10 Year Old Carrier Furnace - Limit switch keeps tripping

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Old 01-17-13, 08:04 AM
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10 Year Old Carrier Furnace - Limit switch keeps tripping

All the sudden I can't keep my heat on. I keep getting an Open Circuit across the Limit Switch. Any ideas where to look?

Things I've noticed:
  1. Main filter a little dirty
  2. Cleaned out a Mud Dauber's nest in the combustion fresh air intake pipe outside.
  3. A little bit of water dripping out near the bottom of the fire box. I'm guessing this is condensation.

Any tips on what to look for?
 
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Old 01-17-13, 11:47 AM
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Describe the sequence of operation of the furnace in order and in detail when you turn up the thermostat.

Also, use a multimeter to measure the voltage at the limit switch to verify that the burners are shutting off when the limit switch opens. You should find 24 VAC on both sides of the limit switch when the thermostat is calling for heat.

If the limit switch opens, you will find 24 VAC at only one side of the limit switch and the burners will shut off when the limit switch opens.
 
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Old 01-17-13, 01:05 PM
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So far this is what I have noted:
  1. Furnace runs
  2. Limit Switch (250F cutoff 40F swing Part# hh12zb250) cuts out
  3. Blower continues indefinitely OR until furnace supply power is turned off
  4. Power back on at any length of time and Blower runs but no heat (limit switch stays open)
  5. If I remove the limit switch, usually just the movement will cause it to reset. BUT it never resets on it's own.

This leads me to believe something's wrong with the limit switch since it's not resetting itself. It appears to be a thermostatic switch that should open when hot and close when cool. It's not doing that.

Curious here, should it ever open in normal use (i.e. without a problem occuring)?
 
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Old 01-18-13, 11:50 AM
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Replace the limit switch as you suggest.


When a limit switch fails, 99% of the time it will open but not close again just as you describe.

I don't recall ever seeing a limit switch that failed to open when it was supposed to.
 
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Old 01-18-13, 03:05 PM
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You need to fix the underlying cause also. A furnace should not cycle on high temp limit.
 
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Old 10-06-13, 05:22 PM
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Four limit switches blown in less than one year,

I have two train furnaces. One furnace is for upstairs, the other is for downstairs. They are about nine years old this February. I change my filters every three months.

I have blown four limit switches in the last year on the downstairs furnace "alone". Never a problem with my upstairs furnace. When I turn the downstairs furnace on it starts up as it should but the beep code sequence goes from slowly beeping to a rapid set of 4 beeps then pauses and sends out another set of four beep codes then pauses again and so on. Only cold air blows out no hot hair.

When I had my furnaces checked by a technician last month both checked out flawlessly accept the innitors so I had each ignitor replaced. I called the furnace repair shop and explained we have the same problem again with the furnace turning on and blowing cold air but it blows no warm air. The man came out yesterday and warm air blow. Before the technician left he told me how to read the beep codes. Four beeps indicates a upper limit switch malfunction according to the trane owners manual. Does anyone have any idea why my furnace is blowing limit switches left and right? This is the second company I have had tune and examine my furnaces yet the same problem persist with limit switches blowing left and right "only" in the downstairs furnace. My up stairs furnace is positioned at the front of my furnace room vs my downstairs furnace is positioned behind my upstairs furnace, would this make a difference in terms of creating additional heat which could blow the limit switch in my downstairs furnace or should my downstairs furnace flawlessly though it is behind my upstairs furnace?

I read the manual that came with one of my furnaces four beep code is definitely the upper limit switch but the downstairs furnace keeps blowing out the limit switch and every technician I have had out has no idea why this is. Does anyone here have insight as to why this may be? In advance, thank you so much for your time and assistance.
 
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Old 10-06-13, 05:26 PM
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I almost forgot, the last time my furnace technician came out he showed me his gauge that shows how hot the limit switch is running. When it was tested the limit switch was 124 degrees. I believe it is a 212 limit switch. Ann assistance on trouble shooting this would be wonderful. Thank you for your time and for reading.
 
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Old 10-09-13, 04:17 PM
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Any chance the flue or chimney is partially blocked causing hot gas to backup in the furnace tripping a sensor somewhere? I have an old Bryant and that happened to me, all I had to do is clear debris out of the chimney base. Did the technician check the chimney?
 
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Old 10-10-13, 01:48 PM
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bet its your secondary

You might have a clogged secondary heat exchanger especially if its lp gas I've replaced more clogged exchangers from carrier than any other manf it has a 20 year warrany but labor is very pricey. I've got 15 years experience in hvac service
 
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Old 10-11-13, 03:44 AM
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When the furnace repair and maintenance company came out last weekend the technician that originally serviced my furnaces brought his supervisor (whom he had run my situation by) back at their office. Upon inspection of the furnace, the furnace company's supervisor noticed that the company that kept replacing limit switches last winter disabled the blower wiring. He reconnected the blower wires, since the blower was reconnected not one problem.

I asked both technicians about it possibly being clogged vents, clogged gas flow in the gas line then all of a sudden a squirt of gas to powerful hits the furnace and over heats the furnace. They checked my gas line and checked all vents to discount that possibility. Both technicians also checked the duck work for a second time and found all air ways clear. Hopefully re-enabling the blower fixed things. I did change filters every three months. the company that serviced my furnaces this year recommended changing filters every other month since basement based furnaces build up more dust than non-basement based furnaces. Hopefully this is the fix (fingers crossed).
 

Last edited by the_observer; 10-11-13 at 04:21 AM.
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