Hello! I need help hooking my humidifier back up to run only when the furnace runs. While out of town my local HVAC contactor had to make a house call and he changed the wiring so that the humidifier runs regardless of the furnace state. I am now getting water in the blower compartment from condensation (guess) when the humidifier runs and the blower doesn't.

I have a Aprilaire 760 with the original manual humidistat, a Trane XL90 TUC100B948A0 furnace and a Hunter 445500 thermostat. The voltage across the humidifier control leads is 24VAC so it doesn't appear that I need a transformer (?). I tried connecting the humidifier lead 1 to the humidistat lead 1 and then the humidstat lead 2 to the 'B/C' terminal on the furnace and the humidifier lead 2 to the 'W' terminal on the furnace. In this setup, the humidistat turns on the heat. I would rather the humidifier only run when the thermostat turns on the heat for temperature control. Is this possible? Do I need to add a new relay so that the R-W heat signal from the thermostat would close the humidifier circuit but a humidistat signal would not close the R-W circuit and start the heat cycle?

Or am I over analyzing and it is normal operation for a manual humidistat to turn the heat cycle on?

 

I think I have found the answer. I need a GA50 current sensing relay to close the humidifier circuit to tie the humidifier to the fan or a 24V relay hooked up to the R-W circuit to close the humidifier circuit with the heat cycle.

Merry Christmas!

 

A relay is not needed if you wire to humidifier to the W & C on the furnace. Honestly, I'm not crazy about doing that, and I'd rather wire a transformer on the HUM wire in the furnace junction box. the HUM wire powers up when the furnace runs in heat mode.

 

Thanks for the reply Jay. I don't have a wire labelled as a 'HUM' in the junction box or on the wiring diagram but your post prompted me to take another look. What I found was an electronic air cleaner hookup rated at 2.5A 120V (from the wiring diagram). It reads 120V when only during the heat cycle when the blower is on. My Aprilaire is rated at 0.95A so what do you think about just hooking the 120V supply wire for the humidifier up to these wires? Right now they are wire nutted into the main furnace 120V supply wires. That way I could skip the transformer and hooking the humidifier control wires up to the W&C.

 

The EAC will power up anytime the blower is running, not just in heat mode.

What humidistat model # did you get with your humidifier?

 

It doesn't have an Aprilaire model number on it. Inside is a Talltronics humidity controller model 83321

 

If you have a transformer, this how it wired. See diagram for manual control.

 

Thanks for the diagram. Forgive the question but why not just wire the 120V humidifier main power wires into the EAC and skip the transformer? Then I would keep the manual humidistat on the 24V control circuit

 

You could do the R and C/B on the board, and plug the humidifier into the outlet to the EAC.