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System sizing and flow rates in mechanical ventilation with heat recovery in a passive house!
May 2, 2016

System sizing and flow rates in mechanical ventilation with heat recovery in a passive house!

There are set criteria for working out how much air to supply in a Passivhaus; these are provided in the manual for the Ventilation Final Protocol Worksheet, which is supplied with PHPP. Supply air and extract air are worked out differently, but when the system is running they must be equal, i.e. in balance. Supply air is set at 30 m3/h/person – this should be calculated using the actual number of people expected to live in the house, rather than the default figure calculated by PHPP at 35 m2/person. For under-occupied houses (> 40 m2 per person) the backstop rate is 0.3 ACH. This gives the standard airflow rate, which is usually set as the middle fan speed. Extract rates are normally based on individual wet rooms: 60 m3/h for a kitchen, 40 m3/h for bathrooms and utility rooms, and 20 m3/h for a WC. The total extract rate is usually higher than the supply rate – the difference is dealt with by running both fans at the higher rate when cooking or bathing requires extract. The design ‘boost’ rate is either the ‘standard’ rate + 30% or the total extract rate, whichever is larger.

Confusingly, the PHPP calculations of the nominal rate are different: PHPP first sets the maximum flow as the larger of the supply and extract rates, as calculated above, and then sets the standard rate at 77% of this figure. The implications are that for more densely occupied houses the per-person rate will be lower than 30 m3/hr/person and for houses with large numbers of bathrooms the per-person rate will be higher. It’s advisable to default to the manual and the 30 m3/h/person standard rate and adjust extract rates up or down to give a boost of +30%-40%, provided that the building regulations Part F boost extract rates are still maintained.

When it comes to deciding air flow to each room the basic rules are bedroom supply of at least 15 m3/h/person, so 30 m3/h for a double bedroom, and the remainder is available for living spaces. Dining rooms can often rely on air passing through to the kitchen and so don’t need their own supply, but this depends on the layout of a particular house. Extract rates are set according to room type but need to be reduced proportionally from the boost rate to the standard rate, so that the supply and extract rates are in balance.

The two settings for normal supply and boost extract are comparable to the Part F approach, though the backstop figure for Part F works out at 0.4 A air changes per hour (ACH) rather than 0.3. We think the Passivhaus level is adequate since MVHR actually provides a significantly higher fresh air rate in bedrooms and living rooms, and the research basis for Part F was in window-ventilated houses which don’t benefit from fresh air being supplied just to these rooms. The lower figure for Passivhaus also helps avoid over-drying of internal air in winter. Passivhaus design also allows for a ‘low’ fan speed, normally 30% lower than the standard rate, which can be used when the house is unoccupied.

Excerpt from “How to build a Passivhaus: Rules of thumb”, chapter by Alan Clarke, Passivhaus Trust, April 2015