In order to provide enough oxygen we need 0.2 litres per person, per second. In order to exhaust the contaminants as detailed in the indoor air quality section, we need 5 litres per person, per second, and in order to create a feeling of freshness, we need to ventilate at 10 litres per person, per second!
Which is all well and good, but as the unwanted exhaust air leaves the building it takes a lot of heat with it. This might be seen as a benefit in buildings that are too warm, but what of those that are not warm enough? And even in those where heat removal may be a good thing, this is often done using even more energy.
What is more, as we introduce air movement in order to meet ventilation rates, so we then have to introduce more heat to counteract the discomfort felt by the air movement.
With regards to our homes, the ventilation and air change rates required in order to maintain healthy internal air quality haven’t been too much of an issue until recently, since there are enough air-leakage paths in old buildings to adequately supply fresh air, at junctions and joints, around windows and doors, up chimneys, and even just through tiny cracks in brickwork.
The problem with uncontrolled infiltration is that it creates cold spots and draughts, and invariably means an increase in discomfort and in the energy used for heating. In old buildings there is a real need to stop this infiltration so as to better control and manage ventilation, reduce energy consumption, and improve thermal comfort.
In modern and refurbished buildings, or at least in those that have been built or refurbished well, air infiltration is less of a problem because modern construction and materials should mean that there are far fewer air-leakage paths available. The issue then becomes the need to artificially supply fresh air to meet ventilation rates and ensure a healthy internal air quality.
On warm days and nights this isn’t an issue: the heating isn’t on and we can open doors and windows without wasting energy. On cold days, however, we don’t want the windows open for more reasons than one, and so we must introduce some form of managed ventilation.
Mechanical ventilation takes many forms, from the extractor fan in your bathroom or kitchen, to full air conditioning. It is generally always electricity that drives the fans, and electricity is an expensive and carbon intensive fuel. This expense can be justified in the depths of winter, when for the easing of discomfort it is a price worth paying, or the energy used to manage ventilation is less than might have been consumed by losing heat.
This is particularly true of Mechanical Ventilation and Heat Recovery (MVHR), which extracts 90% of the heat in the exhaust air and gives it to the incoming fresh air. For MVHR to be effective however, you need a really well-sealed building.
However, it is still feasible that natural or passive ventilation can be both effective and efficient on cold days. Passive ventilation largely works by utilising the stack effect – air within a building will be warmer and therefore less dense than without, and the pressure difference creates air movement from the inside to the outside, the same as a chimney stack works.
The trick then is to capture the outgoing heat as in MVHR. There are now products appearing on the market that will do just that without the need for electric fans; click here for an example of this.