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Community and Q&A

Indoor air quality in a ventillated NZE house when power goes out

Hobieda | Posted in Mechanicals on

We live in a NZE house of about 34795 ft3, and it has a ACH50 of 0.46 – the actual number was 269 cfm @ 50 pa. The Zehnder ventilation is set to 144 CFM. 4 bedrooms 3.5 baths.
If the power goes out, do we need to worry about suffocation? I would prefer not to open windows, to maintain the heat in the house for as long as possible. – the power goes out in the winter, usually. Has anyone done any studies on the indoor air quality in such a situation?

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  1. GBA Editor
    Martin Holladay | | #1

    If your power goes out, you and your family will not be in danger of suffocation. It's impossible to imagine that there wouldn't be enough oxygen in the house to keep you all alive.

    If the air in the house feels stuffy, crack a window. If the entering cold air bothers you, close the window. In other words, use common sense until the power comes back on. Once your power comes back on, using common sense will become unnecessary, because your Zehnder unit will resume operation.

  2. charlie_sullivan | | #2

    Martin's advice is right on.

    Just for fun, here's some analysis. You probably have 10 to 20 CFM of leakage from wind and stack effect. Wikipedia says people breath out 12 liters per minute during light activity, of which ~5% is CO2. So that's 0.02 CFM per person of CO2 flowing into the house. Steady state, with 10 CFM of leakage, you'd eventually reach 2000 PPM of CO2. With more occupants, it would be 2000 PPM/person, for example 10,000 PPM (1%) with five occupants.

    Is that bad? Some studies indicate you might have a mild decrease in performance at 2000, and mild drowsiness at 10,000, but you wouldn't be likely to have serious problems until you reach 50,000 or more.

    So with five people you'd have mildly unhealthy CO2 levels in steady state, but nothing dangerous. Furthermore, it would take about 2 days for the CO2 levels to build up to that point.

    Bottom line: What Martin said. Except maybe the part about common sense being unnecessary when the power is on.

  3. Expert Member

    "Except maybe the part about common sense being unnecessary when the power is on"

    Charlie. I hope you are wrong. Surely the whole point of building these machines for living in is that we can forgo common sense and live unimpeded by the vagaries of climate and necessity - as tourists in our homes immune from the need to intervene whatever happens.

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