Furnace Limit Switch

[seville009]

throwing this out there for what it痴 worth.....my propane furnace starting giving me problems after about 12 years of living here. It would shut down for no apparent reason in really cold weather (I知 in the northeast).

I found a minute amount of ice crystals formed around the perimeter if the cold air intake outside the house. Couldn稚 really see them, but could feel them with my finger. Evidently that small amount of ice crystals disrupted the air flow enough to shut the furnace down.

I put an adapter on the intake to increase to size. Worked perfect - never had any more issues.

 

[KennyG]

I haven't had this switch trip, but an HVAC guy warned me that it will eventually. Why? My old furnace (20+ years) has a stamped heat exchanger that is riveted together. The rivets are starting to fall out. When it lets a crack open up, it will blow back and overheat the cool side of the heat exchanger. It's working OK but I'm planning for a furnace replacement in the next year.

If it continues to trip, you need to do a heat exchanger inspection.

 

[MoKelly]

View attachment 686514throwing this out there for what it痴 worth.....my propane furnace starting giving me problems after about 12 years of living here. It would shut down for no apparent reason in really cold weather (I知 in the northeast).

I found a minute amount of ice crystals formed around the perimeter if the cold air intake outside the house. Couldn稚 really see them, but could feel them with my finger. Evidently that small amount of ice crystals disrupted the air flow enough to shut the furnace down.

I put an adapter on the intake to increase to size. Worked perfect - never had any more issues.

This is a great point. I’ll look at that today. With the extreme cold, ice and snow, this is a possibility.

Thank you.

MoKelly

 

[user.69169]

Write down model & serial numbers and use search engine. Generally you can bring up service manual.

From der you can generally figure out what up.

You mention cold snap and overheat safety. Pretty good chance furnace sized to house when installed. Cold snap makes furnace run lot longer den normal so exchanger might overheat. Possible dirty filter limiting air flow thru exchanger. Also possible dust buildup on blower not delivering full output thru exchanger.

Lot of furnace "technicians" don't do job dey paid to do. Some even get bonuses fer selling new furnaces. Stepson is one of dem. Best if you get manual so you can supervise proper.

Ice in burner air intake got nothin to do wid heat exchanger, but real good indicator furnaces workin hard.

 

[ultrarunner]

Rental house in WA State was having electric furnace breaker trip but only when tenants went away for several days on ski trips.

House would be very cold and furnace locked on and then trip... service tech said need new furnace.

Anyway... I looked at it and removed one of the 6 heat strips... this was 12 years ago and no issues since.

 

[MoKelly]

Write down model & serial numbers and use search engine. Generally you can bring up service manual.

From der you can generally figure out what up.

You mention cold snap and overheat safety. Pretty good chance furnace sized to house when installed. Cold snap makes furnace run lot longer den normal so exchanger might overheat. Possible dirty filter limiting air flow thru exchanger. Also possible dust buildup on blower not delivering full output thru exchanger.

Lot of furnace "technicians" don't do job dey paid to do. Some even get bonuses fer selling new furnaces. Stepson is one of dem. Best if you get manual so you can supervise proper.

Ice in burner air intake got nothin to do wid heat exchanger, but real good indicator furnaces workin hard.

Thanks. Filter was changed on January 31, 2021. The tech vacuumed out the inside of the furnace including all around the blower motor. It wasn’t that dirty.

I did look at the PCV intake pipes outside. There were huge ice cycles hanging below the pipe. Broke those off. They went blocking the pipe but they were huge. I guess the steam would freeze as it was exiting.

I will try to get the manual online as you suggest and read.

MoKelly

 

[pmsmechanic]

I haven't had this switch trip, but an HVAC guy warned me that it will eventually. Why? My old furnace (20+ years) has a stamped heat exchanger that is riveted together. The rivets are starting to fall out. When it lets a crack open up, it will blow back and overheat the cool side of the heat exchanger. It's working OK but I'm planning for a furnace replacement in the next year.

If it continues to trip, you need to do a heat exchanger inspection.

I would really think this through as old furnaces are a lot more reliable than new ones. Easier to repair.....

 

[zzvyb6]

I had this happen. Cause was the new very high efficiency furnace filter. Feed air was restricted too much. Service techs say this is a common problem. When we replaced the furnace (was super high efficiency), the service tech measured feed air flow rate and replumbed some basement ducting to provide more air flow rate into the furnace. Problem explained and never happened again.

 

[KennyG]

I would really think this through as old furnaces are a lot more reliable than new ones. Easier to repair.....

My over 20 year old furnace is a 95% efficient, pvc ducted unit. I don't think the new ones are a lot different in technology.

 

[pmsmechanic]

My over 20 year old furnace is a 95% efficient, pvc ducted unit. I don't think the new ones are a lot different in technology.

I was thinking you had a 60 year old furnace like I do.