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Bathroom moisture control in HRV defrost mode

Bill_L | Posted in General Questions on

Planning new residential construction in moist and winter cold zone 5A.  Basement/walls/roof are R20/40/60, triple-pane, super-tight, well-drying, direct-vent, and mechanicals all in conditioned space.  I feel like I have covered most angles, and am looking for extra insurance against mold.
I’m planning for 2 mini-splits to handle 3000sf plus another mini-split for a 600sf in-law, and an HRV to not only provide exhaust/fresh air, but with the help of a few transfer grills, to also distribute heat & cooling throughout the fairly open layout.
Kitchen hood will have makeup air delivered right to the cooktop, with a damper that opens when the hood turns on.
I’m expecting ordinary, distributed humidity to be evacuated by the HRV in winter, condensed by the mini-splits in summer, and also buffered by the dense-packed cellulose, but am concerned about the heavy dosage of moisture from a series of hot showers.  I have gone back and forth with the idea of dedicated, exhaust-only bath fans, but I like the idea of balancing pressures with the HRV.  I am envisioning flow restrictors in all rooms that the HRV pulls from except the showers, which would instead have a intake like the American Aldes Zone Register Terminal ( that is wired to the boost switch to open completely and allow increased flow from that bathroom when the HRV is boosted.  If both showers are going at the same time then, yes, the flow pulled by the HRV will be even further divided.
Sorry for the long lead-up to this, but here is my primary concern – at the time when I would most want to eliminate concentrated moisture from the house, the HRV could go into defrost mode and dump it right back into the house.  Am I doomed to use a pre-heater with this approach?
thank you,

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  1. user-1137156 | | #1

    Another option. Install A Panasonic ERV bath fan in each bathroom With separate 4" vent and fresh air inlets to each. They units themselves are about $300. You could also use an ERV instead, they don't have defrost cycles but still move moisture. I've chosen a Renewaire ERV.

  2. Bill_L | | #2

    Thank you Jerry. I considered the Panasonic ERV bath fan, but couldn't get past the fact that it is an ERV. I will look into how much of the outgoing moisture would be coming back into the bathroom, and if insignificant, that could be just the ticket - thanks for spurring me on to consider this. I wonder why they chose an ERV over an HRV for a bath fan with which you would want to expel moisture, not reclaim it (even in summer, I would expect a steamy shower might have more relative humidity than outside air) - is it because ERVs are immune to the defrost problem?

  3. GBA Editor
    Martin Holladay | | #3

    If you are worried about defrost during cold weather, the Panasonic ERV isn't a solution. It doesn't work well in cold climates.

    I think you are overthinking the whole thing. Your bathrooms (and your indoor moisture level) are likely to be fine. It may take a few more minutes for all of the moisture in your bathroom to be exhausted, but the HRV runs all day, and will certainly handle the moisture generated by a few showers.

  4. Bill_L | | #4

    Thank you Martin. If I understand correctly, the idea is that although the moisture-laden air will be coming right back into the house in defrost/recirculate mode, it will be dissipated throughout the house and eventually evacuated during periods when defrost/recirculate mode is cycled off. If so, that makes sense to me, and is reassuring.

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