WIRING A HUMIDIFIER TO A HEAT PUMP

Hooking-up a humidifier to a heat pump can be a challenging task for the following reasons:

• The lower supply air temperature from a heat pump limits evaporation which is necessary for proper operation.
• The humidifier should only operate during heat operation which makes wiring decisions difficult.
  • The "Y" terminal is energized for both heat & cool.
  • "Y" doesn't energize in Em Heat operation.

The ideal method of controlling a Humidifier on a heat pump is to use a thermostat with a built-in humidity sensor. 

• Honeywell TH8321R (The TH8320R does not have a humidity sensor or extra output)
• Honeywell YTHX9421R

Both of these thermostats have the ability to control a humidifier and can be configured to operate the humidifier only during heat operation.

With a standard thermostat, the best option is to use the "G" signal from the thermostat to break through a humidistat and power the solenoid valve on the humidifier.

The drawback of this wiring method is that there is the possibility of powering the humidifier in "Cooling" of "Fan Only" mode.

Important Note:

The "HUM" terminal and the "HUM STAT" selector pin on York air handler boards is NOT for a humidifier. It is a De-Humidistat input that causes the indoor fan to run at a lower speed in cooling mode.