Wiring for humidifier to Goodman gmpn080-4 furnace

There's two ways to wire this up. The first method is to take advantage of the HUM terminal on the furnace control board. This terminal will supply 120 volts when the furnace is running in heat mode, so you'll have to use an additional transformer.

You'll hook the primary side of the transformer to the HUM, and Neutral terminals in the furnace. Then you'll connect the 24VAC terminals of the humidistat, to the secondary of the transformer.

The other method, is to use the furnaces transformer to provide power to the humidistat. However, you'll have to make sure the transformer is large enough to supply the load.

With this setup, you'll connect the 24VAC terminals of the humidistat to the C and W terminals on the furnace control board. That way when the thermostat calls for heat, the humidistat will be powered by the furnaces transformer.

According to the documentation for your unit.

The HUM terminal is a 24 VAC output, energized when the blower is operating during a call for heat.

Connect an accessory 24 VAC, 0.5 amp. maximum humidifier (if used) to the ¼ in. male quick-connect HUM terminal and COM-24V screw terminal on the control board thermostat strip.

NOTE: If the humidifier has its own 24 VAC power supply, an isolation relay may be required. Connect the 24 VAC coil of the isolation relay to the HUM and COM/24V screw terminal on the control board thermostat strip. (See Fig. 32.)

So if the humidifier is less than 0.5 amperes, then you can simply use the HUM and COM terminals. Otherwise you'll need an alternate power supply, and an isolation relay.