Same old question, furnace fan stays on....

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Old 04-28-09, 08:04 PM
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Question Same old question, furnace fan stays on....

I can use some advice on a question that keeps popping up in this forum. Here is the deal: I have an old furnace from about 1974, it's a "JOHNSON CORPORATION" model number HAS105AD. Like the other posts here is what happened. Furnace fan won't shut off no matter what position the switch is in (on or auto). Thermostat is a programmable Hunter which is 3 years old. After reading a couple of posts it sounds like it's a "fan switch" or a "fan relay". Does this sound correct to anyone? Any idea on where I could get one for an old furnace? How much will it cost? Any help would be greatly appreciated.
 
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Old 04-28-09, 09:53 PM
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Fan Stays On

A few pictures of the furnace with the door off might help us help you determine the source of the problem.
 
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Old 04-29-09, 08:50 AM
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It might be old enough they use thermodiscs to control fan and limit, and one of the thermosdiscs is shot.

See if you have any device mounted by the blower fan and above the burner area that look sort of coin size with a wire on each side of it. If you have this, I bet your problem lies with one of these. A very DIY-able replacement.

These things will be marked on them like L220-30, or F130-20, or something like that. The L one stands for the high limit and the F one stands for the fan.
 
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Old 04-29-09, 11:32 AM
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When looking inside the furnace (cover removed) the blower motor is at the left towards the bottom. The blower motor has a device that looks like two rows of quarters side by side with two wires coming out. I'm guessing that's the "thermodisc" you're talking about. Is there any way to test this? Thanks for your help.Beer 4U2
 
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Old 04-29-09, 05:42 PM
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You say you have 2 rows. Do you really mean that you simply have 2 thermodiscs next to the blower fan?

Can you read an F or L and then numbers on them, as mentioned in previous post? It may be that the F one will allow the fan to turn on, but not flex the bimetal in the device (or it became arc-welded together inside it) enough to break contact and stop the fan. It might be conceivable that they both serve the fan and one is for on and the other is for off. [It's been a while since I have dealt with these things, to remember every detail.]See if by tracing wires from the disc, that you can tell which winds up going to the fan, by what ever route, and you then could pull off one of the wires and see if that one stops the fan.

Run the furnace by turning up the stat to say 80. Let it run til the blower fan comes on. Then let the furnace continue to run for say one minute longer. Then shut off the furnace power switch. Remove one of the thermodisc wires from the one serving the fan. Then make sure the dangling wire does not hit any metal of the furnace (put electrical tape on the metal spade connector if you have to). And then turn the furnace back on and see if that stopped the fan. But be sure to plug that spade connector back in place, after thsi test procedure. And remember everything you do about when the current is live and when it is not. Use insulated needlenose pliers if necessary.
 
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Old 04-30-09, 09:02 PM
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Thermodisc(s)

The only time I've seen a thermodisc mounted on the fan is in a downflow configuration & then it was only a limit. It sounds like Shadetree's furnace is an upflow.

I still want to see pictures & the easiest way for us to see them is to post the pics on photobucket.com or similar site & provide a link here.
 
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Old 05-01-09, 08:12 AM
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Grady,

Speculation on my part, but a possibility. I very much was hoping for further feedback, so we could have further discussion. Looks like that might not happen.
 
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Old 05-01-09, 04:50 PM
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More input

EC,

Maybe it's fixed? Hope so, but I wish Shadetree would come back & give us an update.
 
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