Any furnace guru's?

Ron H

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I have a Lennox garage heater ceiling mounted. Looking for any tips on removing the flame sensor rod on this lf24 unit. I've replaced the hi-limit and rollout switches, replaced the blower and motor. All rather easy as there's access to the screws and bolts, EXCEPT the sensor. For some reason, the screws for it are behind possibly requiring some heady work to get it out. Every time I stare at it, not sure what needs to be removed to get it out without disassembling a load of stuff. Like to avoid removing stuff that may not need removing. In the photo the sensor rod is far left, white wire, left of the gas pipe. I've searched the web for any chats on this, so far no luck. The light on the CB blinks 4 times and pauses indicating one of the switches; but after replacing them, could be the rod is carboned up needing cleaning or replacement. Thanks

Garage Furnace 1.jpg Garage Furnace 2.jpg
 

69Bee

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Looks to me that vertical wall between the heat tubes on the left and the components to the right, the component bracket is screwed into the vertical wall. Looks like 2 screws, and it will come out??
 

bm02tj

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You need to remove retainer strip above burners then remove burners
you can clean in place or remove strip that hold burners to get to screws
clean with 0000 steel wool or fine scotch brite GENTLY or if unavailable you can use clean inside of match book with no ink works as well
 

F4R/T

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If you are going thru the hassle you could replace the sensor for peace of mind I don’t think they are too much cost wise.. I did hvac for awhile.. it made me sick what we charged folks on a .50 part. I would recommend always keeping spares due to the poor quality China made stuff out there.
 

Ron H

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Thanks much for your tips. Here's a couple of better photos. The vertical bracketing has a screw top & bottom, this would seem to allow the 'start' of removal. Then there are two screws along (on the right) holding the bracket that connects with the burner ports, they are rings on this one-piece bracket that surround the ports coming off the gas pipe...they 'appear' to be able to slide off those ports, assuming they're a slight press-on fit. Suppose, could be once doing this, the works can be turned enough to remove it or just to access the screw heads to pull sensor out. Weird why da hey they put the screws in from behind considering what needs doing to take the sensor out! Ya know, a part that should be checked and cleaned every few years...in my case it's been there for 20.

Furn 1.jpg Furn 2.jpg
 

Moms68

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What was the initial problem? Why are you replacing all of these parts? If it’s 20 years old i’d be checking the heat exchanger too.

Does the unit light, run, and shut down after a short time? If so that might suggest flame sensor, if not you have to start tracing the 24v from the transformer, and see where it stops.
 
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Ron H

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What was the initial problem? Why are you replacing all of these parts? If it’s 20 years old i’d be checking the heat exchanger.

Google your model number and look for a service manual or hire a licensed tech to be safe.
Have worked with furnaces over the years; but know my limitations. Installed it making up the pipeline from the basement to garage wiring it in 20 years ago. On the CB there is little light indicating system functions based on how bulb is flashing. I found a 4-flash then pause; on the chart this says high-limit or rollout switch. Tap on the furnace it starts up and works fine for a while, until it happens again. Installed new limit switches (inexpensive and easy to do). Worked fine for a few months, then same chit again. Did the blower/motor while working on it as it sounded like a bearing was going…some play on spindle. Further pondering seemed to point to a more root cause, one being the flame sensor that can carbon up or deteriorate causing this. Since its never been out figured a good thing to check. But, as posted this thing is a hassle to remove. So after some web surfing finding nothing and the manual saying nada, thought well there might be a member or two who has one having done the job, or maybe a furnace tech to share advice.
 

RemCharger

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Check for signs of moisture in the tube going to the diaphragm pressure sensor. Or in the sensor. Some like mine have a loop in the tube to act as a water trap, yours not so much.
 

Moms68

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Have worked with furnaces over the years; but know my limitations. Installed it making up the pipeline from the basement to garage wiring it in 20 years ago. On the CB there is little light indicating system functions based on how bulb is flashing. I found a 4-flash then pause; on the chart this says high-limit or rollout switch. Tap on the furnace it starts up and works fine for a while, until it happens again. Installed new limit switches (inexpensive and easy to do). Worked fine for a few months, then same chit again. Did the blower/motor while working on it as it sounded like a bearing was going…some play on spindle. Further pondering seemed to point to a more root cause, one being the flame sensor that can carbon up or deteriorate causing this. Since its never been out figured a good thing to check. But, as posted this thing is a hassle to remove. So after some web surfing finding nothing and the manual saying nada, thought well there might be a member or two who has one having done the job, or maybe a furnace tech to share advice.
Ron

From your description, there may be an issue with that flame sensor. If the fan is running, it is unlikely a high limit As a licensed HVAC guy, I can tell you that we rarely find 2 units the same and usually it becomes a small project to replace parts on some units. You can test the flame switch with an ohm meter to test for continuity (power off). After that, as others have suggested you'll need to methodically pull things apart until you can remove those screws. You can try cleaning the sensor with some very fine sandpaper, but I'd replace it at this point.
 

Ron H

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Ron

From your description, there may be an issue with that flame sensor. If the fan is running, it is unlikely a high limit As a licensed HVAC guy, I can tell you that we rarely find 2 units the same and usually it becomes a small project to replace parts on some units. You can test the flame switch with an ohm meter to test for continuity (power off). After that, as others have suggested you'll need to methodically pull things apart until you can remove those screws. You can try cleaning the sensor with some very fine sandpaper, but I'd replace it at this point.
I had caught a tip while ago with a furnace that keeps running the blower; but no kick to ignite and engage the fan, by rapping near the limit switch. It works every time when doing this. Nice steady flame all ports. And the flash pattern says one of the switches – limit or rollout. But, evidently isn’t the root of the problem finding a troubleshoot saying the flame sensor could be causing this. Been a great unit for 20 years…but 20 years. I was still cute back then. My ‘limit’ after what I’ve done was going to be the sensor; beyond that call an expert. Think I know what needs doing to get to it; but as some have said yank out the wallet and call a tech, lol.
 

Moms68

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Did you replace the fan inducer? There should be a proving switch somewhere near that inducer blower. Some call it a sail switch. That switch proves that fan is running and allows the gas valve/ rollout to start. I’ve seen debris/ nests/ leaves, etc. get in there and prevent that switch from opening and closing.

Yours looks to at the other end of the vacuum hose coming off the blower motor.
 
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F4R/T

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Moms 68 is spot on , you must induce draft in order for it to light even on a 80% efficient unit.. lost of good suggestions from others too! Please check your heat exchanger for cracks! Usually the ones I’ve seen fail are shortly after or around the first bend. Some have lifetime warranty on the exchanger.. I would take a good light with a bore scope or Milwaukee camera and go exploring. 20 years on it is a good run!
 

RemCharger

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The old cracked heat exchanger trick.
I'm all for safety,,, but they started getting retarded around here some years ago.
All of a sudden everyone was doomed unless they cracked out 6000 for a new furnace.
The inspectors were getting pissed off at these assholes.
Sorry for the rant.
 

bm02tj

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Next time it does not start use your volt meter and follow wiring diagram to find what is open
 

Ron H

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Thanks guys; sail switch or fan inducer I'm unaware of. Does this show up in my top photo of the guts, my OP? Will check for cracks on exchanger - my guess is if this is bad it's time for a new furnace huh? Had one put in the house furnace and was pricy.
 

F4R/T

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I see where your coming from Rem, my job was commission based and I had to sell on every job.. six months I walked away.. Safety is first.. A lot of folks can’t fork out that kinda money.. some can’t keep what they have going. It’s a fine line I always believe in looking at it thru their eyes.

If there are cracks it’s time to pay the piper..
 

Ron H

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Ok, as a non-expert knowing enough to be dangerous, I had thought a cracked exchanger could be evident watching the flame? Not?
 

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