This is a followup on a question by Jim Blodgett on 7/5/2009 titled Bathroom/Laundry Ventilation through HRV. If I understood the responses to that question, a HRV can exhaust a full bath without separate ventilation through the roof or wall. There was some reference to boosting the exhaust into the HRV with a fan while showering. This in addition to the boost to maximum of the entire HRV by a wall control.
First some background. Our new tight home will be served by an HRV with maximum 200 cfm. Four exhaust ducts will branch off the HRV main trunk. The duct to the bathroom containing 1 toilet, 1 bathtub and 1 shower should max out at 200/4 or 50 cfm (disregarding the loss due to friction through the duct). If I understand HVI’s bathroom ventilation guidelines, our bath of 110 sf should exhaust at 150 cfm or at a minimum continuous ventilation rate of 20 cfm.
My question, is either the 50 cfm max. rate through the HRV or the 20 cfm continuous rate sufficient to remove enough vapor from the bath during and after showering? If not, what are some solutions that don’t involve a dedicated fan to the outside? Those specifying a separate fan ducted to the HRV please be specific how a fan of greater capacity can override the HRV. Those that do think I need a dedicated fan to the exterior please give specific products that exhaust well while not leaking during nonuse.
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You have two choices: stick with the exhaust through the HRV -- that's what I recommend -- or include both an HRV exhaust grille and a separately ducted bath exhaust fan in each bathroom. While I don't recommend the latter route, some people advocate it.
In no case do you want to hook up a bathroom exhaust fan with your HRV ducting. The HRV exhaust duct should connect to a ceiling grille, not a bathroom exhaust fan.
These days, I see far more examples of overventilation than underventilation -- at least among those building energy-efficient homes.