Heat pump water heater as a shower exhaust
I’m putting in two bathrooms in my basement. One of them will be for a small apartment we are planning to rent out. My current plan is to put in an ERV as the bathroom exhaust which will be 70CFM for both bathrooms and a laundry room. I have a Rheem heat pump water heat in the laundry room, adjacent to the bathrooms. after putting an anemometer i found it moves about 300 CFM. My thought is that the HPWH will act as an automatic supplemental bathroom exhaust and presumably recapture some heat. The HPWH will not be ducted to the exterior but the laundry room where it sits will have a return for the ERV if the coil doesn’t catch all the moisture. Would there be any downside to putting the supply air for the HPWH over the showers as supplemental exhaust?
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Someone asked a somewhat similar question the other day. The response was along the line of "how do you synchronize the HPWH and the other device?" That is, the HPWH will run when it needs to heat water. The (in this case) bath fan will run when/if the occupant turns the switch.
On using the ERV for dehumidification, be cautious. I tried to use a Panasonic ERV to ventilate a bath and it quickly failed. Moisture accumulated on the unit's circuit board and caused a short that required soldering in a new fuse.
At least with a shower there's a reasonable expectation that the water heater would be running when the shower was running, or for a few minutes after the shower ends.
For a while heat recovery drains for showers were trendy, where the cold water pipe for the shower would coil around the drainpipe to recover some of the heat of the water going down the drain. They don't seem to have caught on and last I checked they seemed barely available.