In our final blog post of this series we are looking at the trickier problem of introducing ventilation into a retrofit project. Space for an MVHR unit can be especially difficult to find in domestic housing retrofits. Also, inserting new ductwork into voids between joists can be incredibly intrusive and is better suited to deeper, more extensive retrofit. Forming drop-down ceilings to accommodate ducts is often not an alternative because of the loss of room height and volume. An alternative approach could be to use is a decentralised system.
Q: What is decentralised continuous supply and extract?
A: Ventilation is provided via continuous supply and extract from paired wall units. The units communicate via wifi and cycle between supply and extract depending on factors such as humidity levels. Heat is recovered from extracted air via a ceramic plate that warms up and then transfers this heat to the cool intake air. Cycles are typically 70 seconds and units are designed to be effective for defined floor areas. They should be specified in pairs but there can be more than one pair depending on the size of the house. This arrangement typically suits low-energy retrofit where space for ducts is limited. The incoming air is filtered (typically with washable filters) but not to the degree that you would expect with a dedicated MVHR unit. Hybrid versions are available that combined wall mounted supply with ducted extract. 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: 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.
Q: Can I open the windows?
A: The windows can be opened for comfort control and cooling.