Various Problems - Limit Switch and Others


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Old 03-03-09, 06:02 AM
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Various Problems - Limit Switch and Others

All: I've tried several online HVAC groups to solve this problem. The techs in the online forums all seem to harumph in unison when I tell them that the utility company has the service contract on my unit. I'm gonna try again with all of you. Thanks for your patience.

From time to time, not all the time, the blower will continue to run after the heating cycle has completed. The thermostats aren't calling for heat, but the blower continues running, and will not stop until the commencement of a new heating cycle. Then, after that heating cycle completed, the blower MAY stop running or it will continue as before until the next heating cycle. I checked the indicator light, and it indicated "fault - limit switch." I asked the service company to check it, but the tech told me that he couldn't diagnose the problem unles I could recreate the problem, and I couldn't. So, I'm living with it.

A new problem has started this heating season, and it has happened three times as of earlier today. The thermostats will call for heat, but the furnace will not come on. The humidifier is running, but the furnace is not running. I have to flip the on/off switch, wait a few seconds, then the furnace will start the heating cycle, and will run normally, except as noted above. This happened the first time during the first cold snap early in the winter. It happened again about a month ago, and then again this morning. I didn't check the indicator light, so I don't know what the system was showing. I've been reading articles and posts about "furnace lockout", and it seems that my furnace is doing the same thing...I have to flipt the on/off switch, wait a few seconds, then the furnace operates normally, except as noted above.

I'm figuring that if the safety systems are working correctly, that's a good thing. BUT, the problem that the safety system is addressing is not a good thing, and that bothers me. For what it's worth, I tried an independent hvac contractor to address the limit switch issue, and when he attempted to diagnose the problem, the furnace was working correctly, so he pronounced the patient cured.

The furnace is ten years old. It's a Frigidaire brand. I recently had ductwork done...increasing the size of the supply ducts (that are accessible in the basement), adding a dump zone, adding another return duct (from an upstairs bedroom). I have two thermostats, and motorized dampers to make a faux two zone system. It was originally a Honeywell Trol-A-Temp system, but I had the dampers replaced when the supply ducts were enlarged. The new dampers are compatible with the system, and are functioning normally.

The problem with the limit switch was happening from the initial heating season when we moved into the house. The control board was replaced twice, and the limit switch was replaced three times since we've been in the house. The ductwork was done last year when we had to replace the central air conditioner (compressor and coil).

What can you guys/gals tell me, based on my explanation of the problems? I believe simply that the furnace was defective from the get-go, and that it should be replaced. Thanks for your input.
 
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Old 03-03-09, 09:09 AM
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What is the model number of the furnace, which you can get from the rating plate in the burner compartment of the furnace?

And what is the make and model of the control module or circuit board if you can get it?
 
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Old 03-03-09, 04:13 PM
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Circuit Board -- Nordyne 624631-A

Furnace Model Number G6RC100C-16
 
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Old 03-03-09, 06:08 PM
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Originally Posted by eapples
The thermostats will call for heat, but the furnace will not come on.
Does a draft inducer motor come on? If not, it could be a blower compartment door partially ajar and causing interlock switch to not always make good contact. Or it could be a pressure switch, if you have a 3-wire pressure switch and it is not closing the NC to C switch. Or, it could be a bad inducer relay. Or if you have draft damper, that is not opening first, that could be it. This is a start.

Also make sure you have 24 volts down in the furnace, that has returned from the stat at the white wire connection. And see if the 24 volt safety circuit is energized.
 
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Old 03-03-09, 06:31 PM
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Thank you Mr. Ecman51. I appreciate the info, although it's a bit over my head. BUT, at least now I have something to fall back on when it comes time to call in a technician. I generally ask the techs to explain the nature of the problem and the fix. I'll compare what they tell me to what you told me.
 
 

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