Exhaust air heat pumps (EAHP)

Exhaust air heat pumps recover heat from the stale air your home is already venting. They make sense only in very airtight, mechanically-ventilated properties, typically Passivhaus, modern timber-frame or apartment-block new-builds. In a draughty terrace they can't meet the heating demand and shouldn't be installed.

How it works

Modern airtight homes use Mechanical Ventilation with Heat Recovery (MVHR) to extract stale air from kitchens and bathrooms and bring in fresh, filtered air to bedrooms and living rooms. The extracted air is typically 20 – 22 °C, relatively warm.

An EAHP sits in the extract air stream after the heat-recovery exchanger and uses that warmed air as its heat source. The refrigerant cycle is the same as any other heat pump, but the source is the steady stream of warm extracted air rather than cold outside air.

The catch: the volume of extract air is small. A typical home extracts 100 – 200 m³/hour of air; this caps how much heat can be recovered. EAHPs typically deliver 3 – 5 kW of output, enough for a small well-insulated dwelling but not for a draughty Victorian terrace.

Benefits

• Compact and integrated. An EAHP can combine with the MVHR unit in a single package, useful for apartments and small footprints where wall space matters.
• No outdoor unit. Everything sits indoors in a utility room or service cupboard.
• Quiet. The unit is acoustically isolated and inherently quiet (small compressor, no fan facing outside).
• Stable source temperature. Extract air at 20 °C year-round gives consistent performance regardless of outside weather.
• Recovers heat that would otherwise be vented. Captures latent heat from cooking, washing and body heat, a genuine efficiency improvement on top of MVHR.
• Reasonable up-front cost compared to GSHP or WSHP, with no ground-works expense.
• Eligible for the £7,500 BUS grant when installed in eligible owner-occupied properties.

Disadvantages

• Only works in airtight, MVHR-equipped homes. Without MVHR there's no extract stream to harvest. Retrofitting MVHR is expensive (£4,000 – £12,000) and intrusive, making EAHP rarely the cheapest retrofit option.
• Low output ceiling. 3 – 5 kW is enough for a small modern dwelling (~70 m² well-insulated) but not for larger or leakier properties.
• You may still need a supplementary heater. Electric immersion or small additional electric heating may be needed during the coldest weeks. Some EAHPs include an auxiliary element.
• Air filter maintenance. The extract air carries kitchen oil, dust, and moisture. Filters need cleaning monthly and replacing annually. Neglected filters dramatically reduce performance.
• Tied to MVHR commissioning quality. If the MVHR isn't properly balanced, the EAHP suffers. Expect a competent installer to commission both together.
• Smaller installer pool. Most ASHP installers don't do EAHP. Specialist installers tend to also handle MVHR.
• Limited brand choice. Mainly NIBE, ResVent, and a few European brands. Less variety than ASHP.

Best for

• New-build properties built to current or stretched airtightness standards (post-2022 Building Regulations, Future Homes Standard, or Passivhaus).
• Apartments and flats in low-energy developments where MVHR is already specified.
• Custom self-builds where MVHR + EAHP is designed in from the start.
• Small dwellings (1 – 2 bedrooms, < 100 m²) with annual heat demand under 5,000 kWh.

Less suited to

• Pre-1980s housing stock, solid wall, single glazing, draughty floors. Heat demand exceeds what an EAHP can deliver.
• Larger detached homes, output ceiling is the limit.
• Properties without MVHR (you'd need to install that first; combined cost rarely beats ASHP).
• Owners replacing an end-of-life gas boiler, an ASHP is almost always the better answer.

EAHP vs ASHP for new-builds

For a Passivhaus-spec new-build, both EAHP and a small ASHP can meet heating demand. The choice usually comes down to:

• Space. If you want zero outdoor units, EAHP wins.
• Hot water. ASHPs are generally better at delivering DHW at competitive efficiency. Most EAHPs need a supplementary element for high-volume DHW.
• Cooling. ASHPs can reverse for cooling; EAHPs cannot (in most configurations).
• Cost. EAHP-MVHR combined units are often cheaper than ASHP + separate MVHR for small footprints.

Real-world considerations

Filter discipline

EAHPs are sensitive to filter condition. Set a monthly diary reminder to check the filters; annual cassette replacement is usually a service item. Skipping this is the single biggest cause of underperformance complaints.

Cooking-heavy households

Heavy kitchen extraction (frying, woks) leaves oil residue on the EAHP coil over time. Some installs include an extra filter stage before the heat pump, worth specifying if your household cooks a lot.

Bathroom moisture

High humidity from bathrooms is condensed on the EAHP's coil. The condensate drain must be routed to a proper waste, clogged drains cause overflow.

System sizing

EAHPs are not infinitely scalable. If your heat-loss calculation comes out above ~5 kW, either you're over-airtight target or your property isn't quite airtight enough, recheck the inputs.

FAQs

Can an EAHP heat my whole house?

In a small well-insulated property, yes, through the MVHR supply ducts. In larger or leakier homes, no, it's a supplemental rather than primary source.

Do I need radiators with an EAHP?

Usually a small underfloor heating loop or a couple of radiators serve as supplementary distribution in cold spells. Pure all-air heating works in Passivhaus-spec dwellings.

Can I retrofit an EAHP without MVHR?

No. Without an existing extract stream the EAHP has nothing to harvest. You'd be installing MVHR + EAHP together, which rarely makes economic sense vs ASHP.

How loud is it?

Quiet, typically 30 – 38 dB at 1 metre. The unit is indoors so it's louder in the utility room than from neighbouring rooms. Modern installs include vibration isolation.

Will it work with a wood stove?

Yes, but the wood stove must be a room-sealed (externally-vented) model. Open fires interfere with airtightness and can disrupt the MVHR/EAHP system.

Specifying EAHP at the design stage

EAHP works best when it's designed in from the start rather than retrofitted, because the MVHR ductwork, extract volumes and heat pump capacity all need to be sized together. If you're working with an architect on a new-build or a deep retrofit to Passivhaus standard, raise EAHP as an option during the SAP or PHPP modelling stage, not after the ductwork is already specified. Changing to EAHP later usually means re-running the ventilation design, which costs time and, in some cases, requires re-routing ducts that were sized for a standard MVHR-only system.

How EAHP compares on running cost

Because the extract air source sits at a stable 20°C year-round regardless of outside weather, EAHP running costs are unusually predictable compared with ASHP, there's no winter efficiency dip to plan around. In a genuinely airtight, well-insulated small home, annual electricity use for heating and hot water often lands under 2,500 kWh, translating to a modest bill on either a standard tariff or a heat-pump-specific one. The trade-off is that this low running cost only applies within EAHP's narrow output ceiling, it isn't a technology that scales to a larger or leakier property the way ASHP or GSHP can.

One more nuance worth knowing: because EAHP shares its heat source with the ventilation system, opening windows regularly defeats part of the efficiency case, you're venting the warm extract air the heat pump relies on. This is rarely an issue in practice for well-designed MVHR homes, since the mechanical ventilation is designed to make window-opening unnecessary for air quality, but it's a genuine behavioural adjustment for anyone used to opening windows freely in an older property.

Worked example: a new-build apartment

A 65 m² Passivhaus-spec apartment already has MVHR fitted as part of Building Regulations compliance. Adding an EAHP alongside it costs around £7,200, well below the £13,000-plus an equivalent ASHP-plus-separate-MVHR setup would need, because the ductwork and heat-recovery core are shared. After the £7,500 BUS grant (where the development qualifies), the net cost can fall to close to zero, though most EAHP installs sit in the £0 to £4,500 range once radiator or underfloor top-ups are included. Annual heating demand for a flat this size and this airtight is typically under 3,000 kWh, which the EAHP's 3 to 5 kW output comfortably covers.

Compare with other types

Air source (ASHP) · Ground source (GSHP) · Water source (WSHP) · Hybrid

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