This week we are looking at looking at Mechanical Ventilation with Heat Recovery designed to the Passivhaus Standard. Also, we consider how the building envelope should be designed and constructed to comply with the standard.
Q: How is Passivhaus Certified MVHR different from Building Regs MVHR?
A: The mvhr in a building built to Passivhaus Standard needs to be to a certified standard. It will typically be more efficient than a Building Regs standard unit. The intake and extract ducts will be insulated to prevent both heat gain and heat loss. There will be additional noise attenuation on the supply and extract ducts adjacent to the unit and overall, the system will be quieter. Ducts can be plastic but will typically be round, galvanised steel for improved noise and durability.
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. This information will then be inputted into the Passivhaus Planning Package software.
Q: How is 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.
Q: What is uncontrolled ventilation and why is it important?
A: Uncontrolled ventilation is significantly reduced in the Passivhaus Standard. The maximum amount of permitted 'leakiness' is well below a even the best Building Regulations level. The building envelope will be designed to minimise the risk of air leakage and careful consideration should be given to how the building is constructed and whether what has been designed can actually be achieved on site. Air leakage can be controlled with construction tapes at abutments and corner junctions but it can also be achieved in conjunction with gypsum plaster and coated boards.
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.