Furnace Limit Switch

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A. Spruce
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Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 12:04 pm

How does one go about testing a limit switch? Yesterday, the furnace decided it wasn't going to shut itself off at the desired temp. This switch has been replaced at least 4 times in the last 20 years, twice due to catastrophic failure as we had yesterday and twice because the system will cycle on/off every few minutes as it's coming up to temp. FWIW, the system has been cycling as mentioned for the past few years. Owner isn't big on maintenance, even though she has live with a contractor for the past 16 years. :roll:

So, to confirm it's the switch AND to figure out which switch it is, I'd like to test it. Anyone know what kind of voltage or resistance I should read across the terminals? I am assuming that it will be one of these two switches. The paper tag you see laying under the switch on the right says "cooling speed", it was laying in between the terminals when I opened up the unit, so I assume it was once attached to this switch.
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Aaron
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 12:42 pm

A. Spruce wrote:
Thu Dec 20, 2018 12:04 pm
How does one go about testing a limit switch? Yesterday, the furnace decided it wasn't going to shut itself off at the desired temp.
You already ruled out the thermostat not calling for heat after the desired temp was reached?

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 1:01 pm

Thermostat was replaced a year or so ago. In 2015 I had a company out to replace a section of flex duct that got flooded when the weep tube clogged. They cleared the tube as well. I had an injury at the time, which is why I didn't do the work. I told them that the system cycled, he tested everything out and said that the thermostat could be partly to blame, which is partly why it was changed. He tested out the rest of the system and couldn't find any issues.

Past repair receipts show that the limit switch has been replaced multiple times.

Just watched several vids on YT about limit switches, they either have continuity (good) or they don't (bad). One of the videos also said how to identify the limit switch, for those who may need this in the future, it has a number on it that indicates it's limit temp and it's reset temp. In my case the number is F110-20F, it kicks in at 110*F and resets when that temp drops by 20*.

To test the switch, with the furnace cold turn off power, pull the two wires off the switch and put an ohm meter across the terminals. A reading of 0, or no resistance, indicates the switch is closed. Infinite resistance indicates that the switch is open. My switch is infinite/open.

I'm a little irritated that the service techs couldn't diagnose the cycling problem to the limit switch, the couple vids I just watched stated that the most common reasons for failure is restricted airflow, usually from dirty filters, and cycling. One tech wanted to replace the gas pump to the tune of $400, which didn't sound like the right course of action when he said it.

I should have a new limit switch tomorrow, I'll update when it's installed.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 1:46 pm

But your actual problem is that the furnace stays on, even if the room temperature is exceeded on the thermostat? It's like your stat is not opening the circuit made by the control wire (R) and the heat call wire (W). If you use your volt-ohm meter, you should test for the presence of 24VAC on the control board between the white and blue/black/common terminal screws (from the stat). If you have your stat set for say, 70F, and you know it's like 80F inside, then your stat isn't working or is not set correctly. Or maybe there is a short in the thermostat wire between the stat and the furnace (that has happened in my lifetime).

I don't know how a limit switch would keep your furnace running if the stat is working correctly. AFAIK, the limit switches will open if the bimetal contact inside exceeds a certain temp. They're sort of like thermal fuses. But that shouldn't happen in the first place if your stat is shutting off your furnace, which it sounds like it isn't.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 2:36 pm

Ok, let me finish lunch and I'll check. I assume I need to be checking these wires at the furnace, not the thermostat?

Thermostat is a battery powered head unit that unplugs from the wiring harness screwed to the wall. Batteries seem fine, since the numbers are legible. Weak batteries results in the numbers blanking out, the thermostat has worked fine otherwise. should this make a difference in the testing of the wires above?
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 3:21 pm

Honeywell RTH2300/RTH221 series thermostat
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 4:24 pm

Well the stat has to be mounted on its plate so you can diagnose what calls it's sending to the furnace. You do that testing at the furnace control board.

If you determine there's 24VAC on the white and common/black/blue terminals but there is no "heat on" indicated on the display, then the next step is to lift the white wire from the control board screw, and test 24V from that lifted wire and the common terminal on the board. The presence of 24V would rule out your furnace (as it's doing what it's supposed to). If there is no voltage, then test for 24V on the now-vacant white screw on the board. If that registers voltage then there is something screwy with your board.

You can isolate the thermostat by removing it from the plate. Then see if there's continuity from the white to red wires at the control board. There shouldn't be. If there is you have a short in the stat wire. If not then your stat has like a stuck relay shunting the heat on.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 4:33 pm

Ok, just to make sure I'm reading this correctly, pull the stat, probe the wall connector white to black/blue to see if there is 24v. If yes, then plug in stat and go to furnace and test white to red wires for 24v.
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 4:40 pm

If you look at the pic of the wall mount, you'll see that it's using red as the common, white, yellow, and green. There is 30v across w/y/g, using red as the common.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 4:43 pm

A. Spruce wrote:
Thu Dec 20, 2018 4:33 pm
Ok, just to make sure I'm reading this correctly, pull the stat, probe the wall connector white to black/blue to see if there is 24v. If yes, then plug in stat and go to furnace and test white to red wires for 24v.
No. Leave the stat alone--mounted to its wall plate with your desired program set. Do all your volt-ohm meter testing at the control board inside the furnace.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 4:47 pm

Ok, stay with me here, testing now.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 4:50 pm

White is heat, yellow is A/C, and green is the fan. The stat will always send a call for the fan along with either heat or A/C usually. If you're using heat (which you ought to be in December), you should never see any voltage on yellow at all. Red is constant 24vac. The blue is usually the common.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:00 pm

Yeah, I have to figure this stuff out because the heat and ac are separate units so the stat wires are split at the furnace.

With the stat off, I'm getting 24v from red to w/g/y
With the stat on, no call for heat same as above
raise stat for more heat, red to w - nothing, red to g/y = 24v
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:03 pm

Pic #2 above shows the wiring at the furnace. There is no control board, just wires. Ok, maybe there's a control board under a shield somewhere, but it's not right out there saying "hey, look at me!!" :lol:
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:05 pm

No control board, just relays and transformer in a box that the wiring diagram sticker (see above pics) is attached to.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:06 pm

A. Spruce wrote:
Thu Dec 20, 2018 5:00 pm
Yeah, I have to figure this stuff out because the heat and ac are separate units so the stat wires are split at the furnace.
Well the A/C condenser control wire is usually tied in with the yellow at the control board (two wires under one screw).

With the stat off, I'm getting 24v from red to w/g/y

Where? At the furnace board?
A. Spruce wrote:
Thu Dec 20, 2018 5:00 pm
With the stat on, no call for heat same as above
raise stat for more heat, red to w - nothing, red to g/y = 24v
Again, where? I don't have details of where you're testing this...

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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:09 pm

A. Spruce wrote:
Thu Dec 20, 2018 5:05 pm
No control board, just relays and transformer in a box that the wiring diagram sticker (see above pics) is attached to.
A control board in the furnace looks like this:

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You may need to remove another panel to see it. All those screws on the bottom left in that pic above are where your stat connections are terminated.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:12 pm

Tests were at stat wire connections at furnace.

When I turn the stat up for more heat, I can hear it kick the relay on. Turn it down, can hear relay click off. Stat has a click as during these functions
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:14 pm

No control board. This is a 20 year old trailer, mobile home if you want to get all fancy, and the furnace is the cheapest of the cheap Coleman, it was crap when it was installed and it hasn't gotten any better with age. Not to say that it functions any worse than it ever has, only that every expense that could possibly be spared is spared in the manufacture of trailers, et.al.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:16 pm

A. Spruce wrote:
Thu Dec 20, 2018 5:14 pm
No control board. This is a 20 year old trailer, mobile home if you want to get all fancy, and the furnace is the cheapest of the cheap Coleman, it was crap when it was installed and it hasn't gotten any better with age. Not to say that it functions any worse than it ever has, only that every expense that could possibly be spared is spared in the manufacture of trailers, et.al.
Okay, but there still has to be terminations where the stat wires are connected to it. I assumed you had a control board, but whatever... the troubleshooting logic of everything I described above is exactly the same. Just substitute "furnace connection" for every instance I wrote "control board".

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:22 pm

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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:23 pm

Looks like these may be your state connections to the furnace:

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You can test with the wire nuts removed.

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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:25 pm

Does your thermostat have an indicator on its display to tell you when the heat is on? On my Honeywell it reads, "Heat On" when the furnace is running.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:32 pm

Yes, those are the incoming stat wires.

Stat wires as they connect to furnace:
White to main heat/ac relay
Red to inline fuse, changes to blue and connects to transformer
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 5:34 pm

Yes, when the call for heat goes out, "heat on" appears on the screen. When system at idle, no indication of setting, you have to look at the manual switch on the bottom of the stat to see that it's either in the heat or cool position.
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Re: Furnace Limit Switch

Post by Aaron » Thu Dec 20, 2018 5:35 pm

Wait a sec... Am I seeing your white and red wires wirenutted together at the furnace?

Can you sent me a close-up pic of those connections? It's blurry in that original shot:
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:37 pm

Aaron wrote:
Thu Dec 20, 2018 5:23 pm
Looks like these may be your state connections to the furnace:

You can test with the wire nuts removed.
That is where I tested the stat wiring. I redid all the connections, since the stranded furnace wires had rat nest ends on them.
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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:38 pm

No, the wires are individually connected.

White to white
Red to orange
Green to green
Yellow switches to red out to the AC, returns yellow and into the furnace yellow
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Re: Furnace Limit Switch

Post by Aaron » 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.

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Re: Furnace Limit Switch

Post by A. Spruce » Thu Dec 20, 2018 5:45 pm

Here's the control panel. ;) :mrgreen:

Going by wiring diagram, I'm guessing that far right is the main heat/ac relay, center is transformer, left relay is fan control.

There is also a relay down where the stat wires enter the unit, not sure what that one is about.
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