Waste not, want not

Exhaust air heat pumps (EAHPs) work in a similar way to other types of heat pumps, but rather than using heat from the air outside or the ground, they recycle the heat from inside a building. This means EAHPs are best suited to new-builds, flats and smaller properties, where energy efficiency and controlled ventilation are a priority.

“EAHPs work especially well in airtight, well-insulated homes built to modern building regulations,” explains Rick Clarke, product manager at NIBE. “They’re also a strong option for developments with limited outdoor space, as they often require zero infrastructure compared to ground source or large air source heat pumps.”

Role of MVHR

While most buildings rely on natural ventilation in the form of ventilation grills, air bricks and gaps between walls, floors and the roof, Passivhaus and other highly thermally efficient (low energy) new-builds are built airtight to prevent heat escaping. These buildings typically use mechanical ventilation with heat recovery (MVHR) to bring in fresh cool air and blow out stale warm air.

In order to minimise heat loss from removing the stale air, the MVHR extracts the warmth and uses it to heat the incoming cool air. The MVHR ensures that buildings are well ventilated by circulating air around a system of ducts and grills.

There are two main types of EAHP:

• Air-to-water system: Heat is transferred to water, directly or indirectly, by integrating the heat pump with a hot water cylinder.
• Air-to-air system: Heat is transferred to the air for use in conjunction with an MVHR system.

EAHPs are designed to recover energy that would otherwise be lost from a home by extracting warm, stale air from kitchens, bathrooms and utility rooms and passing it through a heat pump. As EAHPs extract heat from ventilated (or exhaust) air leaving a building, they’re usually combined with an MVHR.

“The heat pump upgrades the energy from the air and uses it to provide space heating and domestic hot water,” explains Clarke. “At the same time, fresh air is circulated back into the home, improving indoor air quality. This means an EAHP not only provides heating and hot water, but also acts as a ventilation system, ensuring comfort, efficiency and healthier living conditions.”

Not only do these types of heat pump save energy while heating buildings, they also remove the need to install wet central heating systems with radiators or underfloor heating, creating further savings.

Refrigeration cycle

Using heat from the air extracted from the property, the EAHP uses a principle called the refrigeration cycle to provide heating and hot water.

The refrigeration cycle has four stages:

1. A component called the evaporator contains liquid refrigerant that is colder than the ambient air. The refrigerant has a very low boiling point, which means the heat transfer causes the refrigerant to boil and turn it into a vapour.
2. The refrigerant vapour moves to the compressor, which then results in an increase in temperature and pressure. The high temperature, high-pressure refrigerant vapour now condenses and gives off heat, turning it from a vapour into a low-temperature liquid.
3. The heat emitted by the condenser is transferred to the home’s heating system.
4. The refrigerant passes through the expansion valve, where its pressure is reduced. It has now returned to its original state: a low-temperature, low-pressure liquid. The cycle then starts again.

Fit for purpose?

Whether an EAHP will be able to provide all of a home’s heating needs will depend on the size of the building, how much hot water is needed and how airtight the property is.

“In the right type of property, an EAHP can easily provide the entire heating and hot water demand,” says Clarke. “By continuously recovering and reusing indoor energy, the system maintains a stable, efficient supply of heating and hot water.

“Generally, EAHPs are installed in apartments, however they can also be suitable for larger properties. As with all heat pump systems, correct design is crucial and will determine the product’s ability to provide the full heating and domestic hot water requirements.”

What is MCS certification and why is it important?

MCS (Microgeneration Certification Scheme) is the UK’s quality assurance scheme for renewable energy technologies. Both products and installers must meet its technical and safety standards.

Keymark certification is an equivalent European quality mark, recognised within the UK. This means products are listed on the MCS database and can be installed by MCS-certified installers, giving homeowners peace of mind about quality, safety and performance.

“For homeowners, the important thing is that heat pump products are independently tested and listed on the official databases that demonstrate compliance with MCS requirements,” explains Rick Clarke from NIBE.