Water runs when furnace is off, but stops when furnace is on.
We just moved into this house, and it had the Aprilaire 760 installed, but disconnected. When I put it back together and turned the water on, the water kicks on when the furnace is off and the humidstat is turned up. When the furnace comes back on, the water cuts off. It's clearly wired backwards, but I can't figure out what's wrong.
There is a 24V transformer near the humidstat, and one of the feeds runs through it. It has constant power being fed to it. The plug for the humidifier is plugged in to an outlet on the furnace that only has power when the furnace is running. The brown wires coming off the humidifier are wired to the humidstat (one of which has the 24V feed).
Re: Water runs when furnace is off, but stops when...
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.
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Hope this solves your problem, look for the humidistat, it should be located on cold air return duct near furnace, make sure it is not set in the on position, if it is, then this is why it is staying on, please set humidistat any where from 20 to 35%, never over 40%. If the humidistat is set properly and is still running, you may have a electrical problem, humidifier circuit should only receive power(24 volts) when the indoor blower motor of furnace receives power, also if you have galvanized plumbing in your home, you may have a piece of rust or lime deposit in the humidifier solenoid valve preventing it from closing, turn water off to humidifier and disconnect water supply line entering solenoid valve inspect & clean protector screen. Good luck.
A simple check you can perform on your aprilaire 760 is the following:
1) locate the two brown wires coming out of the 760 (these wires will be wired to tell the 760 to turn on when the hvac bower is on and the humidistat is calling for moisture).
2) temprarily disconnect the two brown wires from the external wiring and then short the wires together. All you're doing here is bypassing the your thermostat and bower-on indicator.
With the 70 unit plugged in to the wall outlet and the two brown wires connected together, both the fan and the solenoid valve (water inlet valve) should both be activated.
Disconnect the two yellow wires. The water should stop. If it does you have a bad (or miswired) humidistat. If the water does not stop the solenoid valve is bad. These valves are known to be expensive. You might want to consider replacing the whole unit.
Good luck HVAC Teacher
I called the service company that put in my new furnace (FAmily Heating and Cooling in Detroit, very highly recommend these people)
Anyway they sent out a second service tech who determined that I would be better off having the humidifier wired to go on whenever the furnace fan was running instead of using the humidifier signal from the circuit board on the furnace. After he changed the wiring the unit runs fine. It is pretty likely that the circuit board was bad also.
You need to determine whether the problem is in the solenoid or in the furnace electronics. You could put a voltmeter on the solenoid connection to see if the furnace is still holding the solenoid open after the blower stops. This will tell you whether it is the solenoid or not.