Gas furnace help

[Guest]

My gas furnace turns on, runs a little while, and then shuts down before hitting the temp I set on the thermostat. It'll start back up and then shut down again. What is wrong with it? Help. Thank you.

[critter69]

It could be a number of things. If it lights and don’t stay lit long, it could be flame sensor. It also could be pressure switch. The flame sensor is more common though. The flame sensor, keeps your gas valve open. It will be one wire, to a rod opposite of where your igniter is. Usually one or two screws, looks like a small metal “ wand” that sits in the flame, if you remove it and use one of those green or purple scrub pads, sand it and replace it. It’s the easiest thing to start with, and very common.
How long is it running before shutting down ? If it’s running a while, it could be shutting off on a high limit switch. ( getting to hot). Post a video if possible.

[SabineHunter]

It could be a number of things. If it lights and don’t stay lit long, it could be flame sensor. It also could be pressure switch. The flame sensor is more common though. The flame sensor, keeps your gas valve open. It will be one wire, to a rod opposite of where your igniter is. Usually one or two screws, looks like a small metal “ wand” that sits in the flame, if you remove it and use one of those green or purple scrub pads, sand it and replace it. It’s the easiest thing to start with, and very common.
How long is it running before shutting down ? If it’s running a while, it could be shutting off on a high limit switch. ( getting to hot). Post a video if possible.

It runs for about 3 min or so

[bowtecmike]

Check/change filter. Sounds like it is cycling on the high heat limit.

Sent from my SM-A716V using Tapatalk

[critter69]

Watching, was it a dirty filter ? If the flame burns for three minutes it’s most likely not the flame sensor. But your induced draft fan will make it sound as though it’s running longer then it may be actually lit. Remove the door over the burner compartment, to watch what it’s doing.

[60 Deluxe]

FYI, gas furnaces are controlled much the same as steam boilers. It will have a pre-purge to push any combustible gases out of the system as a safety precaution, then an ignition, gas valve opening, and a short test period to verify that there is a good flame and that the air is moving and the temperature is within parameters. If something goes awry, the fuel valve is closed and the furnace goes through a post purge to vent out any remaining combustibles. The whole sequence can take a few minutes to complete, so the three minutes sounds reasonable. Boilers use light sensors, either uv or infrared to "see" the flame. Furnaces generally use a thermocouple which cost a fraction of the light sensors. I only say all this because it helps to know the process when trying to diagnose what is going on. The other two controls on a furnace are an air switch and a thermistor. The air switch won't make if the fan is not running or if the filter is so clogged that air flow is not sufficient. They are a normally open switch that closes and lets control voltage pass when there is sufficient air flow. The thermistor is a normally closed switch and will pass voltage up to a certain temperature at which time the switch will open and control voltage to the gas valve solenoid is lost. It is located somewhere on the exhaust side of the furnace. The safety controls are wired in series so if you know the sequence, you can track through the circuit with a voltmeter to find which switch is not letting voltage pass. Modern furnaces usually have a controller that has an LED that sends codes that will tell you what is wrong. If yours has the controller there will be a chart glued to it or somewhere close that tells you what each sequence of led flashes pertains to.

[bowhntrmatt]

Typically there is a little window in the lower door where you can see what code it’s flashing. This will tell you why it shut down.

My guess would be a limit switch or possibly, but unlikely, an inducer pressure switch.

Check for anything that would impede airflow, such as a dirty filter or coil.

Don’t hesitate to call a pro if you feel over your head. The safety of everyone in the home depends on it.

[SabineHunter]

Typically there is a little window in the lower door where you can see what code it’s flashing. This will tell you why it shut down.

My guess would be a limit switch or possibly, but unlikely, an inducer pressure switch.

Check for anything that would impede airflow, such as a dirty filter or coil.

Don’t hesitate to call a pro if you feel over your head. The safety of everyone in the home depends on it.

I think I'm going to go with a pro based on your last sentence. BTW, the fan doesn't run when I switch from auto to on.

Thank you to all that responded with great info.

[SabineHunter]

Thank you to all for your help. It turns out I would not have been able to fix the furnace. $900 and a new circuit board later, I'm good to go. Ugh.......

[sqiggy]

We are having issues with ours as well. You can hear the furnace light then when the fan kicks on, it kicks right off, like it tripped off. Does this off and on thing for a while. Then it eventually stays on. When it first started 3 or 4 years ago, I had a guy come check it out. Never could figure the problem and it wouldn’t do it while he was here. Doesn’t do it all time but at night, when we go to bed, it really gets annoying when it starts.

[flywise]

My electric one did that this summer. The fan motor was going out. I replaced that and has been fine since.
Also please make sure you have a co2 detector

[flywise]

Check the filters

[SabineHunter]

Check the filters

Yes, filters are always first.

[bowhntrmatt]

We are having issues with ours as well. You can hear the furnace light then when the fan kicks on, it kicks right off, like it tripped off. Does this off and on thing for a while. Then it eventually stays on. When it first started 3 or 4 years ago, I had a guy come check it out. Never could figure the problem and it wouldn’t do it while he was here. Doesn’t do it all time but at night, when we go to bed, it really gets annoying when it starts.

Sounds like a dirty flame sensor. Clean it with a small wire brush or sandpaper.