Furnace Limit Switch

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emtnut
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Re: Furnace Limit Switch

Post by emtnut » Sun Dec 23, 2018 1:31 pm

A. Spruce wrote:
Fri Dec 21, 2018 1:21 pm

When the unit cycles, it is the fan turning off and on every few minutes, the burner stays lit. This is the problem it's always had.
I thought the problem with the cycling was happening at the end of the heat cycle. ie, heat clicks off, but then the fan would turn on once or twice till it was cooled down.

If the burner is still going, that can be either of the limit switches, the aux switch, or either of the 2 relays, or any of the wiring between those.

Obviously the gas valve is shot now (new problem).
Hard to troubleshoot the intermittent fan problem ... well, unless you get out the parts cannon !!

Hopefully this guy is a good tech and fixes you up !
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Aaron
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Re: Furnace Limit Switch

Post by Aaron » Sun Dec 23, 2018 1:41 pm

Reminds me of working at the phone company and replacing parts "on suspicion". You replace a big expensive part but then you might also change out some cheaper smaller parts too because they may or may not have contributed to the failure of the big part.

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Re: Furnace Limit Switch

Post by A. Spruce » Sun Dec 23, 2018 2:57 pm

emtnut wrote:
Sun Dec 23, 2018 1:31 pm
Hard to troubleshoot the intermittent fan problem ... well, unless you get out the parts cannon !!
That's what the tech said this morning as well, but he seemed like he probably knew the cause. The way he talks, he's very familiar with Coleman systems, so I'm hopeful that things get resolved quickly and easily, and I also want to know exactly what parts they replace to fix the problems.

Like I said, the cycling has been a recurrent issue for the entire time this house has been in existence. The first guy I had look at it spent 4 or 5 hours chasing down the issue and couldn't tell me with any certainty what had caused it, since he'd changed several switches/relays, and such. I believe this will be the first time the valve has been called into play.
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Re: Furnace Limit Switch

Post by A. Spruce » Mon Dec 24, 2018 1:43 pm

HVAC guy just left. He was here about 2-1/2 hours, didn't really find anything.

1 - Replaced thermocouple
2 - Replaced fan capacitor
3 - Lowered the fan speed
4 - Fiddled with a doohickey

Tested gas pressure, it was low, as expected. He fiddled with a doohickey that is related to using propane vs natural gas and said that it was missing an actuator pin. He flipped doohickey over to propane side and adjusted the pressure manually, then taped it in place. This helped the burner flame intensity greatly and should reduce the fan cycling.

He knows it's not fixed, we are now trying to determine if the runaway burner is an electrical or mechanical issue, so I've been given instructions on how to monitor and deal with it should it run away again. Yes, the fan cycled with him playing with the system, so he understands the complaint.

That's where we stand at the moment. We've got heat now, so far with the doohickey fiddling the fan isn't cycling.
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Re: Furnace Limit Switch

Post by emtnut » Mon Dec 24, 2018 5:55 pm

Glad to hear he got you fixed up !

Joking aside, it was probably fairly cold in there, unless you had the oven running !
You're highs are only in the low 50's this time of year, aren't they ?
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Re: Furnace Limit Switch

Post by A. Spruce » Mon Dec 24, 2018 9:10 pm

Yup, it's been 55*F +/- for the past week by day and mid to upper 40's at night. Tomorrow and the next week are supposed to drop into the mid 30's at night, that is when we start feeling it. The house has been around 60-64* when we get up in the morning, so it hasn't been too bad.
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Re: Furnace Limit Switch

Post by Aaron » Mon Dec 24, 2018 10:42 pm

Our December so far here in Minn has been nicer than November was.

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Re: Furnace Limit Switch

Post by Shannon » Mon Dec 24, 2018 11:35 pm

Glad it is at least functioning right now, you guys have been getting some cool weather lately.
So are you saying you are running on propane and not N.gas? If so it sounds like the wrong size orific was used originally?
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Re: Furnace Limit Switch

Post by A. Spruce » Tue Dec 25, 2018 12:45 am

Nope, natural gas, furnace has a gas nozzle, not LP, though it can be set for either type of fuel.
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Re: Furnace Limit Switch

Post by Shannon » Tue Dec 25, 2018 8:56 am

Oh ok It sounded like he changed the nozzle but I was mistaken. Anyways you will be warm for xmas.
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Re: Furnace Limit Switch

Post by A. Spruce » Tue Dec 25, 2018 12:47 pm

No, that is the doohicky thingie. It's a little black cap on the back corner of the valve, when screwed on in one direction it's for LP, when screwed on in the other it's for NG. Inside is a pin for the NG side that depresses the internal valve/plunger and regulates gas pressure. He flipped this cap from NG, which wasn't doing anything, to LP, which would reach the plunger so that the gas pressure could be modulated. Adjusted it until he got the pressure correct, flame instantly improved. He taped the cap in place for the time being.
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Re: Furnace Limit Switch

Post by A. Spruce » Fri Dec 28, 2018 5:16 pm

Since the tech was here on Monday the fan has not cycled and the house warms up much more quickly. It remains to be seen if the run-away has been fixed, we'll continue to monitor that for a while.

Fingers crossed, we're good to go now and the cycling is permanently fixed with the increase of gas pressure/burner intensity.
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Re: Furnace Limit Switch

Post by Aaron » Fri Dec 28, 2018 6:06 pm

So are you still running a jury-rigged furnace or are you still waiting for a part to get it fixed for good?

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Re: Furnace Limit Switch

Post by A. Spruce » Fri Dec 28, 2018 9:14 pm

Still the same, I expect to talk with the tech next week sometime to determine what his final plan is. I'm hoping the part is replaceable and not have to replace the entire gas valve. We may leave it as is, while I don't like the tape holding the adjuster knob, nobody gets in there, so it's not really a problem. Aside from that, we're waiting to see if it runs away again to determine if it is an electrical or mechanical issue. I suspect that it won't run away again any time soon.
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Re: Furnace Limit Switch

Post by Aaron » Sat Dec 29, 2018 4:37 pm

I suppose so long as it's not dangerous then it should be good enough.

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Re: Furnace Limit Switch

Post by A. Spruce » Sat Dec 29, 2018 9:43 pm

I don't believe it to be dangerous, the only problem would be if another tech comes out and doesn't know what's going on. Before this is all said and done, this will be a point that is clarified with the current tech.

I noticed this morning that the fan cycled at least once. For whatever reason, the duct in my room no longer creaks and clunks when the heat comes on/off, so it's hard for me to know when it's running. The furnace is at the other end of the house from me, so I can't always hear the system running if there are other noises going on in the house at the time, which tends to be the norm.

At any rate, the house is warming up much faster and the fan is certainly cycling much less, if not infrequently now. Both big pluses at this point.
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Re: Furnace Limit Switch

Post by emtnut » Sun Dec 30, 2018 12:27 pm

The cycling with the low flame makes sense. If it was borderline hot in there, the switch would have been shutting it down.

Nothing to explain the valve staying open thou. I guess time will tell if it does happen again.

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