imgoin4it wrote:
We used to call them a "humidastat". Mounted on the wall like a thermostat. Used them in sensitive computer environmental areas where if the humidity would get too high it would bring on the air conditioning to condense water from the air to lower humidity and if temperature was too cool would bring on reheat to warm the air control temperature. Haven't seen or heard of one in years, but could try heating/air-conditioning supply sources.
They're not uncommon for residential forced hot air heating systems, which tend to dry the air out and so are often equipped with a humidifier. Not all of the humidifier designs allow for controls (many are just wicking material in a pan that's kept filled with water by a float valve), but some do and those will have a humidistat somewhere. I guess other heating systems also dry out the air, but it's harder to incorporate a central humidifier in when there is no ductwork to circulate and distribute the air.
Apparently there are also such things as dehumidistats, which I guess differ mainly by when their contacts are open and closed. Home supply places (and HVAC places) have or at least can order both readily.
