How mechanical ventilation and heat recovery works
With tighter new buildings, whole house heat recovery ventilation is the most effective type to satisfy the current building regs (document F) and SAP ratings under (document L).

Mechanical Ventilation with Heat Recovery (MVHR) has been designed to continuously extract humid air from wet areas such as kitchens and bathrooms using a heat exchanger, transferring the heat to filtered incoming outside air which is then redistributed into living areas and bathrooms. “The heat exchangers are up to 93% efficient which can significantly  help reduce household heating bills, plus the affects of condensation are dramatically reduced”