The ASHRAE Ventilation Recommendations Are Too Low

With winter on its way, I've been thinking a lot about heating and insulation -- specifically, about what the minimum level of power required is given the need to heat up outside air in a properly ventilated home. This led me to the American Society of Heating, Refrigerating and Air-Conditioning Engineers, who provide guidance as to what the outside airflow rate should be to ensure acceptable air quality. Unfortunately, having done the maths regarding CO2 production, their recommendations are far too low.

Healthy individuals take 12-20 breaths per minute. Each breath exhales 0.5 litres of air, so each person will exhale 6-10 litres every minute, which at a CO2 level of 4%, contains 100 times the CO2 of ambient air. In order to dilute this to a doubling of CO2 over ambient, around 800ppm, 600-1000 L (10-17 L/s) of outside air would be required, assuming the inhabitants are not engaged in physical activity that would raise their CO2 production significantly.

The recommendations from ASHRAE are 2.5-5 L/s, plus extra based on the area of the building. But even including that results in recommendations as low as 3 L/s*person for conference rooms, and 4 L/s*person for sleeping areas. These ventilation rates will result in conference rooms risking having CO2 levels above 2000ppm. CO2 levels above 1000ppm have been shown to decrease cognitive perfomance and cause health problems. Which is undesirable if people are expected to make decisions.

I do not know the reasoning that was used to calculate these guidelines. It is quite possible they were focused on factors other than CO2, such as maintaining comfortable moisture levels. The UK Health and Safety Executive consider a place to be well ventilated if it has 10 L/s*person of outside air, but this should probably instead be considered to be the bare minimum for ventilation. My conclusion is that targeting 15 L/s*person, and taking the upper expectation for how many people will be in the building, is the right path to follow for calculating required ventilation rates. Assuming this air has to be heated by 15 degrees celcius to be comfortable, it would require 300 W of heating, a third of which can come from the bodyheat of the person themselves. This is an energy cost well worth paying and which we can afford.


Popular posts from this blog

Twittiquette -- The Rules I (Try To) Follow

The Subtle Future