Exhaust air heat pumps

An exhaust air heat pump transfers heat from a ventilation system to warm air that heats your home. It can also be used to heat water stored in a hot water cylinder for your hot taps, showers, and baths.

An exhaust air heat pump boosts the heat that the mechanical ventilation heat recovery (MVHR) unit extracts from the warm, stale air. This means that as well as saving energy, the ducting system can be used to heat the building using warm air. This removes the need to install a ‘wet’ central heating system using radiators or underfloor heating, which can save you money.

For further information on how heat pumps work in general, see our in depth guide to heat pumps.

Not sure where to begin? Visit our heat pump hub to get to grips with what they are and how they work.

• Exhaust air heat pumps combine ventilation, heating, and hot water in a single unit.
• The smallest standard heat pumps (such as air source heat pumps) are typically rated at 5kW, which may be too large for very low energy buildings. Exhaust air heat pumps can deal with smaller heating requirements for very low energy buildings, avoiding the problems associated with oversized heat pumps.
• They remove the need to install a wet central heating system or install a gas connection to a new-build property.

As exhaust air heat pumps extract heat from ventilated (or exhaust) air leaving a building, they’re always combined with a mechanical ventilation system.

It’s important for your health that stale air is continuously removed from your home and replaced with fresh air. Most homes rely on this happening naturally, with air changing through:

• ventilation grills
• airbricks
• gaps between walls, floors and the roof

Passivhauses and other highly thermally efficient (low energy) new builds are built airtight to prevent heat escaping from the building. These homes can’t rely on natural ventilation, so typically use a mechanical ventilation heat recovery system. The MVHR unit sucks cool, fresh air into the building and blows out the warm, stale air.

To minimise heat loss from blowing out the warm air, the MVHR extracts heat from the stale air and uses it to heat the incoming fresh, cool air. The MVHR ensures the building is well ventilated by circulating air around a system of ducts and grills in each room. These ducts are usually hidden in ceiling voids (the space between the ceiling and roof).

In some cases, an exhaust air heat pump can deliver all your home’s heating needs, although this depends on:

• the size of your home
• how much hot water you need
• how airtight your home is

An experienced installer will be able to advise if an exhaust air heat pump on its own will be enough to heat your entire home. If it isn’t, you may need to install an additional heater.

An exhaust air heat pump can deliver heat to a hot water cylinder. In many models, the hot water cylinder is built into the case of the heat pump, working as a fully integrated system. In others, the hot water cylinder will be separate.

In both designs, the hot water cylinder will be fitted with an immersion heater. This is to either top up the temperature or supply additional hot water if you run out.

• Could lower your fuel bills
• Will cut your carbon emissions
• Can heat your home and hot water

Looking for an installer? Microgeneration Certification Scheme (MCS) have a database of certified installers and systems to help you find an installer.

An exhaust air heat pump delivers more heat into your home than a MVHR system on its own. This helps reduce the running costs of your heating.

If your new build home needs a MVHR system, you may want to consider combining the function of heating and air handling into one unit. This will save on upfront and installation costs compared to a separate MVHR and heating system.

The efficiency of an exhaust air heat pump is impacted by:

• The outside temperature.
• Your heating needs, including your room heating and hot water.
• How airtight your building is: the more airtight, the better the heat recovery

The designed seasonal performance factor (SPF) of your exhaust air heat pump will indicate its running costs. You can find out more in our in-depth guide.