Trane XL80 TUD100B948A0 - burner cycling while heating

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Old 10-28-11, 10:54 AM
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Trane XL80 TUD100B948A0 - burner cycling while heating

I have been observing an issue on our Trane XL 80 furnace where during operation, the burner cycles off frequently, but always relights within 20 seconds. Model number is TUD100B948A0, and yes, it is 20 years old this year - originally installed in 1991.

More specifics to the sequence of events, and "problem".
1. The thermostat calls for heat and the furnace turns on.
2. The exhaust fan turns on and shortly after that the burner lights.
3. The limit switch (Honeywell L4064A2881) can be observed as the it heats up - it passes the 80 "OFF" set point and continues to the 120 "ON" set point, at which time the blower fan turns on.
4. The temp reported on the limit switch then backs down and hovers around 100 while the blower operates. The LIMIT OFF set point is at 190, but the switch never rotates up to this level during operation.

---> **Problem** <---
5. Out of the blue, the burner cuts off. One can hear the gas valve click off, cutting the gas supply to the burner, and see the flames go out.
6. Within 5 seconds, the control module clicks and the ignition begins to glow.
7. Within 20 seconds, the gas valve clicks back open and the burners relight.
8. The furnace continues to heat the house, noting that neither the exhaust fan nor the blower ever stopped during burner cycling.

9. Steps 5-8 repeat often (every 5-10 minutes?), until the thermostat reaches the set point and turns off the furnace.
10. Furnace shuts down as expected - burner off and exhaust fan off first (together), and the blower continues until the limit switch triggers the OFF set point of 80.

Airflow does not appear to be an issue, given that the filter was just changed and the limit switch reports ~100 during operation, yet the problem still occurs. The control box red indicator light is not flashing at all, so it appears to be 'healthy'.

What is the correct order of troubleshooting at this point - inducer? flame sensor? exhaust flow? gas valve? other? And how does one go about checking these items? Voltage checks and/or ohm readings I can do. I do not own a manometer...at least not yet .

Thanks,
Ryan.
 
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Old 10-28-11, 11:13 AM
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Sounds like the flame rod needs cleaning. Remove and wipe clean with steel wool, dollar bill or rag. Try this before anything else.
 
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Old 10-28-11, 12:14 PM
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Originally Posted by mbk3 View Post
Sounds like the flame rod needs cleaning. Remove and wipe clean with steel wool, dollar bill or rag. Try this before anything else.

Could be the flame sensor. Cleaning that is routine maintenance that should be done each year in any case. It should be removed, cleaned with a wire brush or emory cloth or fine sandpaper and replaced.

The flame sensor can get coated with invisible oxides.

More likely I'd guess is that the pressure switch is opening. Use an AC voltmeter to see if one side of the pressure switch is opening when the burners shut off.

Also, check for diagnostic lights that might be flashing on the circuit board. They may be identifying why the burner shuts off.

If the pressure switch is opening and shutting off the burners you probably have something plugged up in the heat exchanger or venting system.


Good description of the sequence of operation you are observing in your furnace. If more people took the time for that they would get better and faster solutions to their difficulties.
 
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Old 10-28-11, 01:46 PM
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Okay, I know right where the flame sensor is. Have never cleaned it in the 8 years we have lived here. Sounds like that is a good #1.

Also know where the pressure switch is in the furnace. I can check it at idle, then when the furnace turns on, and I'll monitor it for voltage changes when the burner cycles. Please confirm in the wiring diagram shown that it is 24VAC, in the upper left, labeled "PRESS. SWITCH". Or is it the "VENT RELAY" below the pressure switch, which has both 24VAC and 120VAC on either side?

I have found the control box (circuit board), with the red light. The light remains off - no flashing at all during operation, during burner cycling, nor when not running.

 

Last edited by redlion4; 10-28-11 at 01:56 PM. Reason: adding image, grammar
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Old 10-28-11, 03:24 PM
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yes, it is 24VAC. Across the press switch terminals you should read OV with the inducer running. If the pressure switch opens it would read 24VAC.
 
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Old 10-29-11, 01:22 PM
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Saturday update. Removed and cleaned the flame sensor. Doubt it has ever been cleaned. We have lived here 8 years and this is the first time I have done it.

Have not yet taken voltage readings on the pressure switch, but I tinkered with it and am ready to do that next. Figured out the difference between the pressure switch and vent relay (that I was confused about above) after studying the components in the furnace.

Will watch/listen the the furnace for more burner cycling, and test the pressure switch later this weekend, then report back.

Thank you again!
 
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Old 10-29-11, 02:42 PM
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If you have a mercury bulb thermostat the heat anticipater could be bad. A quick & easy test would be to remove the stat & jump the wires connected to R (could be marked Rh &/or Rc) & W terminals.
Observe the furnace during this test & if it works normally, the stat is your problem.
 
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Old 10-31-11, 07:06 AM
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Monday update:

Thermostat is a digital LUX 5/1/1 type unit. Can't recall the exact model number, but can confirm it is not a mercury bulb type. That was my very first "home improvement" after moving into this house .

Cleaned the flame sensor, and the problem still occurs. The pressure switch reports open with 24 VAC when the burner kicks out, then moves back closed (0 VAC) as the ignition turns on and burner relights.

Just did the pressure switch test quickly this morning. Found, but did not write down nor take a photo of, the pressure switch sticker with model number and specs. Will snap a photo of it tonight and research it.

Next question - how does one determine if the pressure switch is 'bad' versus it is detecting a true problem with the exhaust? Is the appropriate action to first replace the pressure switch, then if the problem continues to troubleshoot the exhaust fan and venting? Or should one start by taking off and cleaning the exhaust motor and fan?

Thanks again,
Ryan.
 

Last edited by redlion4; 10-31-11 at 08:19 AM.
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Old 10-31-11, 09:07 AM
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If the pressure switch opens it will shut off the burners, so you want to check and see if that's the cause of the burners shutting off.

You need to use an AC voltmeter to check for approximately 24 VAC between chassis ground and the two connections on the pressure switch .

The pressure switch should close and remain closed as long as the inducer motor is running.

So measure the voltage at both of the pressure switch connections while the inducer motor is running. Both should be about 24 VAC. Then check the voltages when the burners shut off. See if they are both 24 VAC when the burners have shut off --- if they are, the problem is not the pressure switch.

If one side stays at 24 VAC and the other drops to zero when the burners shut off, the pressure switch is opening and shutting off the burners.

The odds are good that you will find that the pressure switch is shutting off the burners.
 
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Old 10-31-11, 10:16 AM
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Originally Posted by SeattlePioneer View Post
If one side stays at 24 VAC and the other drops to zero when the burners shut off, the pressure switch is opening and shutting off the burners.
This is what was detected this morning, except I had measured across the switch itself rather than from switch to ground. Reading across the switch was 0 VAC while the burner was running, 24 VAC when the burner kicked off, then back to 0 when the ignition kicked back on. So, I do believe the pressure switch is tripping.

The next question is "why?". Is the pressure switch bad? Or is the switch good and an actual pressure problem in the exhaust fan / inducer system is present? Any suggestions on testing these? I have blown out the vent line connection between the two.

Exhaust fan / inducer unit appears to be part BLW00024 ($200-300 online). Still trying to find the exact pressure switch part number - sticker is on unit, but I have not yet copied it down. Looks like pressure switches run $25-80ish.

Exhaust fan / inducer unit is showing its age on the outside - peeling paint off the exterior of the metal that houses the fan. Can only image what inside looks like. Appears fairly straight forward to remove, inspect, and clean. Any problems I should know with pulling it and having a look inside?

edit - from a 'safety' perspective, our house has CO detectors on all levels. If there is a true exhaust issue, it has not (yet) been severe enough to trigger any of the CO detectors.
 

Last edited by redlion4; 10-31-11 at 10:22 AM. Reason: added CO detector comment
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Old 10-31-11, 12:48 PM
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Remove the pressure switch hose from the port near the inducer and verify that it is not partially plugged NY putting a small drill bit or like item through the port.

Co alarms are not detectors and will only alarm well after there is a hazardous event happening. DO NOT rely on you CO alarm only to verify there is a problem with an appliance in the home.
 
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Old 10-31-11, 01:22 PM
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The port coming out of the furnace box has a few inches of copper tubing first (not removable), followed by a nipple on which the flex hose mounts and continues to the pressure switch. I'll have to find something flexible to check the copper tube, since it is not straight.

Agreed with the CO alarm - thank you for the clear position on that. I don't yet believe their is a problem with the core, however I think at this point is will be a good idea to pull off the inducer and look. This will also give me easy access to better clean the port where the vent tube stems from (from the inside, rather than outside).
 
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Old 10-31-11, 03:48 PM
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Yes, your test identified the pressure switch opening as the reason why the burners shut off.

It's possible but unlikely that the inducer motor or pressure switch are defective.

Far more likely is that there is an obstruction or defect in the heat exchange, venting system or drainage system plugging up the free flow of air through the furnace.

Of those possibilities, the most likely is an obstruction in the venting system--- I'd check that first.


Unfortunately, there are lots of things that can be causing the pressure switch to open that a DIYer is not going to be able to identify.

If you are unable to identify a problem, the smart move is to have a good repairman diagnose the problem for you. Replacing parts like the pressure switch or inducer motor and hoping that's the problem is usually an expensive waste of time.
 
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Old 11-04-11, 06:38 AM
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Plan to open it up this weekend and look for anything obvious - obstructions, blockages, cracks, etc. Found an exact match pressure switch for a good price that I will also swap, just to confirm.

No desire to call a repairman. If the problem persists with the new pressure switch and nothing turns up within the core, then I'll likely replace the furnace. At 20 years old, I don't see it as cost effective bringing in someone to tell me the core is bad and the furnace needs replaced anyway...been there, done that.
 
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