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We received the following question about indoor air quality during a power outage in a Zero Energy Home recently and thought we’d share the findings.

Question:

We built a Net Zero Energy home, and the blower door test meets the Passiv Haus standard for air tightness. The final blower door test was 269 cubic feet per minute (cfm) @ 50 pa – which give an ACH50 of 0.46 (PH=0.60 ACH50). It’s an all-electric house with no combustion.

The Zehnder ventilation system is set to run at 144 cfm. The floor area is about 2700 square feet with 9-foot high ceilings and a full basement of about another 1000 square feet, so the actual house volume is about 34,791 cubic feet. There are 4 bedrooms, 3.5 baths.

My question is: If there is a power failure, and the ventilation system shuts down, should we be concerned about the air quality in the house without the ventilation? Should we worry about suffocation?

Thanks! –David

Answer:

open-window-jill-regerBelow is an answer from Bob Lorenzen, a Certified Passive House Consultant from Lorenzen Energy Consulting, LLC, who lives in a Zero Energy Home that he designed.

That’s a pretty good question, and not one that many people think of. I have not seen any studies of the impacts on indoor air quality in a Passive House during a power outage. The only information that I have come across was a discussion on resilient design on the BuildingGreen web site which said that during power outages, you simple crack a couple of windows to let in fresh air.

By definition, suffocation is an inability to breathe due to a lack of oxygen. With about 35,000 cubic feet of air volume, I don’t think that suffocation is the issue. The bigger concern would be if the carbon dioxide levels in the home would elevate to dangerous levels. (I am assuming that the materials and equipment selection in the home has limited other pollutant sources.) Without the ventilation system running, the CO2 levels would rise, so the advice would be to crack open a couple of windows while the power is out, one downstairs and one upstairs (or one on each end of a single story house) cracked open about an inch would provide adequate airflow without letting in too much cold air.

Or you can set up a bank of batteries that get charged from the solar system and can be used to run the ventilation system during a power outage.

Here is another answer from Ryan Shanahan, Green Building Consultant with Earth Advantage in Portland, Oregon:

This is a great question. The short answer is yes: a client in a very airtight home should be concerned with air quality in the event that a power failure prevents the mechanical ventilation system from operating.

Luckily, this can be easily solved by slightly opening windows. I would suggest opening windows at the highest and lowest points of the home to utilize the natural buoyancy of warm air to rise to provide the distribution of this fresh air through the home. However, if the heating system is also electric, the clients will most likely want to keep windows closed to retain the heat energy inside, but I would sleep with bedroom doors open to prevent carbon dioxide from building up overnight.

Do you have a question on any aspect of Zero Energy Homes that you would like to submit to us? Leave a comment below or send us an email.