Gas furnace limit switch fails. Only works when door open

nfilippakis

Furnace is 2011 Rheem 80%. Worked fine since 2016  (when we moved into newly Reno’d home).  Recently, furnace starts and runs intermittently then shut down, and repeats. 3
blink codes (limit switch/clogged air flow). System works fine when compartment door is open. 

Called tech and checked heat exchanger (fine) and replaced pressure limit switch. Gas pressure at recommended setting (3.5). 

Flue has good draft. Registers and returns aren’t closed or blocked. Filters changed every 2-3 months. 

Anyone else experience a similar issue??

Jamie Hall

Did the tech bother to find out why air is blocked when the compartment door is closed but not when open? Seems to me that would be the obvious place to start looking...

EBEBRATT-Ed

Check inside the blower compartment for any plastic bags etc.

Why did the tech leave with it hitting limit? Obviously something is wrong.

He could have lowered the gas pressure a smidge.

There are only so many things it can be

1. plugged returns or supplies...restricted air flow
2. dirty filter
3. blower motor running the wrong direction or on the wrong speed
4. Bad limit control
5. dirty AC coil (if you have one)

Did the tech check the temperature rise across the furnace? He certainly should have done that. At least check the air temp on the discharge

JUGHNE

A dirty blower wheel could cut down on the air flow enough to trip the high limit if the ductwork was just border line to begin with. It is very common to have return air duct undersized.

nfilippakis

so so last night I noticed the flame light would sometimes flash codes indicating sensor failure. I sanded the sensor and now everything seems to work fine. No failure codes, no intermittent shut offs, no cycling on and off. How Would the flame sensor cause a limit switch code?

Ironman

nfilippakis said:

so so last night I noticed the flame light would sometimes flash codes indicating sensor failure. I sanded the sensor and now everything seems to work fine. No failure codes, no intermittent shut offs, no cycling on and off. How Would the flame sensor cause a limit switch code?

The flame rod wouldn’t cause a limit switch code. Are you sure that you were reading the code correctly?

Also: you shouldn’t sand the rod; that will cause it to fail again very soon. Get a new one and if it reaches the point where it needs cleaning, use a dollar bill.

EBEBRATT-Ed

What @Ironman said

nfilippakis

good to know about sanding the flame rod…If it fails again I’ll call the tech and have them replace it as it’s covered in the service I pay for. And yes codes were read correctly. “Flame” LED was off “OK” LED 3 blinks, followed by a 2 second pause. 2 tech’s confirmed the fault code, however they didn’t notice the flame code. Flow chart and trouble shoot map state the following:
airflow limits, rollouts, over fire… all of which the tech checked and ruled out. He also replaced the limit switch… not sure why cleaning the flame rod worked but it did. Set thermostat to call for heat and the furnace worked perfectly, even fired up faster, with no codes and stopped when heat called ended. 

Thanks for the tip about sanding the rod. I really appreciate it. 

I’ll keep you posted about status 

thanks again to everyone for their thoughts and insights. Much appreciated!

Ironman

IDK about your particular furnace, but normally when a limit switch is open, the furnace will only run the blower and inducer to cool the heat exchanger down. No ignition can take place until the unit cools down and the limit switch closes - whether manually or automatically.

There may have been a previous high limit fault which was left in the board’s memory and the current ignition fault didn’t appear or wasn’t seen.

nfilippakis

I have no clue… but it seems to be working now. If it happens again (and I hope it doesn’t) I’ll take a video of the LED flash codes 

unclejohn

I would not worry about using sand paper on flame rod.