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Do I need MVHR?

Mechanical ventilation with heat recovery (MVHR) takes the heat from the air leaving the building using a heat-exchanger in the roof. It then passes it to incoming fresh air which is pumped to the ground floor. MVHR is a must if you are achieving Passivhaus levels of airtightness. It can also be a blessing in an older home that has persistent issues with damp.

A whole-house MHVR heat exchanger with insulated ducting in a loft space.

A whole-house MHVR heat exchanger with insulated ducting in a loft space.

Before you install MVHR

Building owners are often told to install whole-house mechanical ventilation with heat recovery when it is not required.

A building needs to be very airtight and weatherproofed before this becomes necessary, unless there is a known serious problem with damp throughout the building, which cannot be treated with the usual damp-prevention methods, the use of insulation or insulating plasters.

Plan for a simple MHVR layout in the attic.

Plan for a simple MHVR layout in the attic.

As a rule of thumb, installing whole-house MVHR is only worth it if:

  • you have eliminated all thermal bridges
  • the walls have a U-value of 0.12 to 0.15
  • and the air permeability of the thermal envelope (building) is at or below 1.5 air changes per hour (ACH) when tested at 50 Pascal (Pa) (equivalent approximately to 3 m3/m2.h @ 50 Pa for average dwellings) (NB: the Passivhaus target is 0.6 ACH @ 50 Pa).

If the building owner has gone to the expense of attempting to create an airtight building, then the cost of a blower door test (which determines the air permeability) will not be prohibitive, and indeed will confirm the degree of their success.

How an MVHR layout could work in a house

How an MVHR layout could work in a house

However, they would do well to read the caveats on the AECB forum.

An alternative to whole-house MVHR

An alternative to whole-house MVHR would be to install MHVR in wet rooms only. This can take moist air from showers, baths, or cookers, expel the moisture, and reclaim most of the heat, adding it to incoming air which is blown back into the room.

They would only need to be working when required, so the energy and financial operating costs would be low.

The lowest-possible power-consuming pump should be used for the job, to save energy. Kinks and bends in the ducting should be avoided.

© David Thorpe Jan 2013. David is the author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency


eco renovation - talk to homeowners

Also see:
MVHR at free eco open house events in September
Assessment of MVHR systems in 10 zero carbon homes
Mechanical ventilation with heat recovery in new homes
Draught-proofing
MVHR DIY