Ventilation calculation consternation
benrush
| Posted in Mechanicals on
Are the following ventilation calculations correct?
Per (2013) ASHRAE 62.2 we need 3 cfm per 100 square feet plus 3.5 cfm per person.
So, a 3000 sf home with 5 occupants requires 127.5 cfm.
That’s 7,650 cf per hour…
or 183,600 cf per day.
If the 3000 sf home has 8′ ceilings, the total volume is 24,000 cf.
183,600 / 24,000 = 7.65
So, we’re supposed to ventilate enough to change all the air 7.65 times per day?
That seems like a lot of air to heat/ cool/ humidify/ dehumidify. It’s hard to comprehend that this would be an improvement over my old, drafty, poorly insulated house. What am I missing?
Thanks,
Ben
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Ben,
1. Your calculations are correct.
2. Before ASHRAE 62.2 was developed, ASHRAE's longstanding recommendation for ventilation was 0.35 air changes per hour (equivalent to 8.4 air changes per day), so your calculation of 7.65 air changes per day sounds about right -- in line with longstanding recommendations and rules of thumb.
3. Some energy experts think that the recommended ASHRAE 62.2 ventilation rate is too high. While "best practice" recommendations call for homes to have equipment capable of ventilating at the ASHRAE 62.2 rate, nothing stops the homeowner from ventilating at a lower rate if that's what the homeowner wants to do. Note, however, that there is increasing evidence that the ASHRAE 62.2 rate is good for health and mental alertness.
4. Don't worry, ventilating at the ASHRAE 62.2 rate won't break the bank -- and it's a lot better than living in a leaky old house. The rate is controlled -- something you can't say about an old leaky house, which depends on wind and temperature differences to determine the ventilation rate.
5. This controversy is an old one. If you want to read more, here are some links to relevant articles:
Ventilation Rates and Human Health
How Much Fresh Air Does Your Home Need?
Is Your Ventilation System Working?
A moderately insulated house might take 24 hours to cool to ambient temperature if you turned the heat off. That makes it sound like changing the air out for cold outside air 8 times a day would be 8 times worse than that. But in fact, the furniture, walls, floors, etc. have a lot more thermal mass than the air. So you aren't really starting over from scratch 8 times a day.
But it can still help to use a heat exchanger (HRV or ERV) to cut the energy use associated with the 8 air changes by a factor of 10 or so.