WHY MECHANICAL VENTILATION WITH HEAT RECOVERY?
We ventilate our homes for many reasons, firstly, because we want to comply with building regulations, but mainly because we wish to maintain a healthy and comfortable indoor environment. We could adopt a seemingly simpler and cheaper approach, such as holes in the walls and simple bathroom fans to take out the warm moist air, but as we know this is not the best option in the long run.
We are very familiar with moisture in the air outside. It is an integral part of our maritime climate in Ireland and the UK. The relative humidity is high and indeed for a lot of the time it is in excess of 88%. For a comfortable indoor environment the humidity should ideally be between 40% and 60%.
The concept of heat recovery, using the outgoing warm stale air to increase the temperature of the incoming fresh air, will help achieve this. In summer, when indoor and outdoor temperatures are similar, no change in relative humidity occurs, this won’t create a problem as there aren’t any cold surfaces for condensation on which to occur. In Ireland particularly, it is normal to have indoor relative humidity of over 70% during the summer months.
With the price of fossil fuels on the rise we, as homeowners, need to consider all options and methods of minimizing our energy requirements. We do this by ensuring that our homes are not just highly insulated but are also draught free. Draughts aren’t just a matter of cold air coming in and warm air leaking out; they also have negative effects on the insulation and prevent it from performing its rated maximum. Draught proofing is classed as being so important that there is now a provision in the Building Regulations (Part L) for measuring the level of draught proofing or air tightness as it is commonly referred to. See Air Tightness FAQs.
It is important that any fresh air entering the house does so in a controlled fashion and that exhaust air has had its heat taken from it first and goes out at low temperature. In other words, the heat is recovered from the exhaust air and then used to warm the fresh air going into the house.
In this way, fresh air from outside, no matter how cold it gets, will always enter your house at a comfortable 18 degrees or so. This formula of Insulation + air-tightness + Heat Recovery Ventilation = comfort + health + energy efficiency, and forms the basis for the ultra-low-energy strategy used in building passive houses.