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Question: I am building a new home and would like to install a 600 CFM range hood fan. My builder tells me they use a passive HRV system in the new home and that "the system balances itself and will not allow a negative pressure system to develop". My concern is whether a passive HRV system will be safe without make-up air, with the hood fan operating at full speed while a gas burning fireplace is operating. I basically want to make sure that such a system will not require an additional make-up air unit.
Answer: First, here is some useful background information on HRVs. A passive HRV does not have its own fans and therefore uses the fan of a forced air system (furnace or air handler). The return duct of a system is under negative pressure and this can be used to draw air from outdoors, through the HRV and into the return of the system. The supply side duct contains positive air pressure and a portion of this air is allowed into the HRV and exhausts to the outdoors.
The passive HRV will only provide ventilation when the furnace or air handler fan is "on" and the ventilation rate will also change when the fan speed changes. Typically the system will be balanced by using dampers in the duct, adjusted to exhaust the same amount of air that is brought into the home. In this way the passive HRV will not affect any other appliances in the home such as the fireplace.
The range hood is a simple exhaust device that works by creating suction (negative pressure) over the range top, and exhausting air is drawn into it. Quite often the make-up air is drawn though the many small holes in the building envelope such as spaces around exterior doors. Because your kitchen exhaust equipment is approaching the size used in commercial kitchens, you need to pay special attention to make-up air. The hood could operate while the fireplace is operating and this could lead to back drafting issues. Your local building code may provide guidance. For example, more and more municipalities are requiring pressure testing when gas burning appliances rely on air from in the home to be exhausted through a chimney. Where there is doubt the hood should be interlocked with a make-up air system.
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