Mechanical Ventilation Heat Recovery systems

Mike Harvey, HETAS Training and Technical Support Manager offers advice to ensure that your appliance maintains an adequate air supply in all possible conditions when working with Mechanical Ventilation Heat Recovery Systems.

Mechanical Ventilation Heat Recovery Ssytems

Growing popularity of MVHR

Increasingly popular, mechanical ventilation heat recovery (MVHR) systems build on the principal of mechanical ventilation – whereby stale air is mechanically removed from a home and replaced with a fresh stream of incoming air from outside.

In addition, MVHR extracts and recycles any warmth in the outgoing air by use of a central heat exchanger and the heat recovered is fed back into the supply air. This assists in optimising the climate in the building, however it must be noted that MVHR itself is not a means of providing heat to the property.

MVHR negates the requirement for trickle vents and the use of individual extract fans with a single, whole-house solution. It works most effectively in new homes built to modern standards of energy efficiency. Although MVHR can be installed in any building, there is a rule of thumb that its use is not justified unless the air permeability is at or below 3 air changes per hour when tested at 50 Pascal (equivalent approximately to 3m3/m2.h @ 50 Pa for average dwellings).

An example of hot and cool airflow in a Mechanical Ventilation Heat Recovery System.
An example of hot and cool airflow in a Mechanical Ventilation Heat Recovery System

Wood Burning Stoves and MVHR

Wood burning stoves are often installed alongside this technology and can be installed as a primary or secondary heat source, complimenting other forms of heating provision such as a heat pump.

The combination of these tehcnologies does however raise challenges in the design and installation for installers and specifiers who must consider the complexities of air provision requirements for combustion ventilation.

Systems that act by pressurising the building (as opposed to depressurising it) are to be preferred when a system is designed to include open flue appliances such as a wood burning stove. Pressurising MVHR systems are much less likely to interfere with the safe functioning of the chimney system. Having said this, systems should be designed so that air requirements for open flued combustion appliances are maintained in the event of a power failure or filtration blockage i.e. when MVHR fans are not operational or the system is otherwise compromised.

Stove air supply

Conscious of the fact that any extract ventilation systems potentially lower the air pressure within a building which consequently has the potential for spillage of products of combustion to enter the living space; HETAS deem it imperative that any air required by the stvoe is supplied separately from the MVHR system. It may be argued that adequate air is deemed available via MVHR provided positive pressure is maintained, however it must be borne in mind that this would largely be dependent on the MVHR system itself being strictly maintained and serviced to ensure all filters remain clear and air is not impeded. MVHR is usually serviced at six monthly intervals dependent on manufacturers and installation guidance, however any service and maintenace advice surrounding the MVHR is not within the scope of a HETAS registrant.

Flue termination point must also be considered, being mindful of terminal positioning in relation to air intake and extract.

The following excerpt is taken from Direct External Air Supply (DEAS) guidance in HETAS Technical Bulletin #8.

However it must be borne in mind that not all DEAS appliances air requirements are met by the ‘direct air supply kit’. It is imperative that any appliance specified obtains 100% of its air requirement externally – manufacturer’s advice should be sought in mind of this.

HETAS Technical Bulletin 8

Appliance

When the correct appliance is specified it should beinstalled, tested and commissioned, along with handover to the end user, including advice on appropriate fuels and service and maintenance.

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