Friday, February 7, 2014

The Great Ventilation Debate Rages On

The best minds in the building science field have not been able to yet agree on a ventilation rate for residential buildings.  The Energy Vanguard Blog is the best spot to witness that ongoing debate.

In my view, setting a standard for the ventilation rate is completely the wrong approach.  

As an analogy, consider the heating temperature control for a house.  If the temperature drops below a user-defined comfort setpoint, then heat is added to the space.  That's very simple, easy, cheap, and effective. Thermostatic control of  space heating can hardly be improved upon.  Why does it work so well?  Because it MEASURES, then CONTROLS.

In space heating, what we DON'T do is add a fixed number of btus per hour to the space during the winter season.  Yet that is what the ASHRAE or BSC ventilation standards propose to do*.  The current "cfm/person" approach is pseudoscience and should be abandoned in favor of  measurement and control.

What should we measure?  We're not sure yet, but if cost were no object, we would measure CO, CO2, methane, humidity, radon, and VOCs.  We may find one of these that can be a proxy for some of the others.

With that "air quality control" in place, the occupant can dial in a preferred setting, then forget about it.  The house will then get only what it needs, and there will be no money wasted on over-ventilation.

Ah, but we can't trust air-quality-ignorant occupants to know the best setting, right?  No, we can't, because many of the airborne contaminants cannot be detected by human sensory systems.  There is an obvious solution to that also, just ensure that the controls have scientifically derived maximum concentration settings for each contaminant.  Back to the space heating analogy:  thermostats have a minimum setting of 45F because bad things (freezing pipes) can happen below that setting.

*A standard ventilation rate can't account for all the variables.  Consider this:  what if the outdoor air is actually worse than the indoor air?  In that case, ventilation is the wrong answer, and a smart control would shut off the ventilation.


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  2. Well here's a $200 particulate air quality monitor, so technology seems to be catching up with consumer's needs.