The water runs constantly, not just when the furnace turns on.

Check yor wiring, make sure you have sail or pressure switch wired in series with humidistat to humidifiers 24V power. Pressure or sail switch will disconnect humidifier when furnace fan is off. Humidifier (110V) could be connected to any closest electrical outlet.

The automatic shutoff switch from the furnace to the humidifier has probably gone bad and will need to be replaced. However If the humidifier has a float valve in it like a toilet tank, it may be hung in the open position,which allows the water to continually run. Take the top of the humidifier and check the float valve's operation. It may simply need cleaning.

Hello, In order for the humidifier to turn on and off you need either a sail switch or current sensing relay. A sail switch senses air flow and will turn on the humidifier when the blower motor kicks on and a current sensing relay senses current through the blower motors common wire. If you have either of theses hooked up already and the solenoid valve remains open then the solenoid valve must be replaced.

Can you turn off the water flow with the humidistat is set to off?

in order for the humidifier to come on with the heater there has to be a temperature sense curuit to turn it on/off at a certain temp, the humidity curcuit on the unit will turn it off at set %

sounds like you may have it wired to a continuous power source vs one that shuts off with the furnace. The most common wiring is red to C & white to W

I has the same problem with my HE360 unit. The unit was in fact miswired. The head unit red wires were connected directly to the blue solenoid wires, and activated the solenoid without any signal from the humidistat. As soon as the unit is plugged inn, the water starts running without any signal to the unit. If your unit is miswired, the soleniod blue wires need to be cut and connected to the red crimp connectors.

The furnace should be equipped with a #50 relay to turn the Humidifier on and off when only when the furnace blower is on. You should have a test mode on your humidistat. Do you have water coming up to the solenoid valve? Make sure that the saddle valve is turned on to supply the water to the humidifier. Is your humidistat set high enough to turn the humidifier on? Is it plugged in?
Good luch
HVAC Teacher

the 760 humidifier does not need a transformer wired externally. the 760 has its' own 24v transformer inside the fan compartment. this transformer will provide the power to enrgize the water solenoid as needed. it sounds like to me the additional trasnformer is hot all the time. when the furnace comes on, the unit gets power and the internal transformer is cancelling the power from the external transformer. this would explain why it quits running when the heat is on. since the external transformer is hot all time, when the heat shuts off, the solenoid is getting energized by the external transformer. to make this run correctly, you need to get rid of the external transformer. leave the humidifier high voltage wires hooked up so they are only hot when the furnace runs. if you have a humidistat that has a r and c terminal, you will need to wire these directly to the r and c on the furnace control board and move the switch inside the humidistat to "power". make sure it is not on "bypass". then wire your 2 brown wires from the humidifier to each of the "h" terminals on the humidistat. if you have a humidistat that only has 2 terminals, wire one brown wire to each of the terminals. this should allow it to run properly.

Thanks for your question,
Sounds to me like you have a bad solenoid valve in your Humidifier, this will need replaced, the solenoid is supposed to open and close when there is a call for humidity only when the furnace is running, then when the proper amount of humidity is sensed by the humidistat, then will it shut off, with the solenoid not working it will let water flow through like an open faucet, the solenoid is the componet that the 1/4in copper tubing connects to at the humidifier, has two wires running from it, you will need to shut the power off before changing this out, also shut the water off at the valve at the other end of the 1/4in copper line, or you might get soaked,please be careful when installing the new solenoid, making sure the water line connection is secure, or you will have leaks, hope this was helpful, please leave a rating, thanks again for your question,
mr.grzz