Mechanical ventilation with heat recovery recovers heat from extracted air and use it to pre-warm the replacement fresh air from outside of the dwelling. The effectiveness of this process is known as the unit's 'heat exchange efficiency', the proportion of otherwise wasted heat recovered by the process, and this is usually expressed as a percentage. A heat recovery unit of the MVHR system reduces the amount of energy (from a heating system) needed to heat up the incoming air to the desired room temperature. Th is benefit should be balanced against the electrical power required to run the system. Th ese systems both extract moist air from wet rooms and transfer a significant proportion of the heat contained in that air (through a heat exchanger, i.e. no mixing occurs) to incoming fresh air which is supplied to the habitable room. These systems work best (i.e. are cost effective and energy efficient) in highly insulated and airtight dwellings.