MVHR and ventilation
PHPP generally assumes a ventilation rate of around 0.3 air changes per hour (ACH), which is based on German property sizes and occupancy rates. For most UK homes, which are smaller and more densely occupied, higher ventilation rates are required.
Multiple MVHR units
If your design includes more than one ventilation unit, for example, in a terrace of houses or block of flats, you must use the Additional Vent sheet which is included in the more recent versions of PHPP. Although this sheet is challenging to use, it is vital to do so, as without it you will underestimate duct losses significantly.
A test to see if you’re using the ‘Additional Vent’ sheet correctly is to fi ll in the ‘Ventilation’ sheet for a single ventilation unit, with associated duct lengths. Change the flowrate to represent the area served by that ventilation unit. The efficiency should be similar to the overall value in the ‘Additional Vent’ sheet.
MVHR duct data
The fresh air and exhaust ducts from the MVHR to the outside environment are essentially outdoor spaces within the thermal envelope and can add significantly to the heat loss of your design. Be realistic about the duct lengths, diameters and the amount of insulation which will be fitted. Where the MVHR is located right against an external wall, duct lengths of >1.5m are still likely. 160mm diameter ducts and 25mm insulation are typical values for a domestic installation and closed cell insulation should be used to avoid condensation problems.