I found info at this thread, and would have placed this question there, but I can't comment since I'm too new.

I have HUM terminals on my control board, but they're line voltage (120VAC). I don't have a 24v transformer, so I hooked up the humidifier using the W and C terminals. It does work, but only when the air is being actively heated. It turns off when the heat distinguishes. At this point, the fan still runs to circulate the heated air that's already in the ducts.

My question is - if I were using the HUM terminals, would the humidifier still run as the air is being circulated but the heat is actually not firing?

I'm wondering if it's worth it to wire in a new transformer. It's not expensive ($35) but if it's redundant, I'd skip it.

  • Hello, and welcome to Stack Exchange. You did right by asking your new question here; a classic beginner mistake is to post a question as an answer. – Daniel Griscom Jan 31 '17 at 16:12
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    I think it would probably depend on the furnace -- post a model # and maybe someone will look it up. – gregmac Jan 31 '17 at 17:14
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    I think it's pointless though. If you have an evaporate humidifier, cool air isn't going to be nearly as effective. Given the furnace only runs for a few minutes after the burners shut off, it would only even be useful if your humidifier is currently on 100% of the time, and your relative humidity is still too low (but a few minutes more would help). I'd say your time is better spent fixing air leaks in the house (root cause of low humidity), or going to a larger humidifier because maybe yours is undersized. – gregmac Jan 31 '17 at 17:17
  • An easy way to find out, would be to put a voltmeter on it and see. Although I think gregmac has a good point, is that extra minute or so really going to make that much of a difference? The warm air should still be blowing across the moistened water panel, the humidifier just won't be adding more water. – Tester101 Jan 31 '17 at 18:49
  • Furnace is a Tappan # FG6RA 120C - 20C. This is my first home and I have no experience with furnace humidifiers so I'm not sure if the extra minute or so will make a difference. I honestly just don't know how the normal operations are supposed to work, and just asking since I bypassed the HUM terminals in order to get it working. I don't believe I've checked the voltage of those terminals while the burners were off but the fan is still running. That'll probably give me my answer, correct? – Evan Sears Feb 1 '17 at 17:39

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