Ventilation rates criteria for MVHR

The ventilation rates provided  in Approved Document F 2010 are explicitly stated as being the ‘minimum’. 25% as a boost should be factored in as a matter of good design. If systems are so critically sized that an increase of 25% requires larger MVHRs, and larger ducts then this suggests there was no flexibility in the original design/sizing and AD-F was deemed to be the target ventilation rate.

Correct sizing of systems is  especially critical for flats where we have the worst problems of under ventilation and where making changes retrospectively may be costly. MVHRs should not be operating above 50% fan speed in background mode, so the ability to achieve boost should always be present, regardless of dwelling type.