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My humidifier runs all the time, even when the furnace is off. What could be the problem?

Question: My humidifier runs all the time, even when the furnace is off. What could be the problem?

Answer: Being able to interface the Aprilaire humidifiers to turn off and on with the furnace blower or gas would depend on whether you have “HUM” (humidifier) hookups on your furnace’s control board. Sometimes the “HUM” hookups are high voltage (110 volts) and sometimes they are low voltage. (24 volts). Please see the pictures of furnace control boards below. Sometimes if you have an older furnace you probably will not have any “HUM” hookup at all. If you do not have a “HUM” hookup you would need to purchase an A50 current sensing relay to make the humidifier cut off and on in conjunction with the furnace blower. The A50 acts like a light switch. When the furnace blower is on it senses the current and closes the connection between its two wires. When the blower goes off the A50 opens the circuit and turns your humidifier off. You can purchase an A50 on the following page for $27.00 plus shipping. We sell the A50 current sensing relay on the following page: Please click here if you are interested in seeing the A50 current sensing relay we sell. The A50 relay would be wired up in series with one brown wire that comes out of the bottom of the 700 humidifier. On the Aprilaire 600 & 400 humidifiers the A50 current sensing relay would make and break the circuit in the “H H” terminals of the 56 humidistat. If you are installing an Aprilaire 700, and if you have a high voltage hook up on your furnace’s control board, then you can use the high voltage hookup on the furnace’s control board, wire up a receptacle (plug) attached to the side of your furnace so the Aprilaire 700 can plug in. I mount the 110 volt receptacle using self tapping sheet metal screws to the side of the furnace. If you have a low voltage hookup on your control board…you can run this up to the “R & C” terminals of the Aprilaire 56 humidistat. It would be wired, Low voltage “HUM” on your control board up to “R” on the 56 humidistat. Low Voltage “C” (Com) on your control board up to “C” on the Aprilaire 56 humidistat. Polarity must be observed or it will burn up the humidistat and void the warranty. Also make sure these are low voltage, not 110 volts are it will burn the humidistat up. Please look at your furnace’s wiring schematic or test with a meter. The low voltage hook up wires for the humidifier are usually located on the same row as your thermostat’s low voltage wires. Once you get the humidifier wired using the A50 current sensing relay or through using your furnace’s control board then you humidifier should turn off and on with your furnace’s blower or gas valve. We sell many humidifier parts and supplies on the following page:

Below is a picture of a Carrier Bryant board with a low voltage “HUM” connection on the board. You can tell this is a low voltage connection because the “HUM” hookup is on the same row as the low voltage thermostat wire hookups.


The board pictured below has a “HUM” hookup near the low voltage wires, but not on the same row. I would test the “HUM” terminal with a Volt meter to see if it is a 24 or 110 volt hookup.

Control Board 325878 751 Conversion Kit all

Below is the wiring diagram for the Aprilaire 550:

Wiring for 550