Pilot is Lit, burner won't light

[Friday]

Intertherm MGBA-070ABFC-01 gas furnace. Earlier this summer the blower kicked on by itself during cooling season (AC was not running at the time, compressor was out). I couldn't figure a way to shut the blower off, so I flipped the breaker to the furnace. Fast forward to last night. Decided to start furnace up for first time since last year. Remembered I had flipped breaker, so I first turned it back on. Went to the furnace and checked pilot, it was still on from last year. Switched thermostat to heat. Temp in house at the time was 66. Thermostat was set at 70. Furnace wouldn't kick on. Left it alone for the night. Furnace never did kick on. Pilot light looks good. What else should I check? Unit ran fine last winter.

[hvac1000]

Your high limit control may have tripped during the very hot weather like mine did or for any other reason. Near the blower on top of the furnace you should find a round switch with wires on it, There should be a red push button in the center of the switch. Push the red button down to reset and see if the furnace comes on.

[Friday]

Nothing like that on my furnace. There is a pair of wires connected to something on top of the furnace, near the blower motor, but it doesn't have a reset button on it. Any possibility this item is inside the control box?

[Robert]

Hi,

Look just above the gas valve burner for a small Flame Rollout Limit with a red reset button.

Reset it and see what happens.



If nothing still, go to the Aux. Limit you mentioned, remove the wires and ohm it with meter to see if it's open or closed.


Also, alot of those furnaces are designed so that when high heat raises temp in house such as hot summer and no a/c. That along with the pilot burning builds up heat in furnace chamber and the blower motor starts up as a safety.


Take care and best wishes,
Robert

[Friday]

There is no Flame Rollout Limit with a red reset button. I cannot check the ohms on the aux limit switch, my multimeter has seen too many years, and I can no longer read the switch setting, they have faded. I can see some of the ac settings but not the dc or ohm settings anymore. Is it possible to 'reset' this type of limit switch by cutting power to it? Or is it a disposable (once tripped-dead forever)?

[Robert]

Hi,

Your unit should have this:

http://www.mobilehomerepair.com/media/i ... 626352.jpg



If it doesn't, then a call for a Tech is needed.



You can jumper the aux. limit to see if it helps any.




Take care and best wishes,
Robert

[Friday]

Thanks Robert. My furnace doesn't have a switch like the one you show. It looks just like that one, without the push button. I will try jumpering it to see what happens and let you know. What type of wire should be used when jumpering this type of item?

[Robert]

Just trying to be sure we're not talking about two different switches.


The Aux. Limit does not have reset and is near the blower motor housing.


The Flame Roll Out should have reset and is above and close to gas valve and burner at front area.


The old original may not have had the reset button though.


Use 12awg wire to jumper for short period to see if furnace works.



Thanks,
Robert

[Friday]

Robert:

I'm going to try jumpering the system tonight when I get home (about 2 hours from now). If my system has a flame roll-out, wouldn't there be wires running to it? The only wires near the gas valve run to a black switch(?) on top of the gas valve itself. Then there are the two limit switches inside the control panel area. Nothing else.

[Friday]

Robert:

Jumpered the two wires that go to the aux switch. Nothing happened. Here are the steps I took: Set tstat to off. Shut off breaker to furnace. Pulled two red wires off aux limit switch. Using a short piece of 12 wire, jumpered the two red wires together. Turned breaker to furnace on. Set tstat to heat. Raised tstat setting to 75 degrees (tstat showed current temp to be 72). It has been 30 minutes since I did that and nothing has happened. (also, last night I set the tstat setting to Fan from auto and the fan ran fine). The troubleshooting guide for this furnace suggests checking for proper tstat operation (how id this done?), a defective transformer, and to check for 24 volts to the gas valve. What should I do next?

[Robert]

Jumper the other limit also. The flame roll out will look like a limit.

By switching from auto to on, you tested the transformer and it is ok since motor ran.


At t-stat, you should show 24-28vac across red wire and white wire connection with NO call for heat.

Then, with call for heat, you should see it read 0vac.


IF you read 0vac there, wait a 2-3 seconds, does the small combustion motor start up and run ?


If not , let me know.


http://www.mobilehomerepair.com/TechTip2.html



Thanks,
Robert

[Friday]

Robert, I will check this all out tonight. Thanks for the diagnostic help.

[Friday]

Robert:

Have not jumpered the other switch as yet. Tstat reads 24vac with no call for heat, and the same with call for heat (ran tstat up to 85 - switch in Heat position). What does that mean?

[Robert]

Turn power to furnace OFF , diconnect red and white wires at t-stat and tightly tape them together.


Turn power back ON.


Does furnace start up and run ok ?



If so, replace your t-stat. Those readings indicate t-stat is not closing on a call for heat.


Thanks,
Robert

[Friday]

Robert:

Jumpering the red and white did the trick. Thanks so much for your help. Short term, can I just go to the local hardware and buy a cheap heat-only tstat to get through the winter?