MVHR – Mechanical Ventilation and Heat Recovery

Having mechanical ventilation is a basic design principle of a Passivhaus building. For a healthy environment it is important that clean, filtered air circulates throughout a home. This is no longer left to the vagaries of uncontrolled natural ventilation, such as open windows or draughts. Outside air is taken in, filtered, warmed, and circulated around the house in a controlled fashion at a controlled speed ideally suited to comfortable living, 24/7. Ductwork throughout the building fabric is designed to ensure no spaces are left unventilated. No longer do homes have hot or cold rooms, damp or mouldy corners, draughts, and so on.

Hand in hand with highly efficient operation, the ventilation unit will recover heat from the outgoing air before expelling it outside. Many units recover over 90% of the heat energy in this outgoing air, using that heat to warm the incoming air.

Heat Pumps

Heat pumps are highly efficient heaters, typically heating air as air is much easier to heat than water. They typically have a small motor to circulate a refrigerant substance around two heat exchangers. One example is an air source heat pump. Here the refrigerant begins as a cool liquid in an outdoor heat exchanger – where it extracts heat from outdoor air. As the refrigerant warms it is then compressed and warmed further into a gas, where it passes into the other heat exchanger – and warms the indoor air. The refrigerant then cools into a liquid again, once again to be circulated into the outdoor heat exchanger. This cycle repeats continually, again and again extracting small amounts of energy from the outside air and using that energy to warm the inside air. In some houses the amount of energy generated can be small relative to the needs of the home, but in a Passivhaus home a heat pump can be sufficient to hear the whole house.

Solar PV

Solar PV panels convert light from the Sun into electricity. As the electricity begins as a DC current, the solar PV system will have a converter to be able to employ it in the homes AC system. Sometimes the solar PV system will have a battery to store the energy and often it will have a computer card to supply unused electricity into the local grid.

Typical solar panel arrays in a detached house might be rated as 4kW of power, and operate at something like 70% of that rating. In terms of power and energy sizing, that is effectively a similar power rating to two hair dryers. Such solar PV arrays, when coupled with battery storage, can be sufficient for a Passivhaus needs but struggle to meet the needs of many traditional houses.