AC vent and exhaust fan in same bathroom?

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digdugger

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I live in a hot desert climate. My Trane roof heat pump is probably 30+ years old. My small house I bought has an AC vent in both bathrooms. One is normal size (not huge), and the other you have to get in and sit on the toilet before you can fit to close the door. The tiny bathroom has a diffuser type register that you can't close, with an old Broan exhaust fan 1 foot away from the AC register. Seems asinine as it will be stupidly be exhausting cooled air (along with any moisture) .

The other register vent in the other bathroom has cantilevers, but even when closed, leaks quite a bit of air. I bought two Delta exhaust fans from Costco I want to install. I want to plug both AC vents in both bathrooms. Each bathroom has one outside wall, but I never notice either bathroom getting too warm. I've read tips about using either plastic or cardboard to cover the vent, but fear when switching to heat mode in the winter, the prolonged blowing of heat on those materials could possibly start a fire. (If you know of any proof that say otherwise, please link to them-- I don't want to risk burning my house down, or run into trouble with a home inspection if I ever sell, although those materials could be easily removed.) I also read of sealing them with metal duct tape. I know that won't burn, but could be a real pain to peel off if I ever had to.

Do they make blank cover plates to take the place of the register? Although if they did, the tricky one would be finding one to replace the diffuser since there's no screw holes, it just snaps in.

My reasons for this question are I feel I need exhaust fans more than AC in the bathrooms, and also air flow is piss poor out of the living room and bedroom vents where we really need it. (as well, as I said it seems asinine to be exhausting cooled air) Can't afford a new unit yet.
 

Jadnashua

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When cooling, the denser cold air tends to fall into the room while the hot humid air rises, so while yes, there will be some exchange when they are close, it may not be as bad as you think. Plus, the exhaust fan probably only runs a small percentage of the time during the day. On forced air, it would be almost impossible to have the air start a fire...it never would get hot enough. Think of it this way...it isn't uncommon to use plastic lined, insulated ducts...the plastic doesn't melt or burn unless there's an external ignition source.

You might be able to install dampers in the ducts to help balance the flow, but that can be problematic, as the increased backpressure can cause less than perfect joints to then leak into unconditioned spaces. Properly sizing ductwork is more complicated than many think to get the needed air flow with the best operation. Much more common is the simple way, which can have problems like you're seeing.
 

digdugger

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Thanks Jim, I appreciate your scientific responses. I removed the humidity sensors from the fan as per vendor support instructions, as I plan on wiring the fan to the light switch. My thought being you want it to remove the stinky air when doing your business, not depending on a humidity detector which would probably only run after a shower. (It's arid here most of the year). About blocking off the vents, in my amateur thinking I guess I don't foresee any excess backpressure issue since already the air flow is piss poor in the other living spaces as is. I did caulk around the gaps of duct/drywall junctions. Guess you're right as far as fire risk sealing registers- I recall now the ducts in the attic laying on the joists are the white plastic coated wire loop kind. I have a giant roll of green U-Haul cling-wrap if that would be safe, only thing is guess it'd look like crap on the front of the register (if I wrapped up the entire register--it's only about an inch or so deep) Guess the metal duct tape may be best, just on the inside of the register.
 

Jadnashua

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A careful inspection of the ducts in the attic may be called for. When I moved into my place, one room didn't get conditioned well. After inspection in the attic, I found that they all had loose connections, and one of them had literally fallen out of the main duct and the condition air was just blowing into the attic! Also, the insulation on those round, flexible ducts isn't very great and your attic temps may be well over 130-degrees...not conducive to keeping that air cool. I put some additional insulation on top of mine.
 
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