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When do you need to add mechanical ventilation?

user-716704 | Posted in Green Building Techniques on

When tightening a house, at what point do you need to add mechanical ventilation?

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  1. iLikeDirt | | #1

    Get a CO2 meter. Anytime it reads more than 650 PPM of CO2, you need some mechanical ventilation.

  2. exeric | | #2

    I would make a totally wild ass guess that any house with lower than an ACH 50 equaling 5 on a blower door test probably needs mechanical ventilation of some kind. I expect in the very near future mechanical ventilation of all kinds will be tied to a CO2 meter. I would think this could be especially effective with bathroom fan ventilation used for house ventilation. Those kinds of fans always require makeup air from outdoors to bring in that low CO2 air. That also brings in cool air and is not very efficient in cold climates. If you tied the running of a bathroom fan to a CO2 monitor you would minimize bringing in cool air when it's not needed.

  3. iLikeDirt | | #3

    My house (1972 vintage) blows 6.8 ACH50 and CO2 rises to 1050 PPM while inhabited by three people with all the windows shut. Turning on the bath fan with the windows cracked 1/4" causes it to equalize at about 600 PPM. As soon as I replace the bath fan with one that uses only 5 watts, I'm gonna say that's good enough and call it quits. A complicated ducted HRV system for thousands of dollars and ten times the electricity consumption or more seems like a ludicrous idea in my climate (5B).

  4. exeric | | #4

    Nate, yeah, that will work and is probably what I'd do. As one moves to a tighter house or to a colder climate then cracking the windows would negate the whole idea of having a tight house. At some point if one goes for a bathroom fan it makes sense to have a makeup air damper and a CO2 monitor switch operating it in those situations. Also, If you go to Panasonics' bath fan website you'll find in the small print that those particular fans can use up to 2 or 3 times the rated ventilation power if it has to overcome a very restrictive makeup air source. So in a tight house with a makeup damper of some sort you can run the fan for shorter times and have the fans using less electricity while operating it, or some combination of those two things.

  5. Expert Member

    That's interesting about the power consumption of the fans. Thanks.

  6. GBA Editor
    Martin Holladay | | #6

    As Nate pointed out, a mechanical ventilation system isn't necessarily complicated. It can be as simple as a high-quality bathroom exhaust fan controlled by a timer. (Timers for this purpose fit into an ordinary wall-mounted electrical box -- the electrical box that now has a switch.)

    For more information on ventilation systems, see Designing a Good Ventilation System.

    If you want, you can follow Nate's advice and buy a CO2 meter. Or you can use common sense. My usual advice: the two most common signs that you need a ventilation system are condensation on the interior of your windows during cold weather, or the existence of bothersome lingering odors. If you don't have these symptoms, everything may be OK.

  7. davidmeiland | | #7

    Most people aren't going to buy a CO2 meter (or maintain it in calibration), but you can easily pick up a thermo-hygrometer and keep an eye on the relative humidity in your house.

    Using your range hood and bath fans almost always works fine. Most people do not run the bath fan long enough, usually because (a) it's noisy and (b) it has a manual switch and they feel compelled to turn it off when they leave the room. Until the towels and the shower enclosure are completely dry, there is moisture evaporating into the air, and if it isn't vented it lingers in the house and raises the humidity level.

    In addition to a quiet bath fan and a long-run timer, I installed an interesting piece of ventilation equipment in our main bathroom, called a "window".

  8. DIYJester | | #8

    If you're a tinkerer you can easily build a CO2 meter using an Arduino for under $100. I am programming one to control all of my hydronic zones, temperature sensors in the rooms, and eventually ventilation based on actual CO2.

  9. davidmeiland | | #9

    I misplaced the photo I took, but I was in an REI store recently, waiting for my daughter to pick out a coat, and started noticing the HVAC controls. The place was well-outfitted with temperature, CO2, and RH sensors installed on structural columns throughout the space, just above the height where you could accidentally bump into them. We could easily be doing similar in houses, but few care enough and even fewer would pay for it.

  10. charlie_sullivan | | #10

    And if you aren't a tinkerer, you can still buy a CO2 meter for under $100 at Amazon. There's one by "autopilot" for "$97.49 & FREE Shipping" that looks cheap but has good reviews, or a better looking one from "CO2Meter", both with temperature and humidity as well. Controllers based on CO2 appear to start at $400.

    There are also some more comprehensive air quality meters for around $200. One from Coasia seems to include everything: CO2, VOCs, PM2.5, as well as temperature and humidity. Foobot and Koto are two that sense most of those but confusingly report VOCs in terms of CO2 equivalent, and don't seem to actually sense CO2.

  11. davidmeiland | | #11

    I would have questions about the accuracy of low-priced CO2 meters, because I've had bad luck with cheap thermo-hygrometers that gave very inaccurate readings compared to a professional tool. I went with a Fluke meter that can easily serviced and certified by the manufacturer.

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