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Please help! Newbie playing with his furnace - relay help?

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New Member

I am a complete novice at electronics, but I know the problem I'm trying to fix can be solved with an easy circuit, or even just one electrical component. Possibly just a resistor or relay?? Can you please read this and offer suggestions?

Overview of the problem:
The problem has to do with a small humidifier I installed that is attached to the air duct next to my furnace. The humidifier, installed exactly by the book, is triggered on and off by the air temperature from the furnance. When the heat cycles off, the blower still runs for about 30 seconds more, and the air slowly cools, thus triggering the humidifier to turn off. And, of course, when the heat turns on, the air temp affects the humidifer's thermometer and powers on the humidifier's atomizing mist jets. What is flawed with the humidifier is the threshold in which the device turns itself off. Unfortunately, the furnance will cycle off and the blower will run for its programmed 30 seconds (by design), but the humidifier device keeps spraying water into the air shaft until its own thermometer cools down enough to turn itself off. By that time, there's pools of water draining from the bottom of my furnace! I've replaced the unit and called the vendor, but the same situation occurs, and the vendor could care less that the thermostat's sensitity settings are not optimized for such a short blower duration after the heat cycles off. I can't choose a different device since I made permanent and specific cuts into my vent shaft to mount the humidifier.

All I need to do is get the device to turn on and off according to when the actual heat triggers on and off on the furnace. After doing some investigation right inside my furnace access panel, I found two wires that register at 24V when the furnace cycles on. The same pair of wires also become neutral as soon as the heat cycles off. This is exactly what I thought I needed to tap into in order to control when the humidifier turns on and off.

The humidifier itself runs on AC, through a transformer wart on the receptacle. My goal is to cut ONE of the two power wires leading from the transformer to the humidifer, and sticking in a "gate" (for lack of knowing the proper electronic component) or relay that will block all current from the transformer until the furnace turns on and its 24 volts trips the relay coil. (or something like that?) This would allow the humidifier's transformer to feed current through both wires past the "gate" or relay and into the humidifier for operation.

Does this sound like I need a relay with a diode and resistor? Are there more simple ways to accomplish this? I have no idea how to proceed and wire the circuit. Any help would be greatly appreciated.



New Member
relay to furnice......do you read???? lolol

Hey, all kidding aside, a simple solution is to go to any large home building supplier or electrical supply company and ask for a solid state 24vac relay that can control a (I'm supposing) 120volt humidifier. Wire this in series with the hot side of the humidifier going to 120vac and it should cut off when there is no supply to the thermostat control. I am not sure if the supply voltage can handle a regular relay, but if so, they make a 24volt relay that energizes on A.C. and can also be used for this particular situation. Find the two contacts that are closed when the relay is energized and wire it in SERIES with the power source going to the humidifier. Hope that helps. [email protected]. E-mail me if you need any help or if this helps. Thanks.
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