Furnace Limit Switch

[MoKelly]

So, this morning I wake up and it is -1 F outside.

Also, no internet.

Also, cold inside.

Check thermostat and it’s 61 degrees. Set at 70 degrees.

Great.

Heater guy shows up and checks furnace. Says the Limit Switch tripped. It’s a safety switch that stops the heater if the heat inside the exchanger gets above 185 degrees. Designed to protect the exchanger from damage.

Guy tests the switch and says it’s good. Checks everything and says he sees zero wrong. Furnace is 6 years old. Says he believes there was a “surge” that probably tripped the switch. Says switch is low voltage. He guesses whatever took out internet may have triggered switch.

Internet came back on (ATT had outage in our area) around an hour after he left. Not sure how internet impacts electric surge?

He reset the switch and heater works. Been working since late morning. Working fine.

I know little about heaters. Does this sound reasonable?

MoKelly

 

[kenmac]

Nope. Been in that business for a looong time. Not an electrical surge that caused this LV limit to trip. They open high temp(higher temp than they are set to open at the factory)

I'm gonna assume this is a gas furnace ?
Where is this limit switch located ? Gas furnaces have many in different locations.
Where is this one located ?

You say he tested the switch. Did he put a known temp against the disc on the switch to test it ?
At what temp does it open ?

 

[gsganzer]

If you tripped a high temp switch, then it might mean you have low air flow. Are your filters clean and nothing that could have blocked your return? All the vents open?

 

[MoKelly]

Nope. Been in that business for a looong time. Not an electrical surge that caused this LV limit to trip. They open high temp

I'm gonna assume this is a gas furnace ?
Where is this limit switch located ? Gas furnaces have many in different locations.
Where is this one located ?

You say he tested the switch. Did he put a known temp against the disc on the switch to test it ?
At what temp does it open ?

Yes, a gas furnace. The furnace is in the basement. The switch is inside the furnace - he took the top front cover off to access. The top cover is about waist high.

He did test the switch by using heat. It switched off right at 185 degrees.

He also looked thru the flu and no blockage. He tested the blower and it worked. He cycled thru the on/off sequence a few times and it all worked. He hung around until the heat went up to 65.

MoKelly

 

[kenmac]

Well, that really doesn't tell me where the switch is located. Is it an 80% furnace ?
If so, is the limit around the vent area ?

It wouldn't be an air flow issue those limits are in the heat exchanger area. Reduced air flow would cause a limit such as this to over heat and trip.
I doubt it's in the heat exchanger as most of them reset themselves once the chamber cools down

I'm not really sure where this limit is located to give you an opinion. But I' assume it has a button to be able to reset it.

I don't know what went on but I can assure you it wasn't a power surge. That would have fried the board

Something got hot enough to trip that limit.

What brand furnace ? Carrier places one at the vent connection

 

[MoKelly]

Well, that really doesn't tell me where the switch is located. Is it an 80% furnace ?
If so, is the limit around the vent area ?

It wouldn't be an air flow issue those limits are in the heat exchanger area. Reduced air flow would cause a limit such as this to over heat and trip.
I doubt it's in the heat exchanger as most of them reset themselves once the chamber cools down

I'm not really sure where this limit is located to give you an opinion. But I' assume it has a button to be able to reset it.

I don't know what went on but I can assure you it wasn't a power surge. That would have fried the board

Something got hot enough to trip that limit.

Thanks. Sorry I can’t be more helpful.

Do you think I should have another guy take a look? Should I wait to see if it trips again?

I can’t imagine why it would trip unless (a) fan was not working allowing the air to just heat and heat with no new air or (b) sensor is defective.

Always seems the problems occur on the absolutely coldest day of the year. Maybe because the unit runs more than normal.

I’ve lived in many houses and had furnace issues - but, this has never been an issue I have encountered.

MoKelly

 

[kenmac]

If a fan (blower fan) A internal limit would trip. You usually can't reset these. They reset themselves once the heat exchanger cools down. You would have to get inside the heat exchanger to access this type limit. That is the reason they reset themselves

If the exhaust fan didn't come on, the PS wouldn't make up and the unit wouldn't fire at all.

To answer your question. I would see if it trips again.
If the temp is printed on the limit, and it trips at ,or about that temp. I would say it's good.

Where ever this limit is located. It got too hot for some reason, or it was just a freek thing

You never stated if it's an 80% ,or 90% furnace

I'm guessing 80% and the limit is in the vent area, and the vent possibly got too hot ?

If so, I would look at the venting. These limits are a safety measure . I have seen HVAC tecs wire them out of circuit instead of locating a problem causing the home to burn down

 

[MoKelly]

You never stated if it's an 80% ,or 90% furnace

I'm guessing 80% and the limit is in the vent area, and the vent possibly got too hot ?

Sorry. I didn’t answer as I didn’t know. We bought the house last summer.

So, I went and looked at the unit. I saw a yellow sticker that said “92.1% efficient” - but that was all I saw. It’s a Trane manufactured in 2017.

Thanks for your help.

MoKelly

 

[kenmac]

Ok it's vented with PVC (or should be)

Sorry I can't help much not knowing where the limit is located. Perhaps it was just a freak happening.
But I'm pretty confident in stating it wasn't due to a power surge.

You'll know if /when it trips again. Perhaps it won't

Good luck

 

[ultrarunner]

Yep... tis the season for heating issues...

Had a bunch in January cold snap (For Me)

All with newer furnaces...

Blower Motor on one

Circuit Board on another.

Capacitor on another

Draft Inducer on another...

Missing batteries on a wall furnace stat...

Oh and mine bad thermostat... when I jump it out works fine... never had a bad mercury thermostat in 35 years... and this is a 5 day, 1 day and 1 day programmable... ordered a Emerson wifi unit with utility rebate... will see.

Only one had the overheat pop... they were away for the weekend... came home... daughter maxed it and then they went shopping... thing must have been running full bore until it tripped... it has a little disc button to reset.

Funny thing is all the old stuff... 1960 and older just keeps running... sure I check the belt and oil the blower motor and check pilot but no parts required...

Getting to be a regular at the wholesale house... owner asked if I wanted to open an account as all happened over a few weeks.