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Should bedroom air return be _outside_ the bedroom for air circulation?

jonathanb | Posted in Mechanicals on

It’s common to put air returns inside the bedroom, even if the bedroom is its own zone. Would it be better for air quality to put the return outside the room?

Possible reason: in a bedroom, if the bedroom is its own heating/cooling zone, the house is reasonably air-tight, and people sleep with the door closed, CO2 levels can rise by a lot.

My new toy is an AirThings View, an indoor air quality measuring device. What I’ve learned is that CO2 levels rise from 750 to 1400 or more overnight with the door closed. Also, the humidity is a bit higher, partly from us breathing. If we moved the air return to the hall outside the bedroom, we’d get more circulation.

Wdyt?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Air returns should ideally be placed in each room that has supply registers, and the returns should be roughtly matched to the supplied airflow to the room they serve. You want BALANCE in these systems. If the airflow isn't balanced between supply and return, you can have annoying issues like doors blowing shut against their frames when the HVAC system comes on, and/or dust getting sucked into things like receptacles and light switches.

    Try to balance the system as best you can. If you absolutely have to have a return outside of a room, consider putting in transfer ducts, or at least cut the bottom of the door extra high to provide a pathway for air movement.

    I actually think you'd be better off with the system balanced, where you'd be less likely to have stagnant areas of air that aren't passing through the central unit's air filter.

    Bill

  2. jonathanb | | #2

    That's definitely the ideal, as I understand it! Each zone is balanced with supplies and returns. If the building enclosure is tight enough, install an HRV or ERV to get fresh air. If the humidity is too high, run a dehumidifier.

    In practice, there are tradeoffs. We have a house with 3 HVAC zones. The primary bedroom is its own zone. It's 350 sq ft. With two adults and a dog, CO2 levels rise to 1400 or 1500 overnight. With an ensuite bathroom, humidity ranges between 65-70%, even with an exhaust fan. The rest of the house hovers mostly in the 58-60% range.

    I've attached an image from the Airthings app. You can see how the CO2 level rises at night. The one night it doesn't was when we tested running the bathroom exhaust fan all night. The humidity drops when the AC kicks on, then rises again quickly after.

    For the CO2, it's probably no big deal to sleep with those levels, even if it wouldn't be great to breathe them all day. The humidity is probably a bigger issue. We aren't having mold problems, but we are allergic to dust mites, and it does sometimes feel stuffy, even with a fan to circulate air.

    A few ideas so far:
    1. sleep with the door open (vetoed by spouse who feels more secure with a locked door; not a solution for humidity)
    2. sleep with the window open (fine much of the year here in Zone 3C, but not all, and not a solution for humidity)
    3. run the bathroom exhaust fan overnight (exhaust-only ventilation; solves CO2, reduces humidity but doesn't fully solve it; the fan we have is a little loud, but we could upgrade to a quieter one)
    4. install an HRV/ERV in the bedroom (expensive, great for CO2, not a solution for humidity)
    5. run a standalone dehumidifier (fine during the day, too loud to run at night; no help for CO2)
    6. install a whole house dehumidifier (like the Ultra Aire recommended by many people on this site) and make sure there's a return or supply or both in the bedroom (expensive, but solves all issues?)
    7. install a dehumidifer AND an HRV/ERV in the bedroom only (solves all issues, matches the best practices for building science, and by far the most expensive solution)
    8. install a dehumidifer AND maybe HRV/ERV for the whole house (not sure how that works if the HVAC system has multiple zones)
    9. move the return to the hallway outside the bedroom (solves CO2, reduces humidity but doesn't fully fix it)
    10. install some kind of air transfer fan + vent between the bedroom and the rest of the house (could solve CO2 and partly help with humidity)

    Anything I'm missing there?

    Comparing the options a little more:

    On a budget, it sure seems like a standalone dehumidifier (option 5) is worth trying.

    Moving the air return (option 9) isn't too expensive, and might get the bedroom humidity down to 58-60%, like the rest of the house. That's at the very upper limit of what's considering acceptable.

    Following best practices with a dehumidifier or HRV/ERV starts to get really expensive, but it's the recommended solution for a reason.

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