All spaces within the insulation envelope in a passivhaus are properly ventilated with a mechanical ventilation unit which incorporates a heat recovery function (MVHR). This ensures a constant supply of fresh air in a controlled way within the airtightness layer, and is an integral part of the workings of a passivhaus building.

MVHR

A mechanical ventilation unit with heat recovery takes the fresh air from outside and passes this over the exhaust stale air from the building via a heat exchanger which recovers any heat that would otherwise be lost to the outside. For a passivhaus the MVHR unit must have a very low electrical consumption (specific fan power) of not more than 45Wh/m3, and be highly efficient with a minimum heat efficiency of  ≥ 75%. 

A typical system

The MVHR unit is used in conjunction with a system of ducts, which ideally should follow as compact and simple a layout as possible to minimise losses of efficiency within the system. Each duct terminates in either an inlet or outlet valve, which either blow air into the rooms or suck air out of the rooms at very slow speeds. The system utilises several different types of high grade filters to ensure than the incoming air is clean and fresh. These are very easy to change and must be periodically inspected (usually by the end user of the property) to ensure maximum efficiency of the system as whole. 

Typically a system will be designed to input fresh air into the living and sleeping spaces and extract it from the kitchen, utility room and bathrooms, using the transitional spaces such as hallways to move the air through the building. Rooms such as kitchens and bathrooms can be fitted with a control panel that allows a 'boost' function for a higher level of extraction over a short period of time should it be required. 

The units themselves are decibel rated; coupled with a low airspeed at the point of the outlets this ensures a virtually silent air distribution into the rooms, and a draft and noise free system as a whole. When designed, installed and commissioned properly this is a highly functional system that does not intrude on the users of the building, and performs much better than a conventional intermittent extract system. 

Contribution to space heating

The MVHR system makes a small yet essential contribution to the space heating in a passivhaus. As the design temperature internally is 20 degrees C, and the maximum temperature of air that can be supplied by the ventilation system is approximately 15 - 16 degrees C, there is only a temperature difference of 4 - 5 degrees C needed to be provided by any supplementary heating system. This leads to only a small amount of additional heating being required, with the exact amount for a specific project determined in the PHPP calculation.