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Old 03-29-2011, 10:05 AM   #1
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Attic fan with both thermostat and humidistat


Not really an HVAC question but close enough

I have a new MasterFlow attic fan, but they no longer come with the combined humidistat/thermostat control for some reason. It came only with the thermostat. I could purchase a separate dehumidistat (or combined hum/dehum) switch.

How should I properly wiring a dehumidistat switch in parallel with the existing thermostat switch? These are line (120v), not low voltage. Would there additionally be a way to add a manual on/off switch?

Thanks,
Dan

Last edited by dhcernese; 03-29-2011 at 10:07 AM.
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Old 03-29-2011, 10:09 AM   #2
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Attic fan with both thermostat and humidistat


Hello and welcome dhcernese, to the best darn DIY'r site on the web.

I am sure the fan is no longer available with the humidistat switch as it has been determined that this is not really necessary, at least this would be my guess to why it is no longer offered.

Fan driven by temperature alone would do the job.

Mark
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Old 03-29-2011, 10:55 AM   #3
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Actually no. Speaking to GAF, the local sales rep claims the humidistat was removed because customers in the South had it set so that it ran so often the motor would fail within the warranty period.. lame reason.

Anyway, I absolutely require an exhaust fan in my attic during the winter because I just spent > $10K on mold remediation in the Northern walls. The primary driver is to remove humidity.

My fall back is to completely replace the thermostat but I'd like to keep it to save on A/C costs in the summer.

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Dan
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Old 03-29-2011, 11:30 AM   #4
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Attic fan with both thermostat and humidistat


As you refer to, "Customers in the south" I take it you are in the north?

Adding a fan to remove damp air from an attic in the winter time should not be the final fix. You need to air seal all penetrations to the attic, ensure bathroom vents to the outside.

You also need to ensure that the soffit vents are open and funtional, as well as either a ridge vent system or mushroom vents, the cause of the mold has got to be air escaping from the conditioned space into the attic, find the escaping air and ensure proper venting and your problem will be fixed, adding a fan is not the fix.

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Old 03-30-2011, 07:47 AM   #5
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Attic fan with both thermostat and humidistat


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As you refer to, "Customers in the south" I take it you are in the north?

Adding a fan to remove damp air from an attic in the winter time should not be the final fix. You need to air seal all penetrations to the attic, ensure bathroom vents to the outside.

You also need to ensure that the soffit vents are open and funtional, as well as either a ridge vent system or mushroom vents, the cause of the mold has got to be air escaping from the conditioned space into the attic, find the escaping air and ensure proper venting and your problem will be fixed, adding a fan is not the fix.

Mark
Thank you for your advice, yes we're in New England. I'm not going into all the conditions and work that was done is part of the remediation. It included the soffit vent work (they are now as physically large as possible), new proper channels and in fact the mold removal warranty requires that we close the ridge vent if we have the attic fan. We have the fan after consulting a ventilation specialist. His opinion is that the fan would move more air in our cape-style structure than passive air movement would ever achieve. We discussed all the combinations of new ridge vent, gable vents, etc.

None of the people we've had inspecting before during or after identified any obvious route for bathroom or other moisture.
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Old 03-31-2011, 01:26 PM   #6
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Attic fan with both thermostat and humidistat


I have same desire to have both a thermostat and humidistat control for attic fans. I used to rewire/jump seasonally, temp for summer, humidity for winter.

I got sick of it so I wired a switch box into upstairs hallway with a 24 hour Intermatic timer switch. It takes seasonal resetting by logic but now I don't have to rewire all the time. I have 2 roof fans being controlled. The plastic box that forms the contained switch back is installed in a metal electrical box with no other devices on the same 20 amp circuit. It gets a little warm but no more so than a dimmer switch in dining room.
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Old 03-31-2011, 01:29 PM   #7
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I have same desire to have both a thermostat and humidistat control for attic fans. I used to rewire/jump seasonally, temp for summer, humidity for winter.
There's this https://www.greenair.com/browse/ct-dh-series.html but it has plug sockets not direct wiring.

I already have a thermostat and a Johnson control W43A, I want to design proper wiring for them (and if I need a relay I'll figure that out too)
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Old 01-18-2012, 07:25 PM   #8
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Attic fan with both thermostat and humidistat


Hi guys, (from Pittsburgh, PA ...northern climate)

I've been retrofitting homes with high performing insulation solutions for several years now. I'm BPI and RESNET credentialed so have learned to analyze homes as systems. I've seen many moldy attics, and most of the time the culprit has been one or several bathroom fans vented into the attic rather than outdoors.

However, during the past year I saw a number of attics that had very damp and moldy roof decking---but properly vented bathroom fans. In a recent case we air-sealed the attic deck with foam, confirmed the bathroom fans had been vented correctly, and then tested the tightness of the attic deck with a blower door---and it was tight. There was little if any moisture coming from inside the home, yet the roof decking was still saturated. It had to be coming from outside. When we checked the shingles they were spider-webbed with deep cracks all over them.

I also worked in another attic this past summer with a major mold issue. The bathroom fans were not the issue. The homeowner (a single, elderly male) had the roof inspected and it was found that the original roofer had not used felt paper and that the shingles were themselves in bad shape.

Both these roofs were relatively low sloping and had obvious shingle wear / lack of felt paper. Both homes were inhabited by 1-2 people--ie not a family of 7 taking 12 showers a day and thus putting lots of moisture into the air. The point is that even though the majority of my experiences have indicated that the moisture in the attic was coming from inside the home, that in these cases it was actually seeping in through the shingles. The worst areas were also the north facing sections of roof. So if you've accounted for the obvious moisture sources coming from inside the home, check the condition of the roof.
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Old 03-15-2012, 01:12 PM   #9
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Attic fan with both thermostat and humidistat


I have some of the same issues mentioned above. Moldy roof decking. Bathroom fans seem to be fine but they do vent through the soffit and I don't like that. Have been plugging penetrations as I find them. I just put a new 50 year roof on 5 years ago. Before that the original roof was laid on the house (built in 93) with no felt. There was a little mold then but has recently exploded. I'm about to spend 11K to remediate. I did add soffit vents this past summer and had ridge vents put in when the roof was done but its a hip roof no not a lot of linear footage. Also have 3 passive vents in the roof facing the back of the house. I was going to add a fan with thermostat and humidistat. Don't know what welse I can possibly do!!!!!!
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Old 02-27-2014, 06:23 PM   #10
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Attic fan with both thermostat and humidistat


Hi, first post here. I know this is an old thread but I was wondering if the OP or others who posted found solutions to the issues of the roof deck moisture, temp/humidity, etc. I may have some answers if anyone's interested in continuing the discussion.

Chewy
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Old 02-28-2014, 08:29 AM   #11
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Attic fan with both thermostat and humidistat


figure I'd update for everyone. Had the remediation done. All the old insulation was removed from the attic. The whole attic was air sealed, I mean every nook and cranny. The vented the bathroom fans through the roof. I went around and made sure all the AC ductwork was sealed to prevent any air leakage from the registers (and wasting AC cooling the attic in the summer). I installed soffit vents all around and put in an attic fan (with thermostat and humidistat) where one of the passive vents(I have 3) were. Blew in new insulation. Added a wireless temperature and humidity meter to monitor the conditions in the attic. In the summer the humidity was like 15% so I thought I was good. However this winter the humidity was never below 90% and when I go up there there is condensation on the sheathing and the insulation feels damp. The humidity in the house in the winter is like 45-50% and outside it's in the 20% on clear dry cold days. How the hell can the humidity be 90% up there. Honestly I have just given up. don't know what else to do. By the way I went around after they blew in the insulation to make sure all the soffit vents were clear. I've been to other forums and they have filled my ear with relative vs absolute humidity which is all nice high school science but bottom line is that I still have moisture in my attic and I can't figure out where the it is coming from!
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Old 02-28-2014, 08:46 AM   #12
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Attic fan with both thermostat and humidistat


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figure I'd update for everyone. Had the remediation done. All the old insulation was removed from the attic. The whole attic was air sealed, I mean every nook and cranny. The vented the bathroom fans through the roof. I went around and made sure all the AC ductwork was sealed to prevent any air leakage from the registers (and wasting AC cooling the attic in the summer). I installed soffit vents all around and put in an attic fan (with thermostat and humidistat) where one of the passive vents(I have 3) were. Blew in new insulation. Added a wireless temperature and humidity meter to monitor the conditions in the attic. In the summer the humidity was like 15% so I thought I was good. However this winter the humidity was never below 90% and when I go up there there is condensation on the sheathing and the insulation feels damp. The humidity in the house in the winter is like 45-50% and outside it's in the 20% on clear dry cold days. How the hell can the humidity be 90% up there. Honestly I have just given up. don't know what else to do. By the way I went around after they blew in the insulation to make sure all the soffit vents were clear. I've been to other forums and they have filled my ear with relative vs absolute humidity which is all nice high school science but bottom line is that I still have moisture in my attic and I can't figure out where the it is coming from!
I take it you have a humidifier? If you do have a humidifier shut it down and continue to monitor the RH in the attic.
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Old 02-28-2014, 08:47 AM   #13
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Nope no humidifier. I was actually desperate enough to think about putting a dehumidifier in the attic
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Old 02-28-2014, 08:58 AM   #14
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Nope no humidifier. I was actually desperate enough to think about putting a dehumidifier in the attic
Where do you live that may help determine some relative information related to ambient conditions.
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Old 02-28-2014, 08:58 AM   #15
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Boston area
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