UNDERSTANDING THE HUMIDISTAT AND HOW THEY WORK AND WHERE TO INSTALL THEM

The difference between the 2 controllers is the conditions that they shut off. The HUB (damp weather shut off) is recommended to be set at 75% RH. The operation of it will cause it to shut down at any RH above the set point. The HUC is the opposite, it will turn on at any condition above the set point. Operation is such that the 2 controllers when plugged together in series will maintain the humidity in the crawl space between the set points on the controllers. For example, the HUB set at 75% and the HUC set at 50% will run at anytime the crawl space has a humidity below 75% and above 50%. If the humidity drops below 50% the HUC will shut off and the fan will no longer run. If the humidity in the space goes above 75% the HUB will shut off and the fan will no longer run. The HUB is intended to be placed in the airstream of the incoming outside air, therefore, if it is raining or humid out and the fan comes on and starts bringing in air above the HUB setpoint (75%), the HUB will turn off and the fan will stop. The obvious down side to this is that if the crawl space rises above the setpoint of the HUB, the fan will not turn on. However, without the HUB, the HUC would turn on at anytime the crawlspace was above its setpoint and run the fan, it could very well bring in more humid air than is already in the crawlspace, such as when it was raining outside.

There are several theories regarding moisture control in the crawlspace. We utilize the theory that moving air will dry moisture. That basically answers questions by homeowners, but let's not leave it there!

The powered system is designed the way we like to see them. Ideally we would like to see all of the powered units connected to 1 Humidistat C. This unit should be placed near the center of the crawlspace. This does what the homeowner needs and turns the fans on when the humidity reaches an undesirable level (50% rh). A Humidistat B, also called Damp Weather Shut Off needs to be placed approximately 3 feet behind a vent on the opposite side of the crawlspace from the powered vents. This unit is set at 75%, and the Humidistat C is connected to it. This takes away the opportunity for humid air to enter. If the outside air is above the setting, the units will not come on, or, if they are running and it starts to rain the units will shut off.

As for closing all of the vents but the powered vents, the powered vents will have to draw air from somewhere. The air will be coming from inside the living area, as well as cracks and crevices around the foundation walls. Crawlspaces are not airtight. The air will still be entering. The problem would come in when the powered vents stopped running in that scenario. There would be no place for the moisture to exit.

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