1

I have a newer (2007?) Trane XL90 (model: tdx2b080a9422aa)forced air gas furnace. The problem I am having is the burner and fan will not start. The thermostat is set at 67 degrees and shows it is calling for heat but no heat. Noticed it started coming on and off intermittently over the last few days. I took a multimeter to the ignitor and its not reading ohms. Have not checked the flame sensor yet. Should I? Also the fan is on on the thermostat, however it will not start on the furnace. Is this a startup sequence issue? the fan should start first I believe. Any help would be greatly appreciated.

  • What error code are you getting? (blinking lights; what's the pattern and number of the flashes?) There should be a switch for the power; turn it off. If not you may need to cycle the breaker to actually reset it. FIRST make note of any error codes. – Mazura Oct 28 '14 at 4:54
  • 1
    Four red flashes. Indicates open temp limit switch. – Nick Oct 28 '14 at 7:31
  • Sounds like the plenum overheat sensor. – Mazura Oct 28 '14 at 7:37
  • Do you live with this person, lol... diy.stackexchange.com/questions/51891/… – Mazura Oct 28 '14 at 7:40
  • I could not find any documentation on your equipment. Could you please include the schematic for the furnace, which you should be able to find in the documentation or on the inside of the service panel? – Tester101 Oct 29 '14 at 10:44
1

I have the same issue... I did the filter change, checked all the thermoswitches then it worked for a few hours then no heat LED flashing 4 times. Then I measured the voltage from the 24V transformer and the voltage would vary from 11V to 19V max. Replace transformer and furnace is working perfectly.

0

Yours may be different, but generally high temp limit switches look like this:

enter image description here

Order the correct replacement online (if you can figure out the right one to get and have verified it has failed by testing for broken continuity) or take it to your local appliance parts supplier. Bring the model number of your unit with.


"well 4 blinks is an open limit or roll out" -Matthew

What else is on the same wire? (test the other parts of the safety chain)

Roll out switch:

enter image description here


After replacing a faulty ignitor, the elusive culprit for the OP was old batteries in the thermostat.

"I replaced the batteries in the thermostat, even though it wasn't calling for new batteries and now it works".

  • 1
    @Nick A limit switch is a switch, it's either open or closed. You can test it using the continuity or ohm setting on you multimeter. If the switch is closed, you'll get a reading between the two terminals. If the switch is open, you won't get a reading or you'll measure infinite ohms. The high limit should only be open if the furnace is over heated. If it's open when the furnace is cold, it's bad and needs to be replaced. – Tester101 Oct 28 '14 at 11:45
  • 1
    OK back from the local HVAC dealer. Took my ignitor and limit switch. Ignitor was bad and replaced. Limit switch was good. Come back home to put everything back together and its still the sam . No blower fan. And its still flashing 4 red lights. Any ideas? @tester101 – Nick Oct 28 '14 at 19:37
  • 1
    Blower does not come on after thermostat is switched. – Nick Oct 28 '14 at 23:21
  • 1
    If the blower does not come on when the thermostat is set to the FAN (or ON) position, there's possibly a problem with the control board. I can't find the schematic for this model online, so I can't confirm this but the blower should turn on despite an open limit. The problem could also be a bad thermostat. Have you tried bypassing the thermostat? – Tester101 Oct 29 '14 at 10:47
  • 1
    There should be no ignition sequence for simply turning on the blower in FAN mode, it should simply activate the blower relay. If you can't even get that to work, the problem is likely "before" the limits. This would point to the thermostat, control board, power source, or possibly even the blower motor itself. – Tester101 Oct 29 '14 at 10:55
0

My Trane XR90 gas furnace was not producing heat. The red diagnostic light was blinking 4 times, which means "Open High Temperature Limit Device".

I did some analysis and found that I have two flexible condensate hoses that drain into two 3/4 PVC pipes. These PVC pipes meet at a Y junction and drain into one PVC pipe that leads to the exterior. The plumbers clay around one of the flexible hoses was old and crumbling, allowing air to enter. This was creating a backdraft, which prevented condensate from draining and causing a sensor to trip the unit. As soon as I removed the old, crumbling clay and replaced it with new, pliable clay, the unit starting heating again. Hope this helps someone.

0

It has been my experience that these units are very air filter sensitive, i.e. if dirty or the wrong filter merv rating is used they will go into a limited mode - - - try changing the filter to a Merv 8 and see if it fires up normally .....

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.