I recently discovered that many manufacturers of MVHR systems in the UK are offering cheaper versions of their systems without summer bypass features. I believe this to be a very bad idea. In many lightweight buildings where there is a high likelihood of overheating in summer months, the addition of a MVHR will make the problem worse if it does not have a summer by-pass. The MVHR will continue to extract the heat from the outgoing air and pass it on to the incoming air. So consider the end of a hot summer day, the house is full of hot air, the outside temperature has not dropped below 24C, and as the internal air is vented, its heat content is being passed on to the incoming air which is already too hot. The result will be a lot of unhappy residents. MVHR manufacturers need to act sensibly on this issue and not allow the sale of such equipment to projects likely to overheat. It should also be built into the building regulations and SAP to ensure that this feature is better understood by contractors and specifiers. They will not be the ones to suffer from overheating problems in years to come, but the MVHR indutry will suffer generally.