Gas furnace won't ignite.
#1
11-18-14, 09:03 AM
Gas furnace won't ignite.
I have a Maytag 97% modulating furnace that doesn't want to ignite. It's only about 5 years old. The thermostat shows a message "Furnace Ignition FLT". When I turn on the power I can hear the inducer motor start and then the hot surface igniter starts glowing but then nothing. I don't hear or smell any gas. My gas water heater and dryer are working so the house is getting gas. Could the gas valve be bad? Are there any checks the system does after turning on the hot surface igniter but before opening the gas valve? I hadn't changed the air filter in a couple of years but I wouldn't expect that to prevent the system from igniting.
#3
11-18-14, 11:40 AM
I have changed the air filter. I do not hear a click from the gas valve. The gas valve should attempt to open a few seconds after the hot surface igniter starts glowing, correct?
#4
11-18-14, 12:10 PM
Yes, but is the control board sending a signal to the gas valve to open?
You will need a voltage meter to check it.
Is the furnace blinking an LED light? That would help find the problem. A high limit switch could have kicked out due to the dirty air filter, but I'm not sure if that would totally shut down the furnace or just stop it from relighting the burners.
You will need a voltage meter to check it.
Is the furnace blinking an LED light? That would help find the problem. A high limit switch could have kicked out due to the dirty air filter, but I'm not sure if that would totally shut down the furnace or just stop it from relighting the burners.
#5
11-18-14, 12:59 PM
I'm having the same problem with my Amana gas furnace. LED error code is showing "open high limit switch" I changed the switch with no luck. Any ideas. Clean filter, air flow good.
#6
11-18-14, 01:02 PM
I have a multimeter but it is not a clamp on. I might pick one up on my way home today. The only blinking leds are after the system fails to ignite in which case they start blinking alternatingly between the red and green. The manual shows that this is the one hour lockout.
#7
11-18-14, 04:00 PM
Sure sounds like a limit switch. Limit switches rarely fail, sounds like it's doing it's job. You won't need a clamp on meter. Post the model numbers on the furnaces.
#8
11-18-14, 07:12 PM
The model number is pgc2mq.
I have confirmed that there is no voltage going to the gas valve while the hot surface igniter is on.
I got a clamp meter and put it around the green wire labeled MVT and 24v on the control board.
I have confirmed that there is no voltage going to the gas valve while the hot surface igniter is on.
I got a clamp meter and put it around the green wire labeled MVT and 24v on the control board.
Last edited by esgeroth; 11-18-14 at 08:18 PM.
#9
11-18-14, 07:50 PM
I checked for continuity across all of the limit switches. Easy to check since they are all in line with each other. I do get continuity so it doesn't look like any of them turned off.
I have included a scan of the wiring diagram if it helps.
I have included a scan of the wiring diagram if it helps.
#10
11-22-14, 12:26 PM
Does anyone know more about these newer modulating gas valves? They are not like older ones that have only two wires. This one has 5 wires labeled MVT, TR, TX, RX, and TH. So I'm not so clear on how to test it or test if the control board is trying to open it. I put my clamp meter around the green wire that is also labeled 24v on the control board and got nothing but I don't think this is a sufficient test.
#11
11-22-14, 03:20 PM
The best way to test a gas valve is volts. Is this a Honeywell valve? What's the model number on the valve?
#12
11-22-14, 03:45 PM
Member
With voltmeter set to volts AC place one meter lead on TR and one on MVT. After the ignitor starts glowing you should have 24V there.
#13
11-26-14, 10:05 PM
Thanks firedawgsatx, thats what I needed. Yes there is voltage to the gas valve. I have also been able to determine that the gas valve is opening. I took the gas valve and manifold off with it still connected to the gas supply. When the valve opens I can feel a small amount of gas coming out of the nozzles in the manifold. Not enough that I can hear the gas flowing out, but I can feel it and smell it if I am very close. Could it be that the valve isn't opening enough or is obstructed somehow? The furnace is only about 6 years old and is nearly spotless on the inside.
#14
11-26-14, 10:28 PM
Member
It is possible the valve is not opening all of the way but that is probably not the case. The only way to tell is to have an experienced/qualified person to test it by connecting a manometer. Some utility companies will come out and test the gas valve pressures for their customers.
#15
11-26-14, 10:46 PM
I was thinking more of the gas valve inside the furnace. I'm pretty sure the gas supply valve on the outside is ok since if I open it a little bit with nothing attached to it I can hear and good rush of gas come out of it. But when the gas valve inside the furnace opens, I can't hear and can barely feel the gas that comes out of the nozzles.
#16
11-26-14, 10:58 PM
Member
I meant the gas utility company in some areas will actually come out and test the gas valve inside the furnace.
#17
11-27-14, 10:49 AM
Rheem classic 90 fails to light
I have a Rheem 90 Classic + natural gas furnace with electronic ignition
It's symptoms are
1 ) Not lighting ( ignition spark was verified both igniters were cleaned )
Limit switch is zero when induction motor is off, then there is a brief voltage jump the quickly tapers off to 3.
Both green lights on the control board are solid green
Amber flame light is off
Using a torch, when the igniters are sparking I can fire the burners for about 1 second then they go out.
I am not sure which contacts to use to verify if the gas regulator is receiving voltage to open .
Thank you for any assistance.
Indoor temp is now 62 out side, well there is snow on the ground, and no heat
It's symptoms are
1 ) Not lighting ( ignition spark was verified both igniters were cleaned )
Limit switch is zero when induction motor is off, then there is a brief voltage jump the quickly tapers off to 3.
Both green lights on the control board are solid green
Amber flame light is off
Using a torch, when the igniters are sparking I can fire the burners for about 1 second then they go out.
I am not sure which contacts to use to verify if the gas regulator is receiving voltage to open .
Thank you for any assistance.
Indoor temp is now 62 out side, well there is snow on the ground, and no heat
#19
11-27-14, 06:06 PM
Member
The Rheem Classic 90 Plus has a lot of different models in that line. To better assist, please provide the model number of your furnace.
Last edited by firedawgsatx; 11-27-14 at 06:37 PM.
