Balancing Act

One of the key requirements of a PassivHaus or in our case something built to the EnerPHit standard, is air tightness. To achieve the thermal efficiency required, we need to be obsessive about cutting out any air leakage and with it our precious heat. Once you’ve sealed your house up and made it air tight you then need a plan to prevent the air going stale, oh and for making sure the occupants can still breathe.

The answer is to install a MVHR. This Mechanical Ventilation with Heat Recovery is another magic box of tricks, this time with fans and a very efficient heat exchanger. It will supply lots of fresh filtered air into our home with the heat exchanger warming the incoming fresh air using heat reclaimed from the exhaust air. The exhaust air gets warm from everything going on in the house such as cooking, heat from the fridge or us doing star jumps.

It works both ways, if the air temperature inside the building is colder than the outside air temperature in the summer then the coolth (apparently this is a real word) is maintained in the building.

Simple in theory, the MVHR must be carefully commissioned to be perfectly in balance:

  • the volume of incoming fresh air needs big enough to serve all the spaces in the house with lots of air changes to keep us fresh and healthy;
  • the extract vents are placed in bathrooms and the kitchen sucking all the smelly air away. Because these rooms aren’t that big compared with the volume of the house, it results in lots of air changes from these smelly rooms.
  • the system is designed so that the volume of extract air means there won’t be any condensation
  • no extract air is re-introduced or re-cycled, so any dirt or smoke in the air (and insects) will be either trapped in the filters or extracted to the outdoors.

This will only work if our house is sealed and perfectly air tight, so the only air movement is that generated by the MVHR – no more period draughts like our old houses.

That said, we’re not really sealed off from the outside as we can still fling open windows on sunny days, the MVHR will adjust. This will be especially useful in long hot summer spells when we can cool the house overnight running the MVHR in reverse to limit overheating during the day.


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