Limit Switch Opening

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  #1  
Old 11-10-15, 02:03 PM
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Limit Switch Opening

Hi - I have a Carrier 58CVA furnace and am having an issue with my limit switch opening after about 20 minutes of running the furnace. Never had this issue before, just started happening this winter time - furnace is about 12 years old.

I've checked the following
1) All vents are open in the house
2) filter is brand new
3) A/C coil was cleaned and still happening
4) There is no blockage on the exhaust pipe, went on the roof and checked

The sheet metal above the heat exchanger gets hot to the touch, not sure if it always did that or not.

The limit switch that is causing a problem is right by the exhaust pipe. Checked continuity. Furnace is 80% efficient.

I ordered a new limit switch to replace, but i'm not sure if thats the issue or not. Any suggestions?

The furnace runs fine for a degree increase, but once it starts to rise 2+ degrees furance goes into a 33 then 13 mode and shuts down because the limit opens.

Anything else I should check for? Can I move the limit switch away from the exhaust pipe a little bit to see if that helps?
 
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Old 11-10-15, 02:08 PM
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You need to check the blower wheel to make sure it is clean.
 
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Old 11-10-15, 02:14 PM
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The limit switch that is causing a problem is right by the exhaust pipe. Checked continuity. Furnace is 80% efficient.
Is that switch on the actual heat exchanger plenum or near the burners ?
 
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Old 11-10-15, 02:25 PM
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its not in the heat exchanger, mounted on the exhaust pipe

See image (yellow arrow): http://imgur.com/u4oPYlI
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Last edited by PJmax; 11-10-15 at 04:02 PM. Reason: added pic from link
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Old 11-10-15, 02:33 PM
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I took the filter out and can see the blower wheel and it looks good
 
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Old 11-10-15, 04:30 PM
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That sensor you are pointing to looks like the DSS (draft safeguard switch). It has nothing to do with the inside air blower. It's a problem in the exhaust ducting overheating.... possibly from a draft inducer blower problem or the burner is overfiring.

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Carrier thoughtfully put all the limit sensors in one string.
This way each one has to be manually checked.
FRS-1 = flame rollout switch (manually resettable)
DSS = draft safeguard switch (automatically resets)
BVSS = blocked vent safety switch (manually resettable)
LS-2 = limit switch (automatically resets)
LS-1 = limit switch (automatically resets)
FRS-2 = flame rollout switch (manually resettable)

Your manual...
hvac partners/docs/1009/public/0e/58cva-3si.pdf
 
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Old 11-10-15, 04:41 PM
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Thanks for the assist! Any suggestions on troubleshooting? Sounds like this is not a huge deal because the furnace itself is not overheating, just the exhaust?
 
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Old 11-11-15, 06:53 AM
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Do you have any way to check the temp of the exhaust vs the operating temp for the switch?
 
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Old 11-11-15, 09:43 AM
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I'll pick up a temperature gun later today and check the differences. I think the switch is supposed to open at 200 degrees
 
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Old 11-11-15, 08:49 PM
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The draft switch gets to 200 when it trips. See image. Exhaust gets to 230. Does this sound right? Just seems like its not pushing enough air out...but why would this be happening now when this has been working fine

http://imgur.com/JbZ0q9U
 
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Old 11-12-15, 05:41 AM
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Sounds like it's not a switch problem. As to why the exhaust is overheating, it's possible the fan motor isn't turning fast enough, there's something wrong with the fan blades (dirty, worn, etc.), gas pressure too high for some reason, or maybe a restriction in the heat exchanger. Those are things I can think of quickly. I'm sure there are other possible causes as well.
 
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Old 11-12-15, 06:33 AM
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Have you done anything to reduce the make up air i.e. Enclosed the furnace in a room?
Just a thought.
 
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Old 11-12-15, 12:20 PM
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Nothing has changed with the location of the furnace
 
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Old 11-12-15, 12:23 PM
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I'll pull the exhaust fan out and see what it looks like. It was just changed 2 years ago though.

I believe my hvac guy checked the gas pressure and it was normal.

Can I just move the draft switch away a little bit? Is there any harm with the exhaust getting that hot?
 
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Old 11-12-15, 12:53 PM
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The limit is doing its job,find the cause, could be poor draft as it was in my case.
 
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Old 11-12-15, 05:04 PM
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Today I didn't have any issues and it ran for an hour, temp by draft switch was right around 190-200. It was 10 degree colder outside today...wonder if that made a difference for the flow of air up chimney
 
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Old 11-15-15, 10:45 AM
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Just to close the loop on this one....there was a blockage in the chimney. The hot water tank flue was sticking all the way through in the chimney and was above the furnace flue. Therefore, caused 50% of heat to have trouble escaping. Not sure why this issue happened now but all is well. Thanks.
 
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