Humidifiers and Dehumidifiers - How a humidstat works with outside temperature.

I am looking at getting a Honeywell humidifier, the one with the outside temperature gauge. I am unclear about 2 things. First, an inside humidstat adjusts humidity as if it were temperature, correct? If the humidty is too low, it kicks on the humidifier, except if the heating fan is not blowing, does it kick on just the heating fan or do I need to have the heating fan running 24/7 so that it is on when the humidifier is on? Second, what does outside temperature have to do with humidity? Both the Aprilaire and Honeywell have outside sensors that control the humidifiers operation. When the outside tempeature drops they are assuming that the humidty drops as well so the humidifiers is turned on more often. Maybe that is a very general overall rule but they are assuming that humidity is directly related to temperature, or is it? Why not an outside humidstat instead of an outside thermometer?

Roger

Roger,

The reason you have an outdoor sensor is to allow the inside humidity setting to automatically change in relation to outside temperature.

The approximate range for human comfort is between 25% and 55% rh.
The problem comes in when the outdoor temperature drops and your window inside surface temp drops below the dew point temperature, the temp that condensation forms.
You then get water running off the windows.
Most regular humidistats have a chart that gives you the recommended inside humidity level that corresponds to a specific outdoor temp.
You are then expected to adjust the dial as the outdoor temp changes.

What you are proposing to buy adjusts the indoor humidty level according to the outdoor thermostat.
This is the way well controlled commercial buildings do it.

"The problem comes in when the outdoor temperature drops and your window inside surface temp drops below the dew point temperature, the temp that condensation forms."

So there is a formula of temperture + humidity = dew point but it only happens when the temperature drops. So condenstaion forms with a lot of inside humidity and a big outside tempeature drop but once the change has occured, and you have dropped the humidity, you can increase the humidity again.

I guess I am not understanding everything. If I boil a pot of water the windows are going to steam up even thought the outside temperature has not changed, is that dew point condensation or plain old humidity?

Roger

The condensation forms because your windows are cold.

The colder the windows are, the lower the humidity inside the house has to be to prevent this from happening.

For example, if the outside temp is 10 degF, the inside humidity should be no higher than say 40% rh to prevent condensation from forming on windows.
If the outside temp is -20 degF, the inside humidity should be say no higher than 25% rh to prevent condensation from forming.

If you had a manual humidistat you would adjust the levels to correspond to the outside temp.
The one with the outdoor sensor will do this for you.
So, if it is 10 degF outside the control will automatically set the inside humidity to say 40%, if the ouside temp goes to -20 degF then the control would change your level to about 25%.

Clear as a bell, thanks much

Roger