Troubleshooting limit switch

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Old 12-27-11, 09:58 PM
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Troubleshooting limit switch

I cannot figure out the problem causing the high limit switch to keep tripping on my Old Comfortmaker furnace (GUHC60A012CIN). It started a couple of weeks ago, so I:
ē Checked the filter (it wasnít too bad, I changed it anyway),
ē Checked the blower fan & housing (dirty, but not bad-I cleaned it completely)
ē Checked the AC coilóitís clean on the outside. I canít access the inside without completely dismantling the furnace.
ē Made sure all of the return & supply vents are open & unobstructed
ē Made sure the flue is clean & unobstructed
ē Pulled the switch out & inspected-I cleaned it, though it looked fine
ē Vacuumed & observed the burners-they look ok (to an untrained eye-theyíre not blowing about much, no flame rollout)
ē Checked the resistance on the switch-0 ohms
ē Checked the voltage of the switch, itís 0 volts across the switch when the furnace is running
ē Ran the furnace:
o without the filter-it still tripped
o without the filter & without the cover on the blower compartment-it still tripped
o with the plenum half removed-it tripped for a while at first, but then inexplicably stopped tripping. I put it back together (but didnít seal the joints) and it worked fine for a week. Now itís tripping again.

Itís been quite temperate here (for a MN winter), so comparatively speaking the furnace hasnít had to work too hard over the past week. But the temp has dropped about 30 degrees today. Things were working fine this morning, it was maintaining 68 degrees. I came home from work to find the temp at 63 & dropping. The furnace was stone cold, but the switch clearly had not reset. It didnít reset initially even after I turned the power off & on. It finally reset after I left the power off for half an hour & the furnace is now working again, holding 68 degrees.

At this point I have to think the switch is bad. But this is new to me, so Iím not remotely sure. Iím thinking Iíll take a step approach & try to replace the limit switch in hopes that itís the problem, then if that doesnít work maybe a new control board?

Anybody have any other ideas? Thanks for your time....
 
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Old 12-27-11, 10:35 PM
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Pros will be along soon, but donts overlook a cracked heat exchanger.....

Do you have CO detectors?

Mike NJ
 
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Old 12-28-11, 05:59 PM
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Yeah, I'm afraid of that.
I do have a CO detector--I brought it right up to the furnace when I started having issues, in case the heat exchanger is shot--but no alarms so far.

I can't access the heat exchanger without completely dismantling the furnace, so I'm hoping to exhaust all other possibilities first!
 
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Old 12-28-11, 07:16 PM
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Is the blower wheel clean? If it is dirty there is a good chance that the evap coil is dirty/plugged. However, the main limit is auto reset. If it takes that long to reset there is a problem with the limit.

ComfortMaker furnaces are KNOWN for heat exchanger failures. Do not overlook this.
 
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Old 12-28-11, 08:00 PM
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The wheel was dirty, so I expected to see a mess of dust on the coil. But there was none to speak of--on the outside of the coil. I imagine the bulk of the mess would appear in the inside of the coil, but I would have expected to see some dirt on the outside. Air flow coming through the coil seems good.

I am going to go ahead and replace the limit, thanks for the tip about that. Do you think I should tear the furnace apart and check out the heat exchange at the same time?

Thanks!
 
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Old 12-28-11, 08:04 PM
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Yes, Comfortmakers are known for heat exchanger problems. It is also fairly easy to check the accuracy of the limit switch by inserting a thermometer to the same depth as the limit. This can often be done without drilling by removing one screw holding the limit & inserting the thermometer there. A bead style thermocouple with a digital thermometer works best.
 
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