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For a house using an HRV, is there a good plan for ventilating both bath and toilet room (separated)?

bobt14 | Posted in Mechanicals on

we are building a single story super tight house with 2.5 bathrooms, each toilet is in its own room, showers each have very high ceilings. we want to ventilate the toilet rooms, and also the shower area. do i have to pick one to be “passive”, or can i have both controlled (switched)? If we put the exhaust fan in only the toilet room, then the moisture from the bath will only be removed by the HRV. conversely, if we put the exhaust in the bath, then the toilet room is not “actively” exhausted when it really needs it!

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  1. GBA Editor
    Martin Holladay | | #1

    There is some controversy on this issue. Some experts advise that you don't need separate exhaust fans for bathrooms that are served by an HRV. Others note that the exhaust airflow rate provided by an HRV is less than most exhaust fans (or code requirements) and that a separate exhaust fan is therefore required.

    Most homeowners who choose to depend on the HRV to exhaust moisture and odors from their bathrooms are perfectly satisfied. Here's how the system works: the HRV pulls exhaust air from all of the bathrooms (or, in your case, the bathrooms and toilet rooms). The HRV operates continuously (or in some cases, 20 or 30 or 40 minutes every hour). This continuous air flow is enough to pull odors and moisture from these rooms -- even if, in some cases, it takes a little longer to clear the odors and moisture.

    Most HRVs include controls that allow homeowners to increase the fan speed for a certain amount of time by operating a switch in the bathroom. You can install such a "booster" switch in more than one bathroom if you want.

    Needless to say, if you disagree with my advice, you can install as many independent exhaust fans as you want -- one in each bathroom, and one in each toilet room, if that is your preference. However, there is an energy penalty to taking this approach.

  2. user-1072251 | | #2

    we've used HRVs with booster switches for exhaust ventilation in several homes with zero complaints.

  3. bobt14 | | #3

    thanks for the quick responses, i like the idea of a booster switch and one system running everything. regarding placement of the intake ducts: can they be split and go to both the small toilet room and bath on the other side of the wall? how does all this get balanced (flow rates thru each duct)?

  4. GBA Editor
    Martin Holladay | | #4

    An HRV system can have as many exhaust grilles and exhaust ducts as you want. The total airflow rate is determined by the blower motor and the HRV specs. The airflow through each exhaust grille is determined by the duct design (duct diameter is largest near the HRV and smaller as the ducts branch out) and is fine-tuned with balancing dampers during the commissioning process.

    That said, the airflow rates are so small that it will be difficult to get pinpoint precision with your exhaust airflow rates. If you are exhausting 80 cfm from 6 locations, the average airflow rate through each grille will be only 13 cfm.

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