Furnace Blower Won't Shut Off (Revisited)

Old 09-23-02, 06:51 AM
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Furnace Blower Won't Shut Off (Revisited)

Hey Big John, et al - Remember me? I had the problem a couple months ago whereby my furnace blower wouldn't shut off. As per your instructions, I hard-wired the stat, etc. Which turned on my ac condenser. We felt it was a bad stat till I went ahead & rewired the stat up. Then it worked. But only for awhile till one day, the blower was on even tho' the stat was off - same problem.

The thread got to be REALLY long. Anyhoo - I can't find the thread. I've put dealing with this aside as we've had perfect weather. Now it's really cold and I have to get this fixed. Tried to turn on the heat and it doesn't kick in. Just the blower blowing room air. My guess is (like the AC), the heat would turn on if I hard-wired the stat.

Can someone help me remember my options? Here's how I remember them.

1) Bad stat
2) Some wierd short in the wire from the furnace to the stat somewhere. Works off & on.
3) A part in the furnace area. I can't quite remember what it was. Was it a "contactor"???

So if I'm faced with the 3 above options - would one first buy a new stat? Then if that doesn't create an instant fix - somehow test that part (contactor?) in the furnace? Third would be to call an electrician?

Would that be the correct order of events to do here? Sorry this is a long post.

Old 09-23-02, 09:29 AM
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I disconnected the outside AC condenser.
Shut everything off, then hard-wired the stat wires to bypass it.
Turned the furnace power on.

Only the blower turns on. No heat (gas furnace).

I guess this means my thermostat is NOT the problem.

Would that be correct?
Old 09-23-02, 12:24 PM
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I just disconnected the stat wires at the furnace. I removed them from a yellow circuit board thing. The blower still runs.
I figured I was missing something so reattached them then totally disconnected them from the stat in the living room with no wires touching.

Blower still blows. So I guess I can eliminate stat problems and shorts in the stat line.

Given this - Any ideas as to what can make a furnace blower be on when there is no juice through the stat?

I'm fearing now my problem might be very expensive.

My husband is a bit impatient with my diagnostic skills and my ability to fix this thing with a $15 part. He says if I don't figure this out soon, he's going to have the electrician come over tonight. The hair is already standing on top of my head at the prospect.
Old 09-23-02, 02:16 PM
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Yes, I rememeber. Modern gas furnaces are configured so that if the limit switch opens, the blower will run full time and the heat will be locked out. Usually, all other circuits are also locked out. Look in the furnace for a limit switch and see if it needs resetting or if it's open. If you have a downflow furnace there may be more than one limit. You should be able to find the limit switches by looking on the wiring diagram and then tracing the wires. Can you tell us what brand your furnace is?
Old 09-23-02, 02:28 PM
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Yea!!! BigJohn! Hiya!

The furnace is a 6 year old Armstrong. Don't have the model off the top of my head (I could get it). Nothing fancy.

I cannot for the life of me figure out where this fan limit switch could be. All I have in there is a basic metal box (looks like an electrical box) with a few thick wires in it attached to the wall of the furnace. No switches, etc.

The other thing I have is a box/system (Honeywell) thing that seems to engulf the gas pipe inside. I don't think this is the limit switch.

Below the fire section, is a big yellow Honeywell circuit board.

OK - here's what I did. I TOTALLY disconnected the stat wires where they attach to this board. Turned on the furnace and sure enough, the blower STILL works! I guess this just eliminated problems in the stat AND the stat wires (shorts).

So now I have a system whereby the blower won't shut off. AND my a/c AND furnace don't fire up.

Could it be the circuit board is bad? Oh! Also, could I be looking for a limit switch that is simply not there? Would my control board (Honeywell ST9120a-2004) CONTAIN the limit switch function?

If my circuit board is bad. LOL There's a zillion wires attached to it (plugs). Would I be able to simply unscrew the board with the wires still on and methodically transferring them one by one to the new board?

Or - Am I misdiagnosing? This board is about $100 and I'd probably have to face a re-stocking fee if it doesn't fix it.

So - what would you do?
Old 09-23-02, 02:50 PM
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Pictures of the guts of my furnace are here


Do you see a limit switch?
Old 09-23-02, 09:37 PM
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Update: Hubby was getting a bit fed up so he called in his electrician friend who brought in his hvac bro.

First they figured the board was messed up. I kept talking about the limit switch and asked for them to point it out to me. First he didn't see it - then he found it. The darned thing was just a little black plastic faceplate. Grr! Then there was an auxilliary limit switch (a safety) thing that seemed to be seriously corroded. He took that apart - it was so bad it broke off. He hardwired it together but the blower still didn't stop. But it made the fire come on at least. I guess this safety thing was telling the fire to not come on because it was screwed up.

It was looking pretty grim till I pushed & pushed for him to fully inspect the limit switch. He pulled it out and there it was! The limit switch I've been looking for! I was looking for a 'box' not a face plate. Oh well. So he thought it looked ok, then reinstalled it.

Poof! Instant miracle. Everything's working great as it should. This cost me $158. Not bad considering they were here for over 2 hours. Tomorrow they come back to put the aux limit (safety) thing back together. Will involved redrilling & adding new plugs/clamps to the wires. They'll charge me $27 plus the part. I was going to do it myself but for that price I thought I'd just toss the ball totally because then it can be properly tested, etc. It's just too scarey the way everything 'talks' to each other and I didn't want to chance it.

You should have seen their amazement at my knowledge of furnace parts! They kept asking me how I learned all this, etc. It was pretty funny. They were quite impressed with me! I was glad they didn't cop an attitude while I discussed things. It was because what I had to say made perfect sense & it was kind of fun going through the problems together.

Thanks BigJohn for making me look good!
Old 09-24-02, 06:06 AM
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Pretty soon you'll be in the heating/cooling business. May I suggest that you don't run the heat without all the safety switches in the circuit? I'll betcha that the corroded limit switch has been the problem all along and that it was intermittent.
Old 09-24-02, 06:23 AM
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Thanks - Yes. I was warned it's not a good idea to play games with this setup. We just ran it a bit to get the chill out of the house. Hopefully today everything will get back to normal. I didn't even check the a/c yet - hope it works ok.

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