Humidity Comparator

[MillWeld]

I have an old (1950s) farm house built with a crawl space of only 12". We are having the insulation replaced and a plastic vapor barrier put on the ground. I have been told by a home inspector friend that such a shallow space cannot be reliably ventilated by natural wind/air flow. I want to add forced air ventilation via 2 small fans that come on when the RH in the space is greater than that outside. I need a humidity comparator controller to do this but have not been able to locate one. Anyone have sources for such? Thanks,
Bob

 

[zzvyb6]

My forced air furnace has a in-stream humidifier. There is a humidity sensor at the cold air return that maybe could be adapted to your needs. its switches at low humidity but I'm sure your could invert the relay to turn off the fans when humidity gets below a set point.

 

[zzvyb6]

Might be even easier to buy a dehumidifier at a garage sale and tap into the fan and compressor controller on it.

 

[Mendonsy]

I don't think anyone makes one. It could be done with two sensors and some sort of controller I suppose.
It would probably be less expensive to just hook the fans to a humidistat and run them whenever the crawl space gets to some preset humidity level.

 

[Industrial Toys]

I want the same thing. I have a fruit cellar that I want to store some fire wood in. I also want to run fans but don't want to suck in humid air. And I don't want to have to manage this myself. There are micro computer kits around with a multitude of input and output devices. You write your own code. I would love to do this but have way too many projects on the go already.

 

[hr3]

These guys have everything but everything isn't cheep... Humidity Controllers and Analyzers

 

[Cord]

What you want is a humidistat. Here's one from McMaster that will fit your needs: McMaster-Carr Remember, you want cross ventilation. So put your two fans on the far side of the house, across from the intakes. Have both fans come on at the same time or you could end up with a short circuit. Use the fans to pull the air out or you could end up pushing the humidity into the house. Make sure the humidistat is out of the airflow stream or it'll read the outside air humidity. I'd suggest setting it between 50-60%. You're down south so it won't be as much of a problem, but up north during winter you'd have to switch the system off because the unit reads % of humidity which falsely high in the dry winter air.

When they add the vapor barrier, make sure they use a heavy plastic (not the hardware sheets!) and seal it to the foundation wall. In that tight of a space, it'll be a totally miserable job.

 

[quicksandfarmer]

It's not humidity that you need to measure, it's dew point. If the air outside is warmer than inside, the outside air can have a lower relative humidity and still contain more moisture than the inside air. If that's the case then you're actually introducing more moisture by ventilating. Dew point is a temperature-independent measure of the moisture content. If the outside dew point is higher than the inside dew point your venting does no good. If your outside dew point is higher than the inside temperature you're going to make it rain inside by venting.

I faced the same problem. Once I started measuring dew point I realized that all summer long the outside air was too humid to be effective at venting. Instead I sealed the cellar and put in a dehumidifier.

You can get a digital thermometer with a dew point measurement for less than $25 on Amazon. Get it now and measure the inside and outside dew point while it's still summer. Or measure your crawl space temperature and get dew point readings from a weather website.

 

[Industrial Toys]

The company my Dad worked for had an enormous automated steel storage facility. They heated that thing all summer to keep the steel dry.

If I open my basement windows on a summer day where I perceive the humidity is low that day, I encounter no problems, despite the basement being cooler. Just nice fresh ventilation. I have also used a small fan in my fruit cellar under the same conditions I just mentioned, and again I had no ill effects. No walls or ceiling sweating.

So although I kind of understand Dew point and the idea that air of a certain temperature can only hold so much water, I still think and hope that the OP and me can accomplish our goal.

 

[arlen4720]

If somebody wants to buy any of the Johnson Controls controllers, I would program them for you. I have all the software, and have 1000's of hours of experience.