Gas and Oil Home Heating Furnaces - Trane XE 80 enters lockout minimal heat

Here is the furnace I have:

Trane XE80 American Standard Model TDD100C945B1
100,000 BTU Natural Gas
White Rodgers 50A55-843 Control Module (new)

The burners are shutting off prematurely before the set temp is reached in the house causing a cold house. I just replaced the control module about a month ago and it had been working fine for the first few weeks but same problems are back. Here are the details:

1. Call for heat from thermostat
2. Hear clicks in control module
3. Vent blower comes on
4. ceramic ignitor glows
5. gas turns on and it lights (everything is normal)

The normal process happens about 50% of the time and the house temp gets up to the set point of the thermostat and then shuts down like normal.

The other 50% of the time the normal light sequence happens but after anywhere from 3-20 minutes the burner just goes out (no gas), the fan stays on and the vent blower stays on. The control module figures out the flame is out and tries to relight. The ignitor glows but no gas flows so it stops glowing and won't relight. After a few attempts the control module shows the error code for lockout and stops trying to relight. Eventually the vent blower turns off and the fan turns off much like they normally would after cooldown but the temperature never reaches the setpoint. I can turn off the power and the control module resets and the whole process starts over.

So here is what I have done. I replaced the control module since the service guy I had come out told me it was shorting out. It is now doing exactly what it did before. I checked the flame sensor and it is clean (looks new), the ignitor glows fine and always lights when there is gas. I took off the vent stack and let it vent into the garage for a few cycles to make sure it was not clogged and still flame went out. I removed the temp limit switch (so it would not get hot) but left it connected to the wiring and still gas shut off. I removed the flame sensor wire while the burners were lit and everything shut down including the vent blower, which indicates to me that is not the type of shutdown it is doing. I also triggered the pressure switch on and off to get a shutdown and that doesn't seem to be what is happening either. Plus the control module has error codes for pressure switch and limit but they never show.

It really seems to me like the gas is randomly turning off due to the gas control. Is that possible, is that how those start to fail? I would think if it was low gas pressure from the meter the flames would look weak but they are consistently strong.

Is the furnace overheating, but why would it start doing that? I did replace the fan blower motor last year and I was wondering if I wired it wrong to where it is not blowing at a high enough speed to get the heat out of the furnance. But then again it does blow pretty good and there are many times it will run for 30 minutes or longer with heat on with no issues?

I plan to replace the flame sensor, pressure switch and limit switch on Friday since they are pretty cheap, but that just doesn't seem to be the issue. Any help is greatly appreciated.

Pretty good diagnostic effort for someone who apparently doesn't own a multimeter!

Actually, I'd recommend buying at least an inexpensive multimeter that measures ohms (resistance) and AC volts at a minimum if you plan to continue your career in furnace repair. A multimeter that measures DC microamps can be useful for the specialized task of testing the flame rectification system, but perhaps that additional cost isn't worthwhile for you.





The other 50% of the time the normal light sequence happens but after anywhere from 3-20 minutes the burner just goes out (no gas), the fan stays on and the vent blower stays on. The control module figures out the flame is out and tries to relight. The ignitor glows but no gas flows so it stops glowing and won't relight. It really seems to me like the gas is randomly turning off due to the gas control. Is that possible, is that how those start to fail? I would think if it was low gas pressure from the meter the flames would look weak but they are consistently strong.



I think you are on the right track here. Once the main burner lights and stays lit for five seconds or so, it should be stable and remain on until the thermostat shuts off the furnace.

This is where you need the multimeter, to check and verify what's actually happening.

For example, when this failure pattern happens you need to verify that the thermostat remains on all the time, and is not shutting off the furnace due to some oddity or defect.

You need to test the voltage applied to the gas valve and find out if that remains on when the gas shuts off, or whether the gas valve is shutting off because the power to the valve is being shut off for some reason.

When the furnace attempts to ignition process again and the ignitor heats but the gas doesn't flow, you need to check and see if you have 24 VAC across the gas valve as you should, or whether the ignition module isn't turning on the valve as it should.

Is there a diagnostic code flashing on the module? If so, that might go a good way towards identifying the problem.

The installation manual for the module is at
http://www.white-rodgers.com/wrdhom/pdfs/instruction_sheets/0037-6265.pdf

and the diagnostic codes are listed on page seven of the manual.

:coffee:
Thanks for the repsonse, I do own a multimeter but was not sure what to test for, I will monitor the gas valve for 24 volts during the shutdown and relight process. So you are saying that if the 24 volts is there the whole time but the gas still goes off then the valve itself may be bad (or gas supply issue). If the 24 volts goes away then the valve is probably fine but something is shutting it down, maybe the thermostat, control module or temp limit switch? the code on the module is one flash which is system lockout. when it goes out it goes through the two retries and then gives up. i would like to try and rule out the module since its new and only showing one flash i figure it is probably fine. also when this happened before i put a jumper on the thremostat to keep the call for heat constant and the flame still went out. so do you think it is basically narrowed down to the temp limit switch, flame sensor or pressure switch because I am thinking i just replace all those and it should be good. what about the transformer or the current sensing "thing" for the fan blower? would they have anything to do with this issue? it is supposed to get down to -9 F tonight.:eek:

:coffee:
So you are saying that if the 24 volts is there the whole time but the gas still goes off then the valve itself may be bad (or gas supply issue). If the 24 volts goes away then the valve is probably fine but something is shutting it down, maybe the thermostat, control module or temp limit switch? :


Yes.

What is the make and model of the ignition control? Is "system lockout" the entire description given for the fault code? If not please post the entire fault code description.

:wall:
the control module is a White Rodgers 50A55-843 Control Module and it is flashing once for system lockout. basically that is what it does after trying to relight four times unsuccessfully. then it resets after an hour and tries again. so it is putting out some heat when it finally gets lit.

ok, so i tested the voltage at the gas valve and it was ~26.5 V when it opened up. here is the interesting part, it was lit fine for a long time and i just monitored the voltage, the flame went out evetually while there was still voltage to the gas valve, then a second or two later the voltage went away. so it seems like the flame just went out due to no gas then the flame sensor picked that up and the control module shut down the voltage to the gas valve. then during the first relight attempt the ignitor glowed, voltage went to the gas valve but only for a few seconds then went away with no light off and the ignitor stopped glowing. everything seems normal except the gas just doesnt seem to be there. it will go through several relights unsuccessfully about 1 out of 5 will light and when lit it seems to stay lit for a good 10-15 minutes. i got my gas bill today for 385 :wall:

also the transformer is showing 27 V instead of 25 is that close enough or should it be right on 25 V? is it possible my transformer is putting out too much voltage and fried my new control module. when i first changed the module i noticed it didnt have the electric hum all the time like the old one. now the new board is sort of humming again when nothing is running. do you think something is frying the control module?

Well, the installation manual for your ignition manual is here:

http://www.white-rodgers.com/wrdhom/pdfs/instruction_sheets/0037-6265.pdf


The specified input voltage is 25 VAC and no maximum voltage is specified. I doubt very much the your slightly higher voltage is causing a problem.

And yes, your observation that the gas shut off when it shouldn't and then the flame sensor detected that flame failure is no doubt correct.

So you either have a bad gas valve or an unreliable gas supply.

Do you have natural gas or propane? Do you have other gas appliances --- if so have you noticed any erratic operation of the burners? If you haven't you should turn them on and watch them carefully for any signs of erratic burner flames.

If I were doing this job, I'd put a manometer on the gas supply line to measure the gas pressure when this flame failure occurred.

Similarly, I'd examine the gas valve for any signs of a defect. If you were fast enough, you might try tapping on the gas valve with the handle of a screwdriver within that 2 second period of flame failure to see if the flame will come back on.

If I were doing this job I'd use a jumper wire to apply 24VAC to the gas valve terminal to switch on the gas valve manually and do that 50-100 times until I decided whether the gas valve was operating reliably or not. Can't really recommend you do that, but a repairman might do that as a test to decide whether the gas valve is the problem.

If the gas supply seems to be the problem, call your utility and tell them you aren't getting a reliable gas supply.

If you think it's the gas valve, a repairman should change it out since the valve need to be adjusted properly to work with your furnace. He should be able to verify that the gas valve is the problem for you.