High Limit Switch

[eapples]

All: I was in this forum some time ago, discussing how to's and what if's regarding the high limit switch on my furnace. Here's a recap of the problem: from time to time, the blower will not stop following a heating cycle. It continues running long after the heating cycle has finished. I understand the function of a high limit switch. But, when you think about it, I wouldn't know if the limit switch was working because I'd think that the furnace was simply cycling on/off as normal. The only time I'll even think about the limit switch is when the blower continues running long after the heat cycle has finished. The limit switch was already replaced, and the problem reappeared about a week after the limit switch was replaced. If the thermostat calls for heat while the blower is continuing to run, it will stop and the heating cycle will start again, and then the heating cycle will complete its run with the blower stopping as it should. Every time I call the service tech (service contract with the local utility), he checks here and there and ultimately tells me he can't figure out what the problem is. The fault indicator on the furnace is telling me that the problem is the limit switch. The unit is relatively new (only 5 years old) and the guts (what I'm able to see) still look clean. I change the filters regularly (once a month). I use pleated filters with a 8 merv rating. Any further input on this issue would be greatly appreciated. Specifically, is the limit switch something that I can order and replace myself? The switch is clearly visible when I open the cabinet, and it appears to be removable with just two screws holding it in place.

[Edited by eapples on 10-24-2005 at 03:55 PM]

[smokin68]

Call a real service company.

[eapples]

Originally posted by smokin68
Call a real service company.

Mr. Smoke: The reason I went with the utility company was that the hvac guy I was working with was kinda sorta expensive...for example, he replaced the circuit board and charged me $XXX. When the problem with the limit switch happened he came to check it but couldn't replicate the problem and told me he couldn't diagnose the problem until he saw it happen himself. I couldn't afford his prices again. I don't want to get into an argument/debate about the merits of independent hvac contractors vs utility company service contracts. My situation is what it is. All I'm trying to do here is to get some objective advice. Thanks anyway for your input.

(No pricing, due to site rules )

[Edited by jultzya on 10-29-2005 at 01:51 AM]

[beenthere]

What model thermostat do you have, has it been replaced this summer.

If it was the hi limit, when the stat called for heat, you wouldn't get any heat, because the hi limit locks out the burner.

[t527ed]

try a cheap filter see if problem goes away. if not call a service co that deals with brand you have. utility guys don't have as much experience with any one brand since they do everything.

[eapples]

Originally posted by beenthere
What model thermostat do you have, has it been replaced this summer.

If it was the hi limit, when the stat called for heat, you wouldn't get any heat, because the hi limit locks out the burner.

Mr. Been: The heating/cooling unit is only five years old. Except for the circuit board, nothing has ever been replaced. The house was built in 2000. The thermostat is made by Honeywell. I have the Honeywell Trol-A-Temp system in my home (2 zones). As I said above, the blower keeps running until a. I reset the unit by switching the power off and then on, or b. one of the zones calls for heat, whereupon the blower will stop and the heating cycle starts all over again. I call the limit switch the "high limit switch" because that is what others called it...furnace documentation refers to is simply as the limit switch. Understand that what I'm talking about is that the blower has been running for a long, long time after the heating cycle has finished...I'm talking about 5, 10, 15 minutes and longer. Sometimes it'll keep running until I reset it or until a zone calls for heat. And as I said earlier, the fault indicator shows that it's the limit switch. I appreciate your input...thanks.

[eapples]

Originally posted by t527ed
try a cheap filter see if problem goes away. if not call a service co that deals with brand you have. utility guys don't have as much experience with any one brand since they do everything.

Mr. T527: I tried the cheap filter and the problem didn't go away. The hvac guy I was originally dealing with, as I said above, was kinda sorta expensive, and I couldn't afford to continue paying his prices. The utility service contract covers the limit switch. I figure that they'll keep coming back to check the unit until someone gets it into their head that the unit needs to be fixed and they send someone to fix it.

[beenthere]

It might be going into a soft lock out.

Are your zones the same size, or is one alot smaller then the other.

Does it have a bypass.

Your evap coil may be dirty and causing the limit to do its job.

[jacob perkins]

[QUOTE]Originally posted by eapples

it...furnace documentation
I appreciate your input...thanks.

What is the make/model number so we can all read along with you.

I will guess that the pleated filters need to go.Too restrictive.

[MikeJ]

Sounds similar to a problem I ran into once. Took awhile for me to realize that when only one zone was calling, there was not enough air flow and the limit would trip (open), causing the burners to shut off and the blower to run until the unit cooled down. Then it would relight just after the blower quit and run for a bit and do the same thing. I opened both zones and didn't have this happen so I deduced that one zone only was too restrictive.

Try keeping both zones calling with a setting of 80 or so. Open your windows and see if the unit will continue to run. Or if you can, manually open both zones.

Another thing is set an appointment with a better company or the utility but let the house cool to a low temp., Then when the tech gets there let him up the temp and see what happens with only one zone calling. Just a thought.

[hvac hero]

If the furnace is only 5 years old, then the high limit is not what turns the fan on & off. Most furnaces have it built into the boards. Therefore, changing the filters isnt going to solve the problem. I'd get a company out there to check it out.

[eapples]

[QUOTE]Originally posted by jacob perkins

Originally posted by eapples

it...furnace documentation
I appreciate your input...thanks.

What is the make/model number so we can all read along with you.

I will guess that the pleated filters need to go.Too restrictive.

Mr. Perkins: The furnace is made by Frigidaire, Model #G6RC-100C-16. Everything worked fine until the winter of 2005 (Jan/Feb) when the unit was rapid cycling on/off. The circuit board was replaced, and that's when I noticed the problem with the limit switch. Right after that I bought the service contract from the utility company, and they started servicing the unit in the spring of '05.

[trane]

Have someone check the temperature rise with each zone individually. This will tell you if there is a airflow problem with any of the zones.