Furnace to start needs unit AC switch shut off then on sometimes

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Old 12-12-12, 07:40 AM
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Furnace to start needs unit AC switch shut off then on sometimes

Must shut off, then on, furnace unit AC switch to start TRANE furnace. Thermostat is OK. Happens a few times daily. Any ideas? Thanks.
 
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Old 12-12-12, 07:29 PM
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Any ideas ...... yeah. I'm confused


Can you re-post with a little more detail ?
A model number would be a big help.

You will need to tell us the exact sequence of what happens.
As a guess....you could have a flame sensor issue.
 
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Old 12-12-12, 08:32 PM
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As PJmax has posted, the model number will help identify the cause.
The furnace door should have a blinking LED visible through the door to assist in diagnosis.

Examples...





 
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Old 12-13-12, 08:48 AM
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Clarification

Thanks for the replies!

The unit is 17 years old. The Model # is TUJ100A960A1. TRANE unit.

More explanation:

Thermostat says HEAT ON. 24 volts is supplied to the control circuit. Unit will NOT start. I go to SERVICE toggle switch and cycle it (turn it OFF then back ON). Unit starts sequence (exhaust motor starts, few seconds later ignitor goes on, gas flame begins, a few seconds later main air circulating blower engages and I have heat until thermostat setting is reached --- everything is OK. Unit may continue to cycle automatically for a few hours before messing up.

NO lights on this OLD model --- unless I am missing seeing them.

Appreciate your help. If you need more info, please let me know.

Vic
 
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Old 12-16-12, 07:19 AM
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So far, so good

Unplugged and plugged some of the connectors at furnace unit about 4 days ago. SO far the unit seems to be functioning properly.

The only thing I can guess that is wrong would be the control board.
 
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Old 12-16-12, 07:35 AM
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By cycling the power to the furnace you are effectively resetting the control board. I recommend cleaning the flame sensor as normal maintenance. You need to describe in detail what happens when the thermostat calls for heat and the furnace attempts to fire, but does not.
 
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Old 12-16-12, 08:28 AM
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Problem is intermittent---hard to diagnose

The problem is intermittent and some time goes by before I notice the temp is not up to setting --- so I am not present at the unit to detail the control/operation trail at failure.

The TRANE unit is 17 years old. I keep a spare ignitor in case it goes. The flame sensor has never been cleaned. Didn't know it was a maintenance issue.
Maybe there are some components that should be changed due to the age of the unit and/or just plain maintenance --- any suggestions would be appreciated.

I understand that cycling the AC Service switch most likely resets the Control Board --- there could be relay/component problems on the board or limit/safety indicators set. Since the problem is intermittent, it is hard to diagnose the circuit board. If I had an oscilloscope, and a schematic of the board, I could let the scope run and give me some idea of what is happening. With a schematic only, I could probe the board (by the way I am an Elec. Eng. with many courses in Mech. Eng.).

Thanks for your help!
 
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Old 12-16-12, 08:52 AM
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The schematic is typically don't know the back of the blower door
 
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Old 12-16-12, 10:04 AM
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Furnace schmatic

The schematic on the blower door is more of a wiring diagram and not a control board circuit diagram. By the way, the unit has an ignitor and does not have a pilot flame sensor.

The only hint I have is that the thermo says HEAT ON and there is 24v at the furnace unit.

Again, if you think that after 17 years there are some parts that could/should be replaced, please let me know. I do believe that some parts should be changed for maintenance purposes ( I know the saying "If it ain't broke don't fix it"). The ignitor is 17 years old and I haven't used the spare yet. That is a long time for the ignitor to be useful --- but I will change it on the next failure, just for kicks!

Like I said before, the diagnoses for this kind of failure is hard due to its intermittent nature. If the failure were constant, I could monitor the start up sequence at will.
 
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Old 12-16-12, 10:11 AM
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Not a pilot flame sensor......a main burner flame sensor.

It is a thin rod that is sticking into the flame when the burner is lit. It is held in place with one screw. It has a single conductor wire connected to it. You remove the screw, disconnect the plug which is just a 1/4" slide connector and clean the rod with a Scotchbrite pad.
 
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Old 12-16-12, 11:42 AM
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I see no thin rods ------

I see no thin rods sticking into burner unit area. There is a small plastic box mounted on bracket where ignitor is attached. I see what look like "thermisters" --- one at top of burner nozzle assembly and one at bottom of burner assembly. That is it around burner nozzle assembly.

By the way, I changed the ignitor while I was in there. It works properly.
 
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Old 12-18-12, 07:27 AM
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Radiant Sensor

I believe that the plastic box mounted on the same bracket as the ignitor is the Radiant Sensor, and since I do not have the thin rod flame sensor, could this box be intermittently stopping the start up sequence? Anyone familiar with the device. I notice that is a 2 position switch and both output terminals lead back to the Control Module.
 
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Old 12-18-12, 01:59 PM
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Yes, your furnace does have a radiant flame sensor. This is obsolete as all manufacturers who used it had multiple and many failures. You need :
[TABLE]
[TR="class: DataTableResults"]
[TH]Part Item Number[/TH]
[TD="colspan: 3"] KIT09660[/TD]
[/TR]
[TR="class: DataTableResults"]
[TH] Description[/TH]
[TD="colspan: 3"] KIT; RADIANT SENSE TO FLAME SENSE. INCLUDED IS...FLAME SENSOR, WIRING HARNESS, LITERATURE, WIRING DIAGRAMS, WARNING LABELS, 1-STAGE CONTROL AND COVER. [/TD]
[/TR]
[/TABLE]
I also recommend a new gas valve when doing this upgrade. It would be a good idea to have this type of repair done my a professional.
 
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