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Lenox G26 - Primary/Secondary limit switch open

tws
tws Member Posts: 5
in THE MAIN WALL
My furnace will run between a few hours to a few days but eventually, it'll stop working. The LEDs indicate it's a limit switch. I've changed out the filters and taken off the squirrel cage fan on the front and shook out some dust but it wasn't what I'd consider dirty (looked like a dirt-dobber next fell in from the exhaust pipe). See pics. Not sure what next steps are? I've never worked on a furnace before.

https://www.amazon.com/photos/shared/qxd2AROzRoerrL078VscKw.cPTOeZciXy3h_861L85gKr

Comments

  • tws
    tws Member Posts: 5
    bump. Any help? Thanks!
  • JUGHNE
    JUGHNE Member Posts: 11,310
    You need to check the flue pipe from the furnace all the way up thru the roof.
    If flue gases are slowed down for any reason, the furnace can overheat and lock out.

    Or if you put a very restrictive high Merv rated filter in, it could cut down on the air flow enough to trip hi limit.....happens more than most imagine.
    tws
  • tws
    tws Member Posts: 5
    Ok, I finally had time to get up on the roof and thread a spinning brush down the pipe. Made it all the way through and only a bit of fine dust came out, so the pipe is clear. I have a MERV 5 filter in it, when I remove the filter it seems to fix the issue. Is it possible that a MERV 5 is too restrictive? If so, what kind of filter do I need? I picked up some cheap fiberglass MERV 2 filters from Home Depot, I guess I'll see if those do the trick.
  • wmgeorge
    wmgeorge Member Posts: 222
    If you have AC my guess is the A coil on the furnace is packed full of dirt on the air entering side.
    Old retired Commercial HVAC/R guy in Iowa. Master electrician.
  • mattmia2
    mattmia2 Member Posts: 11,149
    is the blower running when it happens? is there side or end play in the blower bearings? how old/what quality is the cap for the blower?
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,848
    Look on the furnace name plate, your looking for the "temperature rise" It should be on there.

    That's the difference between the supply and return temp when it is running. It will tell you the min and Max temp rise, It should be 40-50 degrees
  • tws
    tws Member Posts: 5
    wmgeorge said:

    If you have AC my guess is the A coil on the furnace is packed full of dirt on the air entering side.

    We do have AC. I stuck a camera in under the furnace but I can't get in there.
    mattmia2 said:

    is the blower running when it happens? is there side or end play in the blower bearings? how old/what quality is the cap for the blower?

    Not sure what you mean by blower. When it trips, nothing will run, it shuts off and makes what I'd call a 'strained' sound.
    EBEBRATT-Ed said:

    Look on the furnace name plate, your looking for the "temperature rise" It should be on there.

    That's the difference between the supply and return temp when it is running. It will tell you the min and Max temp rise, It should be 40-50 degrees

    What would I do with this info?



    Thanks everyone!

  • mattmia2
    mattmia2 Member Posts: 11,149
    tws said:

    mattmia2 said:

    is the blower running when it happens? is there side or end play in the blower bearings? how old/what quality is the cap for the blower?

    Not sure what you mean by blower. When it trips, nothing will run, it shuts off and makes what I'd call a 'strained' sound.
    Ok, technically in the moment before it locks out, is the blower running and is this "strained" sound coming from the air circulating blower or from somewhere else?
  • wmgeorge
    wmgeorge Member Posts: 222
    edited February 2022
    TWS - Could be the bearings in the blower motor either need oiling or ball bearings if it has are going bad? Regardless there is No easy way to check that A coil for partial dirt blockage is to actually Look At it, sorry but that can be tough one.
    Old retired Commercial HVAC/R guy in Iowa. Master electrician.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,848
    @tws The temperature rise will tell you if the blower is moving too much air or too little air.

    Assuming the burner input is correct a low temp rise means your moving too much air, you would then change the blower speed to slow it down.

    A high temp rise means your not moving enough air, you would speed up the blower. ut more likely the problem would be plugged filters, dirty a/c coil, undersized ductwork, closed dampers or blocked supply and/or return registers
  • tws
    tws Member Posts: 5
    Wanted to follow up with the resolution. It got warm after I asked this question and so I just forgot about the issue. As it's cooled this winter, it needed fixed. I ended up taking the blower fan cover off and tested the blower fan capacitor. It was bad. $10 later it is working again.

    Thanks everyone for the help!

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