This week we look at some options if you don’t want to rely on trickle ventilation through the windows and instead want to think about a mechanical solution that can also recover heat that would otherwise be lost.
Q: What is mechanical ventilation with heat recovery (MVHR)?
A: Ventilation is provided via continuous mechanical mechanism which is usually a central extract system. The mvhr is a unit that has 4no ducts: an intake & exhaust attached to the external wall; and a supply and extract which supply fresh, warmed, filtered air to habitable rooms (such as bedrooms and living rooms) and extract stale air (usually) from wetrooms. The mvhr unit incorporates a heat exchanger that recovers heat from the stale air extracted from the wet rooms and uses this to heat the cold fresh air entering the unit via the intake. Kitchens typically will have a recirculating hood. Trickle vents are not required.
Q: Who calculates the required amount of ventilation?
A: The calculation is complex should be carried out by a specialist consultant or subcontractor and you should expect them to design the system including the unit and the ducts.
Q: How is moist and stale air extracted from the building?
A: Stale air is extracted via a continuous fan from the wet rooms and is exhausted via the heat exchanger (in the mvhr unit) to the outside. The ducts are typically plastic and can be circular or rectangular.
Q: What is uncontrolled ventilation and why is it important?
A: Uncontrolled ventilation occurs through construction gaps in the external building envelope. It is often referred to as a building's "airtightness". It can contribute to drafts and results in the loss of internal heat gained through heating and occupying the house. MVHR will be more efficient the more airtight a building.
Q: How much electricity does it use?
A: Domestic MVHR units typically have two small fans which push the air through the system. The size of the dwelling and occupancy levels determine the size of the unit. For a small dwelling, an efficient unit will typically use around 20W in background mode, which is comparible to a low energy lightbulb (The Green Building Store).
Q: Can I open the windows?
A: The windows can be opened for comfort control and cooling.