Humidstat for bathroom fan: does it go inside or outside the bathroom?


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Old 07-26-12, 12:50 PM
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Humidstat for bathroom fan: does it go inside or outside the bathroom?

I have a baseboard-heated house.

I'm installing a better ceiling extractor fan in the downstairs bathroom.

The fan will have a multi-option timer switch control inside the bathroom, in parallel with a humidistat.

Why do some people say to put the humidstat outside the bathroom?

Should it go outside or inside?

Thank you!
 
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Old 07-26-12, 01:19 PM
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I've never heard of putting it outside the bath. How would that register the true humidity inside the room you are trying to evacuate? Many fans have one built in.
 
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Old 07-26-12, 01:27 PM
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That's true that you can get fans with them built in. Two contractors insist that humidstats never go inside the bathroom, as they tend never to turn off then.
I'd like to track down more reasoning on this, and if it's old lore that is no longer appropriate, what has changed.
 
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Old 08-11-12, 06:17 AM
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The humidistat always goes in the area to be controlled. To do otherwise would be like installing your home thermostat in your unheated garage. While it is true that the fan will run more with the humidistat in the room, it is BECAUSE the humidity is still too high. Wet towels, floor rugs, shower walls, wash cloths, etc take a long time (like overnight) to air dry in a small room. If excess humidity is an issue in your bathroom, you might want to consider removing the wet items to a laundry room or other drying area. Running a 100cfm exhaust fan overnight to dry towels means exhausting 144,000 cubic feet of conditioned air from your home. This means your home has to draw in the same 144,000 cubic feet of outside air to replace the exhausted air and your HVAC system now has to cool/heat/dehumidify that large volume of non conditioned air. This is ten complete air exchanges for a 1800 square foot home and is a costly way to dry towels.
 
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Old 08-11-12, 06:45 AM
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Thanks for stating the "obvious", with reasoning. That's what I want to hear. So maybe someone can say something about when/where people use exhaust fans in the bathrooms to control humidity in a whole section of the house? This is what most of the contractors who visited had in mind. I suppose in my case I would have liked to have a humidistat both inside and outside (set differently) the bathroom, in parallel, since I'm a bit concerned about both the local and general humidity (it's Vancouver, BC!).
Well, in my naive state, really, I would like to have an occupancy sensor, inside humidistat, inside timer/control, and outside humidistat all in parallel.
 
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Old 08-15-12, 06:06 AM
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CeeBeeLee,
I think you may be making the assumption that an exhaust fan should be used for humidity control,...this is not the case. It seems your main concern is humidity control and exhaust fans will do almost nothing for humidity control, they are just air movers. The exhaust fan in the bathroom is basically an air mover designed to move unpleasant odors from a small area. The simplest answer to your dilemma is to run a decent size dehumidifier in the area of most concern. This removes airborn moisture thus controlling humidity. You then can, if needed, turn on the forced air furnace blower to mix the dehumidified air throughout the entire home. If you want to get fancy, you can add a humidistat to your HVAC system fan and it will automatically cycle on and off according to the humidity setting you select. The dehumidifier has it's own humidistat which will cycle the machine on and off as needed. This will work best with the dehumidifier located in a large room so as to have a large supply of room air to continually work with. The only scenario where an exhaust fan could help with indoor humidity is if you live in the desert where the outdoor humidity is extremely low. Exhausting the moist indoor air, to be replaced with 14% RH outdoor air will surely control humidity. In your case however, the outdoor air is probably a higher RH than your indoor air and thus the incoming air will actually raise your indoor RH running an exhaust fan. Remember with any exhaust fan "what goes out (in air volume), must come in (from outside air) in the same volume." Hope this helps.
 
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Old 08-15-12, 07:21 AM
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podunk: Another kind reply. Thank you!
Yes, I have used a dehumidifier, though it hasn't seemed generally necessary. My house has no central heating; see my OP. I rather thought there was a humidity gradient from top to bottom of the house (especially when the weather has changed and it's sunny outside) and the discussion is all about the basement. However, I am largely trying to figure out why contractors are so intent on putting the humidistat outside the bathroom, and maybe, from reading your post, the answer is that they are inappropriately transferring lore from drier climates.
Pulling in higher RH air from outside could still reduce RH humidity inside if it's warmer inside. Hmm, yes, it would be nice if all the HVAC functions were integrated in my house, but they're not...
 
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Old 08-18-12, 03:44 AM
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CeeBeeLee,
Sorry, missed the baseboard heat thing. Unfortunately the lack of air moving ability will allow different areas to build different moisture levels and stagnate there. My next best guess for your situation would be to use the dessicant dehumidification technique. This is accomplished by placing calcium chloride collectors in the areas of concern and dumping the collected moisture from time to time. That is the most economical treatment. If you don't mind getting a bit more spendy, you could install an attic blower fan system with ducts to all rooms just as an air mover. This would keep all areas at about the same humidity level and would allow you to get much better benefit from a dehumidifier. It would also eliminate stuffy feeling rooms and better equalize temperature throughout the home. The baseboard heat is nice and quiet but it's main drawback is the inablilty to process or move air.
 
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Old 08-18-12, 06:26 AM
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podunk, thank you for sharing your expertise!!
 
 

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