Humidifiers and Dehumidifiers - Wiring Aprilaire 500a Humidifier to 120v HUM

Hi there,

Trying to wire a 500A to my furnace. I have wired the 24v transformer to the 120v HUM and Neutral on furnace board since no 24v tabs. Ran 24v output to R and C on digital humidistat. Have outdoor thermo connected to ODT on humidistat. Have both leads of selenoid going to both H locations on humidistat. What other connections do i need to make? I have W/G, Cf, and AB left? Wiring instructions are terrible. Thanks for any help.

I have wired the 24v transformer to the 120v HUM and Neutral on furnace board since no 24v tabs.
Run the low voltage wire from the transformer to W/G, and Cf.


R and C on digital humidistat
Run that down to W and C at the control board. W to R, C to C.

Jay, thanks for the info. Forgot to mention that the thermostat wires are connected to R and W on the furnace board. So I will end up with 2 wires on the W on the furnace board...the white wire from the thermostat and the red wire from the humidistat and then common from humidistat to common on furnace board? I saw in another post you had the rc from humidstat to rc on furnace board. Aprilaire support was terrible and said 8 wires had to be hooked up at all times. Please confirm the R to W and C to C. Thanks for the help!

. Forgot to mention that the thermostat wires are connected to R and W on the furnace board.
That's fine to piggy back the wires on W at the Furnace.


Please confirm the R to W and C to C.
Yes, that is correct.

After several calls to aprilaire and testing on orig could't get to work. Plumber recom a new unit. New unit came could not get that one to work. Aprilaire has stated it wants constant 120/24 power in R&C for memory to work. Plumber said others he knows will not install 500a because of same problems. Recom getting 550 humidistat. HELP

Aprilaire has stated it wants constant 120/24 power in R&C for memory to work.


Run that down to W and C at the control board. W to R, C to C.

Ok, My bad... The wire should be going to R and C at the furnace.

Guess my mind was on another mild that the transformer was always hot.. Your case, the transformer runs on EAC (Cycles on and off)

Wired constant 120/24v trans to R&C on humidistat. Wired 120 Hum/Neutral to 2nd - 24v trans to W/G & Cf. Manual mode resistor to ODT and solenoid connected to HH. So constant power to R&C to power humidistat itself and 2nd power when blower comes on to power solenoid circuit. Humidistat seems to read very high on cold air return 42-45%. On highest setting (7) solenoid will shut off with 45% humidity reading. If outdoor thermometer is hooked up solenoid will not work if temp is above 60 deg. Got around that by installing manual resistor. My advice is to stay with their 550 unit. My plumber says the 550 is the only unit he will purchase for his customers. If you get stuck you can buy a honeywell humidistat at home depot for $26 and use it in place of the 500 humidistat. Jay thanks for your help!