plan need to be considered together, not treated as separate questions. Siting also matters for planning: the exemption for domestic heat pumps in Ireland requires noise in normal operation not to exceed 43 dB(A), or 5 dB(A) above background noise if that is greater, at the nearest neighbouring inhabited dwelling. A manufacturer's figure does not tell you that level on its own, so ask the installer how the proposed position meets it.
What to ask before comparing noise between two quotations
- Is the published figure sound power or sound pressure
- If it is sound pressure, at what distance was it measured
- Does the figure reflect full output, or a more typical running level
- Is the same measurement basis being used across every quotation you are comparing
- Where is the unit actually proposed to sit relative to your own and any neighbouring windows
For where an outdoor unit can be sited and the current planning position for domestic heat pumps in Ireland, see our guide to Heat Pumps outdoor unit positioning.
