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In the condo (2 story semi-detached townhouse built in 2004) I rent, the main bathroom has an interesting vent system.

When turned on, 2 fans are activated. One is a booster of some type and increases the air flow from the floor HVAC register. The second one acts as exhaust, pulling air out through a similar register mounted up on the wall. This second register is connected to a rectangular sheet metal duct that goes straight down from the register and appears to actually be an air return.

Is this sort of arrangement actually allowed by code (specifically, NBC Canada 1995 would have been current when the place was built.)? What are the potential problems (mold?) with such an arrangement beyond code compliance?

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Can you post pictures? That is by far the strangest set-up I've heard of for a bathroom vent. Bathrooms have vent fans to remove humid and sometimes smelly air from the living space, so the air should not be exhausted into the HVAC return. –  Evan Johnson Apr 25 '13 at 15:20
    
@EvanJohnson - What sort of pictures are you wanting? –  Compro01 Apr 25 '13 at 15:28
    
Pics showing where the exhaust vent goes. I suspect it leads to the outdoors and not your air handler. –  Evan Johnson Apr 25 '13 at 18:17
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up vote 3 down vote accepted

This is an acceptable solution for multi-tenant buildings where the exhaust fans from multiple units vent in to a common duct. The booster fan in the floor vent makes sure that the air pressure in the room being vented is higher than the common duct. The booster fan can pull air through the central AC/hear air handler even if it's not running. This lightens the load on the exhaust fan.

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