Furnace Limit Switch

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A. Spruce
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:50 pm

Aaron wrote:
Thu Dec 20, 2018 5:44 pm
Ok. One step at a time, please.

With the stat mounted on its plate, with the heat turned on and the associated "Heat On" indicated in the display, are you testing 24VAC between those white and red splices?

Wait until "Heat On" disappears from the display, and tell me if you still see 24VAC between those white and red splices.
With the stat off, stat switch in the off position (heat/off/cool) I'm getting 24v from red to w/g/y
With the stat on, switch in "heat" position, no call for heat, same as above.
Raise stat setting for more heat, red to w - nothing (assume because white is now energized), red to g/y = 24v
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Aaron
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:53 pm

Okay, I screwed up. You need to test for 24V between any given wire and a NEUTRAL or NEGATIVE or GROUND. If you had a control board it would be the "C" or common screw. You don't have that so I don't know what constitutes that in your furnace... the chassis perhaps? Touch the black lead to the chassis and the red to a connection.

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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:55 pm

For example you should always see 24VAC between the red wire and chassis. That should be constant.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:59 pm

Should it look like the 4th of July in my furnace room right now? ;) ;) ;) ;) :mrgreen:

Let me go do a chassis ground and touch the stat leads, see what I get.
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 6:03 pm

Ok, black lead (tester) on chassis, the red stat wire gives me a reading, I can't say what it is, my analog meter isn't clear on that. When touching the g/w/y wires from stat, with chassis ground, the needle gives the tiniest of flutter.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 6:10 pm

What about the AC control line, the one that goes out to the condenser. That should either be red and white or red and black.

Wired correctly, the black or white is the neutral/ground/negative, and the red is the 24VAC.

The red is usually tied to the yellow from the stat.The white or black should be tied to the neutral/ground/negative source.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 6:23 pm

Stat wires as they connect to furnace:

White to white connects to main heat/ac relay
Red to red to inline fuse, changes to blue and connects to transformer
Green to green - Blower relay
Yellow - out to ac from stat, returns to furnace and goes to the heat/ac relay
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 6:24 pm

Last post on page 1 has pic of the relay box under the wiring diagram which is in the original pix at the top of the thread.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 6:27 pm

I'm thinking that yellow wire from the furnace (NOT the stat yellow) is your ground... you get 24vac from that to the red, right?

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 6:36 pm

the only wire with any discernible reading is the red wire
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 6:36 pm

And that's from chassis ground
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 6:45 pm

[/quote]
A. Spruce wrote:
Thu Dec 20, 2018 6:36 pm
the only wire with any discernible reading is the red wire
And that's from chassis ground
Okay, you're gonna need to get a HVAC tech out there who has a digital meter and tools to troubleshoot your stat circuits... I'm not sure what connections you've got going on. It seems like you have the standard color codes of wires doing the standard things, but those splices with the furnace are screwy to me. You need a negative/ground source to test for the calls.

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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 6:50 pm

What is this?

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 6:55 pm

I have absolutely no idea. Those would be the incoming stat and leg to external AC. Why there is aluminum and tar tape on that spot is anybody's guess. I will dig into it and see, give me a few minutes.
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 7:00 pm

That is a splice in the AC leg, I would assume that the installer shorted his length of wire and added a bit more to get up to the rest of the stat connections.
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Re: Furnace Limit Switch

Post by emtnut » Thu Dec 20, 2018 7:12 pm

I'm not 100% clear on the problem.

You're furnace came on, room came up to temp, and the furnace ran and ran bringing the heat higher and higher ?
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 7:15 pm

emtnut wrote:
Thu Dec 20, 2018 7:12 pm
I'm not 100% clear on the problem.

You're furnace came on, room came up to temp, and the furnace ran and ran bringing the heat higher and higher ?
Correct
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 7:15 pm

A. Spruce wrote:
Thu Dec 20, 2018 7:00 pm
That is a splice in the AC leg, I would assume that the installer shorted his length of wire and added a bit more to get up to the rest of the stat connections.
Okay. Well you have some odd and seemingly unnecessary splices in there, and my suspicion is that you have an incorrect splice or a bad splice. There's some sort of shunt from the red to the white that is keeping your heat on, but you can't really tell with your meter because you have no known reference (neutral/ground) that you can measure the voltage (with the black lead of your tester).

Get an HVAC tech out there with a good digital meter and troubleshooting skills to sniff out where the problem is. Maybe they will do you a favor and label those conductors for you (R, W, Y, G, C).

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Re: Furnace Limit Switch

Post by emtnut » Thu Dec 20, 2018 7:25 pm

A. Spruce wrote:
Thu Dec 20, 2018 7:15 pm
emtnut wrote:
Thu Dec 20, 2018 7:12 pm
I'm not 100% clear on the problem.

You're furnace came on, room came up to temp, and the furnace ran and ran bringing the heat higher and higher ?
Correct
It could be wonky wiring like Aaron said.

It could be that t-stat as well. First make sure it's in manual mode and or that the programming hasn't changed (that's caught me before too)

If all that is good, I'd try to run the furnace again, and see if it still happens. On most of those t-stats, you can hear the relay 'click'. See if you hear the startup click, and then the dropout click when temp is reached.
(remember that the fan will still run until the furnace cools down).

If you get more than a degree or 2 above the setpoint, and the furnace (and flame) are still going, try pulling the stat off (like in the pic you posted of it). See if it shuts down then (flame, then furnace after a delay).


Q- How did you turn it off last time when this happened ?
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 7:49 pm

When it did not shut off per temp setting, first we lowered stat setting, I pulled it the stat off the connector, it continued to burn. I had to turn off the gas to shut down the system. I left everything alone after that, the fan shut down by itself shortly after the flame was terminated.

In playing with it today, the stat and furnace relay clicked when I upped the stat setting and dropped the stat setting in relation to the ambient temp.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 7:56 pm

Well if you removed the stat from its plate on the wall, all those stat wires should be OPEN. No circuits made.

So my hunch is your stat is fine and there is an issue with screwy wiring/splices or whatever control circuitry in your furnace (stuck relay, a short, or something).

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Re: Furnace Limit Switch

Post by emtnut » Thu Dec 20, 2018 8:06 pm

A. Spruce wrote:
Thu Dec 20, 2018 7:49 pm
When it did not shut off per temp setting, first we lowered stat setting, I pulled it the stat off the connector, it continued to burn. I had to turn off the gas to shut down the system. I left everything alone after that, the fan shut down by itself shortly after the flame was terminated.

In playing with it today, the stat and furnace relay clicked when I upped the stat setting and dropped the stat setting in relation to the ambient temp.
OK, it's the gas valve. It's sticking in the open position.
Either a bad valve, or dirt/grease build up, or faulty.

I've heard that whacking them with a mallet sometimes fixes it for awhile.
I've never serviced one, so not sure on the cleaning procedure.
Could try youtube maybe ?
If you can get a pic of it, i'll see if I can find anything online as well.
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 8:21 pm

Just finishing up dinner now. I'll grab a few pix later tonight and get them posted.
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Re: Furnace Limit Switch

Post by emtnut » Thu Dec 20, 2018 8:29 pm

A. Spruce wrote:
Thu Dec 20, 2018 8:21 pm
Just finishing up dinner now. I'll grab a few pix later tonight and get them posted.
OK, I feel like crap today (most of the week, on and off), ... neo citran and beddybyetime :mrgreen:

I'll check the thread tomorrow morning.

If the gas valve is still stuck in the open position, even with the t-stat off/disconnected, the flames will start right up ... followed by the fan shortly after.

I mean, I wouldn't want you to walk around in a sweater or anything :lol:

#CaliforniaHeatingProblems :shock:
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 9:08 pm

I'll do the pix tonight, but it will be tomorrow before I can mess with relighting the pilot and whacking on the valve with something big and heavy. :twisted: :mrgreen:
emtnut wrote:
Thu Dec 20, 2018 8:29 pm
#CaliforniaHeatingProblems :shock:
I know, first world problems, right? :lol:
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 10:19 pm

Coleman Evcon Presidential
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Gas Valve (natural gas)
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ID tag
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 10:20 pm

I will try to fire it in the morning, but I won't have the limit switch, so I don't know if it will go or not. Most that can happen is it goes boom, right? :| :? :shock: :lol: :lol:
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 11:23 pm

Bad gas valve! Really. Like a stuck solenoid. And there's a sensor to detect it so it must keep the burners lit as a safety precaution to burn the gas instead of allowing it to leak. HUH. Learn something new every day.

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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 11:29 pm

BTW, I had a Coleman furnace for 25 years. The one I have now was made by a company called Payne. I am really not so sure there is much difference between brands. They all seem to work with commodity parts.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 11:30 pm

The roommate pulled her file on it and looking at the receipts for the past 25 years, it's had relay's, fan switches, and limiter switch issues. Even saw a transformer in there somewhere. She's taking the file and the limiter switch to the parts house, we'll see what she brings home.
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