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dennismc 01-25-2013 03:31 PM

Help! What is this!
Does anyone know what this is and what it does? Th.
instruction manual states only that it is a PLA 2 Circuit connector.

My twined furnace has two of them and one was bad, they are on the 24v side of the furnace.

I removed this one and connected the two wires, furnaces run fine now. BUT want to know what they are for and is it safe to run without it.

Supplier states they are not sold separately and only part of the twining kit, (300.00 bucks)

He said it was just a connector and was fine, but want a second opinion.

Thank you

ddawg16 01-25-2013 03:39 PM

Depends on where it was. It looks like a Thermal Switch to me....

If it was attached to the side of your heater where the hot air is, it's job is to shut down the unit if it gets too in when the fan blower is not blowing.

I would replace's a safety item.....though, $300 sounds a bit high for a $30 item.

how 01-25-2013 03:46 PM

Sure looks like a 140F thermal limit with a reset button to me.
It's usually there on furnaces to stop house fires.
Most HVAC supply houses would have replacements.
Under $20.00 in my parts chest.

dennismc 01-25-2013 03:47 PM

It was mounted on a bracket in front of the blower, (one on each unit)

24v power come from transformer, to one side of it, out the other to the aux door switch on the RH furnace, to the same connector on the RH furnace, then out of that one back to LH unit, to the controller board basicly

how 01-25-2013 03:52 PM

If a blower compartment gets too hot (above 140F) the main gas is turned off.

dennismc 01-25-2013 04:42 PM

Ok in all reality, what is the likely hood of it getting above 140? It is a shared twined system, with the cold air return underneath.

Furnanes are mounted side by side, competently open to the returns under them.

It will take 4-10 dyas to get the replacement, sent.

how 01-25-2013 04:50 PM

Motor dies/ cage becomes loose of motor shaft/ overfiring gas valve/ sound baffle insulation comes loose and gets in the fan cage/ drapes or furniture gets moved over return air grill/
were just some of my calls in the last month that caused furnaces that seemed OK one day to overheat the next.
"In all reality what was the likelyhood is an excuse that probably brightens an insurance adjusters day.

dennismc 01-25-2013 04:55 PM

as for covering the returns, not a chance here, they are 24" ducks, two of them , 12' off the ground. The furnaces are 2 years old and only started being used this year. :)

They are in a 4600 sq ft building, open

yuri 01-25-2013 05:26 PM

On a twinned unit if one furnace fan fails then the 2nd furnace can cause it to rotate backwards and suck heat downward. The suction of the 2nd one on the common return duct will suck thru the other fan and turn it, that is why they put a limit there. BYpass it , have a house fire and the fire inspector and ins co will be all over you like bugs on a bumper.

dennismc 01-25-2013 05:32 PM


What about running it with one panel open? To ensure the temp never hits 140?

Its in a business that I am struggling as it is to keep open, and waiting 10 days to get the replacement is not an option?

Marty S. 01-25-2013 06:45 PM

It's a manual reset 140 degree limit. Push the button on top and it should reset. NEVER, EVER, BYPASS A SAFETY LIKE THAT! The next issue will be finding out why the one blower didn't come on and the limit tripped in the first place. In my experience it has always been a control board issue and you'll need to get a new control board for each furnace if that ends up being the root problem. The new boards are not compatible with the old ones for twinning,hence replacing both.

Saw the answer in the other thread. Only power to one furnace would do it. Should be able to reset and be good to go

yuri 01-25-2013 06:59 PM

It is VERY rare that a limit like that trips but it could be bad from vibration or a dud. It should manually reset by pushing the button in and I find it very hard to believe they don't sell it as a seperate part. May need to talk to a more knowledgeable parts person at that supplier.

Marty S. 01-25-2013 07:13 PM

Very rare indeed. Matter of fact yesterday was the first time I ever saw one fail just from being filthy.Old neglected unit on a fast food joint and it wouldn't reset. Had them up and running in no time and customer cost on the part was less than $7.

Funny part about that call was they got a new manager and she was not happy with cold bathrooms so called us to find the problem and fix it. Turns out the w wire was never hooked up at rooftop unit.:huh: No wonder that 25 year old heat exchanger looked like new.

beenthere 01-25-2013 07:22 PM


Originally Posted by dennismc (Post 1101961)
Ok in all reality, what is the likely hood of it getting above 140?

Very good.

Other wise that limit wouldn't have tripped. Your system has a problem, and that safety did its job. now that you bypassed it, you have a dangerous situation.

dennismc 01-25-2013 07:26 PM

Well, I went back up to the studio, re-instaled the switch, Held the button in, (which i did earlier with no success) it clicked on once, then shut off, (Status led that is) so I flicked the button a couple times, and it stayed on and working.,

Will know in the morning if the furnace is still running if it stayed on.

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