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richapple 02-05-2013 11:51 PM

Flue Limit Switch removal safe?
I won't go into the saga of how I've arrived at having to ask this question (see QUESTION below to cut to the chase because yes, I have gone into a bit of the saga), but an HVAC service guy who is probably excellent for 99% of his calls had a rough day installing a new Gas Valve on my old furnace. What I think is called a "flue limit switch" had additionally fried, related-to or coincidental-to the gas valve's pilot valve sticking open sometimes, which was the initial problem.

The flue limit switch (shown as maybe OPT on the schematic?) was a smallish plastic thing that once fried, really put out an intense chemical odor (like melting plastic). It sat above all the burner stuff mounted to the round blower housing in its own little sheet metal box, and from what the tech explained, what it does is makes sure there isn't too much heat, and shuts off the burners if there is.

Well, he was going to order its replacement, but in the meantime connected the wires, bypassing the switch, so he could complete the furnace work at hand and get it functioning. Unfortunately he connected the wires exactly wrong, so for hours he was convinced that once the pilot was lit, the sensor was not "rectifying flame", and that's why it would all shut down.

I kept trying to point out that the shutdown was happening way fast (like right away after the pilot lit) compared to how long it previously would seem to "try" to rectify that there was flame before turning the burners on. Finally I guess I said it the right way, pointing out that it was as if that flame check is happening and then some further check is not passing its test, and I pointed up at the jumped wires for that fried/removed piece.

Okay, sorry to have told so much of the saga, but early on he had said that he, as a licensed HVAC guy, couldn't connect those wires to bypass that safety switch, so we basically had a "wink wink" agreement that I connected the wires to bypass the flue limit switch (if that is what it's called). Anyway, when he switched them after my suggestion and considering the schematic again, all worked fine and the plan was that he'd get the part and right after the holidays come back to install it.

Bottom line now is that the part isn't available, and they (local company he works for) sent their invoice (I called several times in January saying there's supposedly a part still on order, and pointing out that I hadn't paid them a dime yet), and on the invoice it reports "system operating safely" and "replacement optional switch is no longer available".

QUESTION: How does this sound to the experienced HVAC people here? Do you recognize that function/positioning of such a switch? ("Flue limit switch" is just the phrase I found online, but seems to be it.) If you do know this backup safety switch, is it something that experienced people would say can be removed and the furnace is still safe?

Or should I not trust the local company saying it's safe and pursue maybe a generic replacement (since the exact isn't available)?

The end of the saga for the service call, of course, is that even a good tech who has a bad day gets pretty expensive, and in his flailing I did also get to buy an ignition control I'm convinced was never a problem. (A universal one from his truck didn't get it to work, and then when he went to the parts store and that ignition control didn't get it all to work either, with all still behaving exactly as my ignition control - which had always lit the pilot - had been. I know the parts store, and there are no returns. Again, switching the jumped wires bypassing the flue limit switch is what got it all working.)

Any insights appreciated, but I won't be able to supply a photograph of the fried and smelly little part 'cuz the tech took it with him...



beenthere 02-06-2013 04:32 AM

Brand and model number of furnace would be helpful. So would a pic of where this switch is/was mounted.

richapple 02-06-2013 02:15 PM


The furnace is a York PIECD12N04901, and it is over 20 years old.

I can take a photo later, but the switch that's now gone was in a small (3X4 2 inch deep?) sheet metal box mounted on the housing, which looks like this (photo just from internet):

...and if it was mounted on this image, it would be on the slightly curved surface just up to the right of the American flag decal. Four heavy wires go into the sheet metal housing where the switch was, two red and two black.

Like I said, I don't have the switch at all 'cuz the tech took it with him, but I think mine must have had four terminals, but the only switches I could find images of on-line show only two terminals. I include the following image of a switch just to show the solid plastic nature of it. The following image just from the internet has a red reset button, and I can't say for sure if the one removed from my furnace had that or not...


Again, I can get a photo of the fan housing, sheet metal box, and the wires running into the sheet metal box later if it would be helpful. Inside the sheet metal box now are just the wires connected (jumpered) and covered with black electrical tape.


bobinphx 02-06-2013 02:43 PM

The flue sensor is there for a reason. Its a safety device. NO NOT RUN THE SYSTEM WITH THIS DEVICE DEFEATED. You are asking for trouble. the switch burned out for a reason. THE FLUE GOT TOO HOT!!! you are putting the house and people in the house at risk. The professional who bypassed this should not have done so. contact the company and report this and request that it be fixed asap.

They may not have the exact part, but it is critical that any replacement parts have the same operating charitaristics. (ie must open at the exact same temp, same voltage rating etc).

I would contact a second company for an opinion and availablity of the part.

Please be careful!!!!

beenthere 02-06-2013 03:40 PM

If it was mounted on a housing like that in the pic you posted. Its not a flue sensor. Its an aux high limit. it may have been a combo limit and fan switch. Any York dealer should be able to get it for you.

PS: Thats a 1, not an I

richapple 02-06-2013 03:49 PM


So the York numbers as in "P1E..." (perhaps I had "pie" on my mind when I'd copied it down)

Thanks for the advice and what you say about it being the combo makes a lot of sense. (number of wires and the fact that when it melted down the blower went on/stayed on and the horrendous melted plastic smell filled the house, which at first I thought was gas. That was when I shut the whole thing down and called the HVAC people.)

I'm going to get all the info I can from the company that did the work, and then see what specs I can get from York directly. If the part isn't made anymore, as the HVAC company said, I'll hope they (York) have a replacement with the same specs or a recommendation.



how 02-06-2013 04:25 PM

The old part will usually have the limit temp stamped on it. Aux limits are there to stop the fan compartment from getting too hot and come in many different temps.. (Dirty air filters, squirrel cages or a/c coils, faulty motors or fan controls are common causes).
4 wires connected to it is different as is having plastic melt in a fan compartment. I've never seen a fan/limit OEM installed inside a fan compartment but it might explain the 4 wires.
Do you know where those 4 wires run back to?

richapple 02-06-2013 04:34 PM


Later I can track where those wires go to, but the switch in question is (was) mounted on the outside of the fan compartment. Not inside.


how 02-06-2013 05:20 PM

The picture shows otherwise. The fan compartment is that whole compartment within the furnace wherein the fan cage is secured.

richapple 02-06-2013 05:31 PM

Hmmm, how,

Sorry - I will have to take a photo of the small sheet metal box mounted within my furnace, but I think on the outside of the fan housing. I'd just grabbed the photo in this thread from the internet to show a fan housing 'cuz that's what I pictured. I'll do the actual photo later tonight, but I guess I was thinking the "fan compartment" is the curved sheet metal within which the actual fan spins...

Thanks, and sorry to maybe confuse with the photo shown!

richapple 02-07-2013 12:20 AM

Perhaps what the removed switch actually is can better be deduced from where the wires that came from it go.

The following photo shows what I was calling the "fan housing" and the small sheet metal box attached to it (in which the switch lived, but now inside are just the wires and black electrician's tape connecting them to each other to bypass the switch).

The red wires from the empty box (had been from the switch) both go into what I think might be a Fan Relay Transformer (has Rheem 47-19807-01 for the main unit), and in the following photo it is in the metal box to the far left with the wires going in the top and other stuff hanging on it to the right. Red wires look to come from there to what might be the "fan limit switch" (beneath the bottom of the round blower assembly).

The black wires go to the gas valve (one to the M-Main and one to the C-Common, zero to the P-Pilot), and the gas valve is pretty obvious in the above photo.

If this cinches it for what the thing I'm looking to replace actually is, that could help with my contacting York, etc. I will still try to find out the specifics printed on it, but that assumes the tech didn't throw it away, the "limit temp" is still readable on it, and the HVAC company can/will give me some more information.

My gut has told me from the beginning that just 'cuz the furnace works without this switch does not mean it's safe, and the one thing I haven't heard as an answer to my question here in this excellent forum is, "Oh yeah, I'm a HVAC guy and you really don't need that switch." So far I've heard the opposite, and yes, I am going to get it handled. More info from anyone who knows would be good. At this point it sounds mostly like that what I've got there (or more accurately - am missing there) is a combo "aux high limit" and "fan switch".

Thanks for any further info,


beenthere 02-07-2013 04:29 AM

Its not safe to use it without that aux fan limit.

richapple 02-13-2013 11:19 PM

An update to this is that with my check to pay the invoice of the HVAC company that did the work and is saying it is safe to not replace the part, I did request any info they had (or the fried part itself if the tech hadn't tossed it) so I could find a replacement or a comparable part.

They sent a pdf of the York parts list for my furnace (doesn't seem to list the part) and a request that I take a picture of the schematic and send that back. Near the top center of this schematic is the part that was removed and hard wired, just down to the right from the hand-circled "ALC" (Auxiliary Limit Control). The part shows "OPT" and has the two red wires/two black wires... The Component Code in the lower left shows that as "Optional Component", and it clearly does stuff based on temperature rising.

The HVAC company is going to ask York directly what that "Optional Component" does and why it would have been in the system, so perhaps it is still possible that it is "optional" and not a safety issue to have had it removed? Seems odd, but we shall see...

hvac benny 02-14-2013 01:32 AM

Good job. I'd also suggest involving your local gas authority.

I recently came across a high efficiency furnace that a local tech thought didn't need the condensate trap/collector box. A quick call to my local inspector and now someone's peanuts are in a vise.

richapple 04-06-2013 12:14 AM

I know this is an old thread, and while I'd like to include a "conclusion", it's still in the works. I'm communicating directly with York after getting photos of the missing part and a likely part number off of it (I say "likely" 'cuz it was a neighbor's identical furnace for both the photos and the numbers, and while he was kind enough to let me remove the housing from the fan compartment, removing the part from the housing to get an even better photo or a "for sure" read of the numbers felt like asking too much).

York (Johnson Controls in Oklahoma) did request the Serial Number so I was back in looking at the data plate inside the furnace, and it turned out that where beenthere had suggested the "I" in the first three characters of "PIE..." was actually a "1" - well, not so much. It is an capitalized "I" for sure. (An I for an I?)

Stay tuned and I'll eventually get to the bottom of it all and find if anyone's peanuts need to end up in a vise. In the meantime, gladly we've got plenty warm weather here on the west coast...

Uh, photo of the same part on a different furnace is a bit blurry, and the guess at the numbers printed on it was "24V *60704A0-*33, *H26-*85-*C55-*105, *87V".



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