MVHR stands for Mechanical Ventilation with Heat Recovery, and it is a type of ventilation that recovers heat energy from waste air, to save energy, and therefore money.
The unit you will need will depend on several factors. Generally you can base this on square meterage, but it can also vary depending on number of rooms, and ventilation requirements.
Certain units need different-sized filters. Our filter page lists each type of filter alongside the units which use it