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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8512
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I want to find a humidity control switch that has dual sensors

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I want to find a humidity control switch that has dual sensors (one for outside and one for inside) to control attic and crawl space exhaust fans. I simply want the control switch to turn an exhaust fan on only when the outside humidity is lower than the inside humidity and if the inside humidity is higher than 55-60% or so. I cannot find a control switch with dual sensors that will sense the humidity in two zones and use those two readings to turn a switch on or off when one zone has a higher humidity than the other does. I live in eastern NC deep within the Croatan National Forest, a sub-tropical rain forest. We have outside humidity that varies greatly throughout the entire year. Standard dual thermostat/humidistat controlled exhaust fans do not keep the humidity in my attic and crawl spaces low enough. It doesn’t help to set the humidistat to exhaust when the humidity reaches 55% or 60% if the humidity outside is 75%. This is a situation that occurs so often that I’m bringing in more humidity during the prolonged rainy periods than I’m removing in the drier periods and results in a significant problem for moisture build up and fungus/mold. I am already familiar with the new technologies to seal in crawl spaces and have priced my options for such. Yet, I can’t afford that right now and I have floor joists that have a moisture reading of 27% right now when the acceptable range is 1% to 19%. I will begin upgrading my crawl space and sealing it in myself, but it’s going to take me a while. I need something to help the situation in the meantime and can install right away. Additionally, when my exhaust fans run when the exterior humidity is higher than the inside humidity, even when the temperature inside is lower than I’ve set. The net effect is I’m wasting electricity on the fans when their work is actually increasing interior moisture and running continuously in the mean time because the humidity is still higher than what I’ve set. The only thing I have found so far is to get two (one for the attic and one for the crawl space) “Total Humidity Controller - Controls Appliances Based Upon Humidity Level by Green Air” found at: ( ). I could install these controllers in weatherproof, dry locations on the outside of the attic and crawl spaces respectively and use them in tandem with my existing dual thermostat/humidistat exhaust fans. The unit has a switch to change from turning a device on when the humidity is lower than a selected setting or when the humidity is higher than a selected setting. I would leave my existing exhaust fan settings as they are, connect the Humidity Controllers to the power sources and then the fans to the Humidity controllers. I could set the Humidity Controller to turn on when the humidity is less than 61% or so. The only problem in this scenario is the result would be an exhaust system that will not run in situations that might be similar to when the exterior humidity is 63% and the interior humidity is 70%. This is why I’m looking for a control switch that will simply turn on when the outside humidity is lower than the inside humidity. I could then place that unit in between my existing thermostat/humidistat exhaust fans and their power sources. Then I could reap the benefit of saving energy by not running the units to pull in higher moisture levels than already exist. Additionally, the fans will only exhaust when exterior moisture is lower than the interior, thus always working to lower the humidity in the attic and crawl spaces whenever possible and only when the interior humidity levels are higher than 55%, or whatever setting I choose to set the thermostat/humidistat controls. Of note, I am more worried about moisture in the attic than I am temperature because I have very good insulation anyway in the attic. Nonetheless, when the humidity outside is higher than the attic inside, it is usually overcast and thus attic temperature is not a problem. So my question is, do you know anywhere I might could purchase a Humidity Controller like I’ve described, or any place I could purchase components to fashion or build a controller devise to accomplish the same purpose? Thank you, XXXXX XXXXX await your kind answer. Sonny XXXXX

Welcome to Just Answer Sonny!.

I understand your logic and the problem very well thanks to your detailed and well argued situation. I have not seen a humidistat arrangement like that in my career, and I have been in the business for 52 years so far.. much of it involving humidity control in sensitive areas such as semi conductor fabrication.

There are several issues... humidistat's are notoriously inaccurate... plus or minus 5% is about the best I have seen in actual practice, many times it is plus or minus 10% accuracy... that rules out many of the designs that a person might think should work...but will in practice not get the job done.

Running 61% Rh air through an attic will take days to dry out wood that has been saturated to 27%... controlling ventilation by those means will not provide the results you are looking for, especially getting the moisture content below 19%... the first damp day or two will see the wood re-absorbing the moisture.

I am picking up your question because none of our other experts have, and that is the primary reason.

About the only way I can see to make a meaningful difference in the moisture content of the floor and ceiling joists etc will be to heat the attic and under the house crawl space ventilation air... or buy dehumidifiers.... either approach will definitely make a difference, but of course will drive your utility costs up.

(when you heat the air, the moisture content remains the same, but the heated air will hold more water, so that its *relative* humidity drops, and it will draw moisture from the wood.)

De-humidifiers cost $500 to $1,000 each. Those are in common use for this situation. Those also use energy, are not cheap to buy, wear out, and require ongoing maintenance. I do not see an easy solution for you in this case.

I can discuss this with you at whatever length you wish and your positive rating for my assistance is optional. but please do not rate my conversation negatively, that hurts on this end... I am just attempting to give you the benefit of my experience in this area.


Customer: replied 4 years ago.

I am so incredibly thankful for your kind, candid and expert answer. Your expertise humbles me.

I understand all that you are saying very well and I am thankful for the detail of your answer. I had not thought about the option of heat! I think that I may be able to use heat as a crawl space interim option pending my sealing it off and installing a dehumidifier built for that purpose. I am going to think hard along these lines. Even if I use a heat option just for a season or two, to get myself into the winter when I can begin laying down a quality liner, etc., I can at least begin to treat the moisture content of the wood in the crawl space until I can eliminate the source of the moisture itself.

For the attic, which does not have a significant structural problem with moisture, humidity fluctuations are really only affecting the things I store in the attic, I think perhaps I may not have any more options than I have already employed: the dual thermostat/humidistat exhaust fan. I am perplexed however at the industry for not realizing that such a fan could run continuously when the outside humidity is higher than the inside. It makes me almost think those fans are almost not worth the bother and that I should only worry about the attic temperature. If you have any further thoughts on that, I would be very appreciative.

Hello again, thanks for the compliment, however you are pretty sharp yourself!

We should discuss the use of one or two HRV's for fresh air intake and venting the air from the house to the attic and under the house to reduce the humidity in those areas.

That will work to some degree, but not as well as it might appear on the surface, because air inside the house, say at 50% rH is drier, but after going through the HRV it its *relative humidity will rise due to the fact that it is cooled in the winter by the heat exchanger... still the air will be a bit drier.


Such an application will definitely reduce the humidity in the attic and crawl spaces when the house AC is running, because moisture is definitely removed from the air by the AC.... in the winter the ventilation effect .... relative to dehumidification in the crawl spaces..... will be marginal. It will be worth testing and you can always choose to heat that HRV exhaust air to the attic and crawl spaces.

For a larger home, and for these purposes you want a 200 CFM or larger HRV or possibly two of them. Cost is in the $1,200 range each.




There are also ERV's these have humidity exchange features.




You can rate my work so far anytime you wish, if you rate it positively I will hold the question open without time limit so that we can continue the discussion.



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