A humidistat senses humidity, thermostat temperature.

The difference is the relay operation.Normally opened or normally closed.If you add an external spdt relay,you could have a humidistat/dehumidistat.

With a temp/rh probe barring that take a humidistat and turn dial when it clicks from closing circuit note the setting and that is humidity surrounding humidistat

Not sure how it works, but it seems that you need to raise the humidistat to make the heat go on. Just raising the thermostat doesn't turn on the heater. Type your answer here...

A humidistat is a device used to control the operation of space conditioning systems such as air conditioners, dehumidifiers and humidifiers. When used with an air conditioner, for example, the humidistat cycles the unit on and off based on humidity rather than temperature as provided with a regular thermostat. A humidistat set at 70 percent relative humidity, for instance, will cycle the air conditioner on when relative humidity reaches 70 percent or above, even if the thermostat setting does not call for cooling.