What is continuous mechanical supply and extract with heat recovery (MVHR)?

Also known as a System 4 ventilation system. MVHR is a form of centralized ventilation scheme that combines supply and extract in one system.

Typically, hot, humid air is obtained by a duct system from wet areas, such as kitchens, toilets and utilities, and is exchanged via a heat exchanger. Fresh air enters through the heat exchanger before being provided to spaces such as the living room or the bedrooms. The air is preheated. There are two airflow rates, trickle and boost.

Other types of ventilation systems are permitted to be used in the UK, such as demand controlled ventilation (DCV) and positive input ventilation (PIV). In order to satisfy the building control body, these schemes need to show that certain performance criteria, such as moisture removal or indoor pollutants, are met.