Furnace Hums At Ignition

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Old 10-29-08, 06:08 AM
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Furnace Hums At Ignition

Issue: Gas furnace hums during ignition … but blower works and output appear unaffected.

Details: I have an American Standard GUF120D E3 H 120,000 BTU natural gas furnace with a bonnet capacity of 96000 and a 1/5 hp motor. When the thermostat calls for the burner to ignite, I hear an electric hum like a motor trying to start. This continues for about half a minute
during the burn, fades out, the burn continues for a short period longer, then the blower kicks in as usual.

I found a website with a flow chart of a common gas furnace mechanism (http://www.mobilehomerepair.com/FurnaceFlowChart.htm ). Though this appears to be from a site dealing with mobile homes and my home is not mobile, I assume there are similarities. According to this chart, there’s an “induced draft motor” – different from a blower motor – that gets triggered in the front end of a burn cycle. If that’s what I have, I’d venture to bet I need a new draft motor.

Anyone have sufficient knowledge to help solve this? I could use a flow chart for my particular furnace as well. I've emailed American
Standard for a manual but if there's something on the web, I'd appreciate being steered to it.
 
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Old 10-29-08, 04:54 PM
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Without knowing what kind of ignitor your furnace has, I would guess the sound to be from an arc type lighter or the step up transformer powering it.
 
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Old 10-29-08, 08:00 PM
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Thank you, Goldstar, for helping me... and being a gentlemen in the process. I now know two more components of my furnace than I did before I came here. If I may trouble you with a follow-up to make sure I got a good picture of the basics here: although the burners ignite the issue could still be with the “lighter” itself because that lighter may be one that generated a electric arc and although it is arcing, the mechanism (the lighter itself I suppose) may be fault because it continue have an open circuit that shouldn’t be and as a result… it hums. Or – with regard to the transformer – which I assume steps up the current to create the arc – the transformer itself may be the culprit. Am I close?
 
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Old 10-29-08, 08:38 PM
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Update - About that igniter... it's a pilot light. So... is a transformer still involved?
 
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Old 10-30-08, 04:50 PM
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Update: Problem diagnosed as faulty gas valve. Replacing.
 
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Old 10-30-08, 06:11 PM
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Who diagnosed it?

Certain good gas is present up to the valve?

Certain of 24 volt MV current when called for at the valve? If yes, has anyone tried to rap on the gas valve with the butt end of a screwdriver?

Is this a plain jane gas valve or a smart valve that has control board inside it.
 
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Old 11-04-08, 06:14 AM
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Qualified tech diagnosed it. And "yes" he wrapped it with a screw driver butt end before concluding it was the $150 value. I assume the cost ($380 complete with installation + $80 diagnostic) made it a "smart valve"... or me a "dumb consumer"!
 
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Old 11-04-08, 04:12 PM
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Ah, a smart valve. Them things can get bad connection issues inside them even, I think. You can jiggle on jacks and get them to work, not work, work, not work again...then you just give up.

....................................

I just thought of something: Seems to my recollection these often use a type of jack like a molex that instead of spade connections they have those skinny metal pins in the jack. And the pins have lengthwise splits in them, by design. You might want to first try spreading open the pins some, to get a better connection! I recently diagnosed and made such a correction on a gas furnace, sparing me an expensive misdiagnosis!!
 
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