Two figures that are both correct — and still confusing

A brochure promises 35 dB. The subsidy database lists the same unit at 55 dB. If you put them side by side, you might suspect false advertising — but both numbers are correct. They just measure two different things.

The difference is not just a technicality for acousticians. In one federal state, it decides whether you get a public grant.

Sound power is the unit, sound pressure is the location

The sound power level (LWA) describes how much sound the unit emits in total. It is a property of the machine, regardless of where you stand. You cannot hear it directly or verify it with a measuring device in the garden — it is determined under standardised conditions.

The sound pressure level (Lp) is what reaches the ear. It depends on the distance, the installation, and reflecting walls. It decreases as the distance increases.

Both figures use the unit decibel, and that is exactly where the confusion comes from. When a data sheet says "35 dB(A)", it usually says somewhere in small print: at a distance of three or five metres. That is a sound pressure. The 55 dB in the database is a sound power. No contradiction, two different metrics.

As a rough rule of thumb from acoustics — it is not stated like this in any of the subsidy sources mentioned below: the sound pressure at a distance of one metre is around 8 dB below the sound power, and with every doubling of the distance, it drops by about another 6 dB. So if you plan for a distance of four metres instead of two, you gain noticeably — more than from the last few decibels in the data sheet.

That is why brochure figures can only be compared if both state the same metric and the same distance — otherwise you are comparing apples with oranges.

In Vorarlberg, the sound power decides on the public grant

Most federal states link their public subsidy to efficiency — to the flow temperature or to key figures like ηs. Vorarlberg takes its own path and makes sound a condition.

The Richtlinie Heizungstausch 2026 (heating replacement guideline 2026) states in § 7 para. 2 lit. b that the maximum sound power level according to the GET-Datenbank must not exceed 55 dB. Up to 60 dB, a system remains eligible for a public grant in only three cases:

1. The manufacturer confirms a throttling that brings the value down to a maximum of 55 dB.
2. The unit has an acoustic enclosure.
3. The system is installed indoors, there is no outdoor unit.

Over 60 dB is the limit — such air source heat pumps are not eligible for a public grant there.

The decisive factor here is explicitly the value from the GET-Datenbank, not the figure from the sales pitch. The GET-Datenbank is the Austrian product database for building energy technology; for the federal public grant and most federal state public grants, a unit must be listed there in the first place. You can check there yourself before you sign.

That is the practical core: for subsidy eligibility, a number counts that is often not even in the brochure.

Two subtleties that are in the guideline and easily overlooked:

The value applies at full load — without defrost mode. The guideline explicitly defines LWAmax as the sound power level at full load operation "without special operating phases such as defrost mode". In everyday life, a well-sized heat pump runs well below full load most of the time and is correspondingly quieter. During defrosting in winter, however, it can briefly be heard more clearly than the data sheet value suggests — this operating state is simply not included in the number.

For air source heat pumps in Vorarlberg, a building permit must be obtained, the municipality is responsible. This is not a sound limit, but it is practically related to it: in the approval process, the neighbourhood becomes an issue, and then it is no longer the database number that counts, but what actually arrives at the property line.

Why the installation location matters more than the data sheet

When it comes to the neighbour and not the public grant, the picture is reversed. Then the sound pressure at the property line counts — and that depends more on the installation location than on the last three decibels in the data sheet.

What helps:

• Distance. The most effective measure and the cheapest, as long as it has not yet been built.
• No corners. Two intersecting walls reflect the sound and can significantly amplify the effect.
• Do not point at bedroom windows — neither your own nor others'.
• Night setback. Many units can run more quietly at night. The night is usually assessed anyway, because stricter guide values apply then.

You can estimate in advance how much reaches the neighbour: our sound calculator calculates the sound pressure at the neighbouring property from the sound power and distance and compares it with the planning guide values according to ÖNORM S 5021. This does not replace an assessment — the evaluation by authorities or experts according to ÖAL-Richtlinie Nr. 3 (ÖAL guideline no. 3) is binding — but it shows early on whether a planned location will be tight.

What you should ask about the quote

Three questions that take ten seconds and save a lot of trouble later:

1. What is the sound power level according to the GET-Datenbank? Not the brochure value.
2. Does this value fit the public grant of my federal state? In Vorarlberg, this is an exclusion criterion, elsewhere it is not — it should be checked everywhere.
3. What arrives at the property line, at this specific installation location? A good installer calculates this instead of guessing.

If an answer is missing, that is no reason to worry about the unit — but a reason to ask. A heat pump stays where it was once installed for twenty years.

A note for context

We deliberately do not name any models or brands here. The values change with every series, and a unit that is inconspicuous in a garden can become an issue in a narrow terraced house backyard. The decisive factor is not the name of the machine, but whether the sizing and installation location fit your property — and whether the value in the database fits the conditions of your federal state.

The information on Vorarlberg comes from the Richtlinie Heizungstausch 2026 (heating replacement guideline 2026), which we read in the original for this article on 28 August 2026. It is limited until 31 December 2026 — for applications, the version valid at the time of application applies. So anyone submitting in 2027 will have to check again; whether the sound limit will then remain unchanged is open.

What applies in your federal state can be found in the subsidy overview — there you can see for each state when we last checked.