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Old 09-16-2020, 08:09 AM   #1
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Do I have a prob?


Here's the scenario: I'm a retired Mstr Electrician. I've done my share of HVAC ckt tracing and minor repair but I'm certainly not an HVAC pro. The heat in my house is gas. Turned it on for the first time this AM. The house was about 4 below the Tstat setting so it came on. It ran about 15 mi then it stopped blowing heat but the fan was still on (flame was out). The LED code from the control board was 'Limit Switch Open'. System has 2, Hi Limit & Aux Limit.
I let it run 5 min or so (but still no heat) then turned off the breaker, checked the schematic and checked pin to pin on the Molex plug (also checked each switch separately). At this point the ckt was closed (<2 Ω). I put it all back together and now it seems to be working properly.
So the questions: Do I have a prob or is it not unusual for a switch to hang up after being off for 5 months? I did remove the connectors from both switches to check them, could this have cleared any oxidation and reestablished the connections? Should I routinely change the two limits? The Hi limit appears to be original (16 yrs or so) but the aux limit has been changed at least once.
Opinions/comments?
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Old 09-16-2020, 08:18 AM   #2
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Re: Do I have a prob?


Post the brand name and model # of the furnace.

Is it a counterflow/reverse flow unit with the air blowing down under the furnace and into a crawlspace?

Aux limits should never get used except when the furnace fan fails so I wonder why yours fail? If they fail from vibration from being on the side of the fan that could be a issue and yeah replace it.

Post a pic of where that one is.

Molex connectors can be wonky. Pull it apart and carefully look at the pins with a magnifying glass and see if one is pushed back too far and not making good contact. Push it forward if so.
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Last edited by yuri; 09-16-2020 at 08:21 AM.
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Old 09-16-2020, 08:36 AM   #3
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Re: Do I have a prob?


Yes it's a reverse flow. All the ducting is under the house and all the vents are floor vents. Take a look at the Aux limit. it is not the original and, as you can see, it is not secured under the tab. It has only one screw holding it down and is simply wedged in place, definitely could be subject to vibration.
I did check the Molex pins, all tight. Sorry about the sideways pics, obviously the left side is the top
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Last edited by stevemiller000; 09-16-2020 at 08:42 AM.
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Old 09-16-2020, 08:43 AM   #4
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Re: Do I have a prob?


Looks like a Nordyne unit and I have seen quite a few of them.

I suspect there is residual heat left over once the fan shuts off and the limit may get brittle and yeah vibration does not help so replace it.

You should be able to find the OEM part and if you are in a mobile home there are sites that specialize in mobile home parts. They also use them in pre fab homes but you can still find the same furnace at the other sites.

Also try supplyhouse.com and americanhvacparts.com
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Old 09-16-2020, 09:36 AM   #5
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Re: Do I have a prob?


Thanks Yuri, ordered both OEM. Fortunately it's really not that cold yet so if this thing goes out again we're ok.
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Old 09-16-2020, 09:44 AM   #6
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Re: Do I have a prob?


Check your chimney where it exits thru the roof. You should have a pipe within a pipe concentric type chimney.

Make sure the intake screen has no debris or spiders webs etc partly blocking it. That furnace is wonky/sensitive and prone to pressure switch tripping problems. The least amount of restriction can upset it.

Lucky 4 U I have worked on a few and know it pretty well.
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Old 09-16-2020, 09:48 AM   #7
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&#x1f44d; Will do, thanks
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