Weird cycling behavior with gas furnace
#1
03-28-22, 07:44 AM
Weird cycling behavior with gas furnace
Hello folks
Bought a house recently and I think this gas furnace is acting a little strange. It is Trane xB model 4txcb036b03hcba, looks to be 11 years old based on manufacture date. In previous homes I had a boiler and then a mini split (both of which I installed myself) so generally familiar with hvac things but this is my first furnace.
Sometimes when there is a call for heat, I'll hear a relay click and the furnace will make a humming type noise. It will hum for maybe 20 seconds and then stop. After a second or two it will start again, for another ~20 seconds or so. This will happen maybe 3x or 4x, and after the last one I hear what sounds like the burner and fan kick on and it runs for a bit, blowing out heat. This only seems to happen sometimes, and other times it will just turn on and fire up like I expect (no humming cycles)
Are furnaces supposed to do this? If not, where should I start my investigation?
Bought a house recently and I think this gas furnace is acting a little strange. It is Trane xB model 4txcb036b03hcba, looks to be 11 years old based on manufacture date. In previous homes I had a boiler and then a mini split (both of which I installed myself) so generally familiar with hvac things but this is my first furnace.
Sometimes when there is a call for heat, I'll hear a relay click and the furnace will make a humming type noise. It will hum for maybe 20 seconds and then stop. After a second or two it will start again, for another ~20 seconds or so. This will happen maybe 3x or 4x, and after the last one I hear what sounds like the burner and fan kick on and it runs for a bit, blowing out heat. This only seems to happen sometimes, and other times it will just turn on and fire up like I expect (no humming cycles)
Are furnaces supposed to do this? If not, where should I start my investigation?
#2
03-28-22, 08:34 AM
Group Moderator
It sounds like it is not igniting properly. Go watch the unit when it is going through a heat cycle and you will see what it is or is not doing. If it times/locks out you can look at the control board for a blink error code.
#3
03-28-22, 01:31 PM
Group Moderator
Very hard to get a problem reading from your statement.
Some helpful info here........ Please read first for furnace repairs.
Some helpful info here........ Please read first for furnace repairs.
#4
03-29-22, 05:54 AM
Thanks both. Read through the 'read first', apologies for not reading that first 
It does sound like perhaps it's not igniting properly. Took off the panels and found control board, will tape down the switch and then run it through some cycles to see what the LED tells me.
It does sound like perhaps it's not igniting properly. Took off the panels and found control board, will tape down the switch and then run it through some cycles to see what the LED tells me.
#5
03-30-22, 10:20 AM
Okay it definitely seems like "intermittent failure to ignite".
The humming noise I was describing was the draft inducer turning on.
I see 5 "trumpets" where the gas comes in the back from the manifold/line and then goes out into the burn chamber, or whatever you call it.
During the cycling I hear the draft inducer run for a bit. Then a glowing from the first two trumpets on the right, which gets progressively brighter over a few seconds. I assume this is the ignitor heating up? Is it like a glow plug? Then a click which I'm guessing is the relay to start the gas. When it fails to light the cycle repeats. When it does light, flames shoot out the ends of the trumpets into the burn chamber and the furnace runs for a while.
The red LED on the board (white rodgers carbide automatic ignition system) is blinking but in no discernible pattern.
The humming noise I was describing was the draft inducer turning on.
I see 5 "trumpets" where the gas comes in the back from the manifold/line and then goes out into the burn chamber, or whatever you call it.
During the cycling I hear the draft inducer run for a bit. Then a glowing from the first two trumpets on the right, which gets progressively brighter over a few seconds. I assume this is the ignitor heating up? Is it like a glow plug? Then a click which I'm guessing is the relay to start the gas. When it fails to light the cycle repeats. When it does light, flames shoot out the ends of the trumpets into the burn chamber and the furnace runs for a while.
The red LED on the board (white rodgers carbide automatic ignition system) is blinking but in no discernible pattern.
#6
03-30-22, 03:14 PM
Group Moderator
I had similar behavior in a furnace in a rental unit once and it did turn out to be the igniter.
#7
03-30-22, 04:55 PM
Group Moderator
It could be a defective igniter if it's not glowing bright enough. It needs to be near white hot.
It could also be rust in the burner blocking the ports that send the gas towards the igniter.
12 years old.... has it ever been professionally cleaned ??
It could also be rust in the burner blocking the ports that send the gas towards the igniter.
12 years old.... has it ever been professionally cleaned ??
#9
04-02-22, 07:41 AM
No home warranty... that's why I'm on this forum Plus I like to know how everything works and do it myself.
"Not glowing bright enough" is possible... definitely not white hot, just bright orange. Watched a few random YouTube furnace videos and it seems reasonably close, maybe a little dim, but hard to tell.
No idea the last time it was thoroughly cleaned/serviced. I did replace the filter (which was very dirty) and will probably relocate from inside cabinet to just outside, into the return duct, so it can be more easily replaced in the future. We are nearing the end of heating season here in Durham NC (and not quite yet to AC season) so I will plan to give it a good cleaning. Some quick YouTubing suggests that should include burners, flame sensor and general vacuuming of cabinet. My previous house had an oil boiler and I'd clean/vacuum the burn chamber... should I clean out the heat exchanger on this furnace?
Plus I like to know how everything works and do it myself."Not glowing bright enough" is possible... definitely not white hot, just bright orange. Watched a few random YouTube furnace videos and it seems reasonably close, maybe a little dim, but hard to tell.
No idea the last time it was thoroughly cleaned/serviced. I did replace the filter (which was very dirty) and will probably relocate from inside cabinet to just outside, into the return duct, so it can be more easily replaced in the future. We are nearing the end of heating season here in Durham NC (and not quite yet to AC season) so I will plan to give it a good cleaning. Some quick YouTubing suggests that should include burners, flame sensor and general vacuuming of cabinet. My previous house had an oil boiler and I'd clean/vacuum the burn chamber... should I clean out the heat exchanger on this furnace?
