Heat pump acoustic enclosures without losing efficiency

Heat pump acoustic enclosure design fails in a predictable way. The acoustician specifies a box that meets the noise limit, the enclosure gets built to the drawing, and the unit inside it runs at a lower output than the heating design assumed because it cannot get enough air. The occupier then turns the flow temperature up, the efficiency drops further, and the noise complaint the enclosure was meant to prevent arrives anyway because the compressor is running longer.

Why an enclosure changes the duty of the unit

An air source heat pump works by moving a large volume of air across a coil. In heating mode it takes heat out of that air, so the air leaving the unit is colder than the air entering it, often by several degrees.

Restrict the airflow and three things happen together. The evaporating temperature falls, which reduces the capacity available at any given outdoor temperature. The coil ices more readily, so defrost cycles become more frequent, and each defrost is a period of no useful output and usually the loudest noise the unit makes. And the compressor runs for longer to deliver the same heat, which increases the total time the neighbour hears it.

So an enclosure that cuts the sound power level by 8 dB and the airflow by a third can leave the receptor worse off over an evening. That is the trade to understand before the enclosure is drawn.

Where the unit serves a dwellinghouse, remember the planning route as well. Permitted development for an air source heat pump sits in Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015, subject to conditions including a noise limit assessed under the MCS planning standard, commonly expressed as 42 dB(A) at the assessment position. Those conditions have been amended more than once, so read the current text of the Order rather than a summary.

Airflow clearance the manufacturer requires

Start from the data sheet, not from the drawing. Every manufacturer publishes minimum clearances, and they differ between models in the same range.

Typical published requirements cover four dimensions: clearance behind the unit at the air inlet, clearance at each side, clearance in front of the discharge, and clearance above. Discharge clearance is normally the largest, often a metre or more, because the fan has to throw the cooled air away from the unit.

Record the figures for the specific model in the submission, with the source. Then check three things against the enclosure geometry.

1. Whether the clearance is measured to a solid surface or to a free opening, because a louvre at the clearance distance is not the same as open air.
2. Whether the manufacturer states a total free area for an enclosed installation, which some do and which overrides the individual clearances.
3. Whether the warranty is conditional on the clearances, which it usually is.

Where the model cannot be enclosed within the available space, change the model or the position rather than the clearances. A unit installed outside its published clearances is an installation the manufacturer can decline to support.

Attenuated louvres and their pressure drop

Acoustic louvres are the usual way of getting sound attenuation and air through the same opening, and the arithmetic has to be done properly.

Two numbers govern. Free area, which for an acoustic louvre is commonly around half the gross area, so an opening needs to be roughly twice the free area required. And pressure drop, which rises with face velocity and which the fan has to overcome.

The pressure drop is the constraint people miss. A standard domestic or light commercial heat pump has an axial fan with very little static pressure capability, so it cannot push air through a deep acoustic louvre at all. Where an enclosure needs louvres on both the inlet and the discharge, the combined resistance can exceed what the fan will deliver, and the unit then recirculates its own air instead of drawing from outside.

Lined plenums perform better for the same resistance. An enclosure with generous openings, an internal absorptive lining and a change of direction in the air path attenuates by breaking the line of sight and absorbing on the reflected path, without the face velocity penalty of a deep louvre. That is usually the design to aim for.

Recirculation and the short circuit problem

Short circuiting is the failure mode that turns a compliant installation into a poorly performing one, and it does not show up on a noise calculation at all.

The mechanism is straightforward. Cold discharge air finds its way back to the inlet, so the unit sees an inlet temperature below the true outdoor temperature. Capacity falls, icing increases, and on a cold still night the effect compounds because there is no wind to clear the discharge.

Three arrangements cause it. An enclosure with inlet and discharge openings on the same face. A discharge facing a wall or fence within the clearance distance. And an enclosure roof close above a top-discharge unit, which turns the whole box into a recirculating loop.

The fixes are geometric. Separate the inlet and discharge openings on different faces, ideally on opposite sides. Duct or cowl the discharge so it exits the enclosure clear of the inlet. Keep the enclosure open at the top for a top-discharge unit, using a baffle above rather than a lid. And where the unit sits in a narrow passage between two buildings, accept that the passage itself will recirculate and move the unit.

Materials that satisfy both duties

Acoustic performance comes from mass and absorption. Airflow comes from free area. The material choices that do both are limited.

Perforated metal facing over a mineral wool absorbent, behind an open louvre or slot, gives absorption without blocking air. Solid panels of adequate mass on the faces that do not need openings give the mass. Timber slats look domestic and give almost no attenuation unless backed by something absorbent and closed.

Fire matters on some buildings. Where the enclosure is fixed to or forms part of an external wall of a relevant residential building, the restrictions on combustible materials in external walls come into play, and a timber enclosure with mineral wool behind it may not be acceptable. Check the building's status before specifying, and for a tall residential building expect the fire strategy author to have a view. Wind loading on a roof-level enclosure is its own question, covered in wind effects of a rooftop plant enclosure.

Anti-vibration mounting sits alongside the enclosure rather than inside it. Mounts, a rigid base and flexible pipe connections deal with structure-borne transmission, and an enclosure does nothing about that path.

Maintenance access and coil cleaning

An enclosure that cannot be opened becomes an enclosure that stops the unit working, usually in year three.

The coil needs cleaning, the condensate route needs checking, filters where fitted need changing and a service engineer needs to reach the electrical and refrigerant connections. Design for that from the start.

• A removable or hinged panel on at least two faces, with fixings a service engineer can undo without specialist tools.
• Enough standing space outside the enclosure for a panel to be taken off and put down.
• A clear condensate route with a defined discharge point, not a puddle inside the box.
• Space to withdraw the unit entirely if it has to be replaced, including the route off the site.

Where the enclosure is on a flat roof, add safe access to the list, because a plant enclosure inside a fall-protection zone becomes a permit job every time someone has to open it.

Specifying the enclosure so it can be conditioned

The condition a council writes decides whether what gets built is what was assessed. Give them the wording to work with.

A useful condition does three things: it sets the numeric noise limit at a named receptor position, it requires the enclosure to be built in accordance with a specified drawing and specification, and it requires the manufacturer's minimum clearances and free areas to be maintained. A condition that says only that an acoustic enclosure shall be provided is unenforceable in any useful sense, and it invites a value-engineered box that meets nothing.

Supply the information that makes the condition work: the unit make and model, its sound power level by octave band, the published clearances, the enclosure drawings with opening free areas dimensioned, the predicted level at the receptor, and the maintenance access arrangement. An Energy Statement should carry the performance assumptions that the enclosure must not undermine, and the acoustic case belongs in the Noise Impact Assessment with the same model and the same figures. Where conditions are drafted loosely, the enclosure is one of the first items to be simplified on site, which is the pattern described in securing an energy strategy through planning conditions.

One honest uncertainty. Manufacturers publish sound power data for defined operating modes, and the level during a defrost cycle is often not published at all. Where night-time performance is the issue, ask the supplier for defrost data in writing and say in the assessment what you assumed if none is available.

PlanScope drafts an Energy Statement and a Noise Impact Assessment from your project details and your own equipment data, with "TO BE CONFIRMED" flags where manufacturer figures are missing. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and enclosure acoustics and heat pump selection both need suitably qualified input.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your scheme is likely to need.