The replacement air is supplied through background ventilators. Alternatively, when the building has a design permeability greater than 5 m3/(h.m2) @ 50 Pa, background ventilators (trickle vents) are not required. The system can either be a centralized system with a single fan connected to various rooms or a decentralised system (dMEV) with air extracted from each room by individual fans. There are two airflow rates, trickle and boost. Air heat pumps for exhaust air would be regarded as a central MEV form.