In a MVHR dwelling what would be the impact on people if the fan failed, assuming the worst case scenario that winter windows were closed etc?

Question from Gareth Davies, via email:
In a MVHR dwelling what would be the impact on people if the fan failed, assuming the worst case scenario that winter windows were closed etc., and or a gas escape or release of any volatile substance? Also, what would be the level of adventitious ventilation in a dwelling built to current regulations?

Answer from Ian Davis, technical manager at Soler & Palau:
If a MVHR unit was to fail, the dwelling’s occupants should be aware that their property’s ventilation strategy will be compromised. Most properties, with the exception of the less permeable constructions, will continue to “breathe” as air transfers via leakages in the building envelope. If there is a temporary power failure for instance, then the reduction in ventilation may go unnoticed by the occupants.

However, should the MVHR unit remain inactive for an extended period of time, there will be a noticeable decrease in air quality; humidity levels can increase, condensation may form on cold surfaces and there becomes a risk to occupants’ respiratory health.

Many MVHR units, such as the Soler and Palau IDEO 325 ECOWATT, have remote indication of the system’s status and warns of product failure. The occupants should take immediate action to provide their property with natural ventilation. This is particularly important in less leaky properties or where there is a risk of volatile compound releases and can usually be achieved quickly and effectively by opening windows in all rooms (where security is not an issue).

The level of adventitious ventilation to a dwelling should be no more than 10m3/h/m2 at 50Pa (the air leakage rate per hour per square metre of envelope area at a pressure differential across the building envelope), a value defined by the current Building Regulations. There is no lower limit when the property is designed with a MVHR ventilation system.