Bathroom vent design
The upper floor of my house has a cathedral ceiling constructed of 2.4″x6″ cedar planks. I’m putting a new roof on it this week with foam board under the metal so it will finally be insulated. One detail in the project is how to replace the bathroom fan. Currently, it is an exterior unit that draws through a 6″ duct. It draws plenty of air, but also leaks heated air. I’m planning to get a new rooftop unit so that I don’t have to build anything inside:
So, my questions are:
1) Is there some kind of (passive preferably) damper that could be put in to reduce air loss?
I’m thinking of something like this, https://foursevenfive.com/product/lunos-ald-r160/, except oriented for exhaust, but I suppose an active damper could be tied into the switch as well. Which would you recommend?
2) Since the bathroom has a cathedral ceiling, I plan to stick with an external fan ( Fantech RE 54 or 6: http://catalogue2.systemair.com/FileHandler.axd?id=43464) What size would you recommend? I did a sizing calculation (LxWxH/8=8x9x8) and came up with 72 cfm, which is probably less than what I have now. I can get 116 cfm at zero static pressure with the 5″ duct-size fan. Or I can get the 6″ fan to stay with something like what I have now. Is the larger fan overkill? Can’t most motors be operated with variable speed control?
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