Everything you need to know about mechanical ventilation with heat recovery (MVHR)

Ventilation is becoming increasingly important as homes become more airtight. Mechanical ventilation with heat recovery (MVHR) is a potential solution can help, as explained here.

As government legislation and new building requirements push for more airtight homes in the pursuit of energy efficiency, ventilation has taken on a new focus. While homes with greater airtightness do a better job of retaining heat – thereby lowering energy bills and reducing carbon emissions – they can result in poor indoor air quality as condensation and pollutants remain trapped inside the home. This can lead to damp and mould.

Mechanical ventilation with heat recovery (MVHR) offers year-round ventilation that improves indoor air quality, while keeping properties warm.

How does MVHR work?

MVHR works by extracting moist, stale air from ‘wet’ rooms, such as kitchens and bathrooms. It takes the heat from this air and used it to warm incoming air that’s fresh and filtered.

The system comprises two independent ducted airflows, each with its own fan. The extraction fan takes the humid air and passes it over a heat exchange matrix, where it recovers 80-90% of the heat before discharging it outside. The second fan draws fresh air from outside, filters it to get rid of pollutions and allergens, then passes it over the heat exchange matrix where it is warmed before being supplied back into the property.

There are two general types of MVHR system: whole house and single room. A whole-house centralised system is typically installed in a loft space and is ducted to the occupied rooms of a property for extract and supply. Decentralised single room heat recovery systems are typically installed through outside walls and resupply fresh, tempered air into the room they are extracting.

Some units have an automatic or manual boost setting which can be used when generating excess moisture, such as when cooking or showering.

What are the benefits of MVHR?

MVHR is chiefly used to improve indoor air quality while generating warmth, however it offers a range of other advantages. It mitigates the need for a wet central heating system and can be used all year round for both heating and cooling.

In optimal conditions MVHR saves more energy than it consumes, which can in turn reduces energy bills. Its process is controlled and consistent, providing ventilation through slow but constant air movements. In contrast, ventilation via other solutions, such as trickle vents, is uncontrolled and subject to a variety of external factors such as wind speeds, temperatures and internal obstructions.

MVHR is also comparable to other heating systems in terms of maintenance: filters need to be cleaned and changed annually.

What are the disadvantages of MVHR?

MVHR provides similar temperatures throughout a property, creating a homogenous atmosphere which might not be to everyone’s liking, while open fires and wood burners will require dedicated external or direct air supply to operate safely with MVHR.

Noise is often cited as an issue, although this generally comes down to the cost of the system. Flexible ducting is cheaper but has a rippled internal surface, which increases resistance to air pressure and creates noise. Rigid ducting has a smooth internal surface, which results in lower noise levels, but this is more expensive.

The main disadvantage of MVHR, however, is that it’s not building agnostic. Properties need to meet a range of criteria to make MVHR worthwhile.

What properties are suitable for MVHR?

In theory, MVHR can be installed anywhere there is room to accommodate the system. However, there’s a lot of ductwork involved, and for systems to be as efficient as possible ducts need to be routed in line with the building layout.

As such, MVHR is better incorporated into the design stages of newbuilds than it is in property retrofit. This also enables ductwork to be hidden away in floor and ceiling joists, studwork and loft areas. In older properties, hiding the ducting within the fabric of the building is usually a challenge.

Secondly, while MVHR is suitable for properties of all air tightness rates, older leaky dwellings will not see the same efficiency returns as newer homes. The more airtight the house, the more effective the MVHR system will be. It’s generally accepted that 3 m³/hr/m² level or below is necessary to see real benefits.

Retrofitting is also more expensive than installing a MVHR system in a newbuild, so it’s often the case that the cost of overcoming aesthetic problems and getting the building sufficiently airtight outweighs any potential benefits.

How much does MVHR cost?

An MVHR whole-home system for a standard four-bedroom house costs from £3,500, while the installation will depend on the room layout and complexity of installing the ducting. For decentralised single room systems, prices start from £600 per fan.

However, there are energy savings to be had. Heating bills will come down modestly (less than 5% per year) in properties with an airtightness level of 5 m³/hr/m². From 3m³/hr/m², savings can reach around 20%, making MVHR a more viable choice for developers seeking sustainable ventilation solutions.