I have two Trane units (installed 2008 and 2009 with complete systems including brand new furnaces). both are gas for heating.
My 2009 unit (R410A) has variable speed for gas furnace. It has a rather odd behavior. When I was in attic installing EZ-trap, I noticed it. I turned off AC from T-Stat. But every a few minutes, the blower would blow and fire flames are burning. First, I thought it was because I was touching the system and tripped something. but I was observing a few times later without me touching anything. How can this possible? For one, it's summer and I don't need heating. secondly, how can it operate without T-Stat turned on?
the other unit (R22, installed 2008) doesn't have the same issue.

I wonder the system 'leak' gas and furnace has to burn it?

 

Do you have a voltmeter? What the model # of the furnace?

 

yes. I do have a multi-meter.
the furnace model is: TUD1B080A9H31BB (Trane)

 

Sounds like you have a faulty stat or a short in the low voltage wiring.

I would see if this problem occurs when you pop the stat off of the wall.
Many thermostats will allow this without the need for unwiring the stat.

 

Thanks Houston204 for great advice as always.

I did a few things including completely replacing the T-stat. but issue is still there.

test 1: popped out t-stat cover. issue still there
test 2: completely removed old t-stat. then I noticed that there were three cut wires. there were not used but exposed closely. so I separted them and wrapped in insulating tape. then I make sure all other 5 wires were seperated with plenty of space. issue still there
test 3: given that the old T-stat is 20 years old. I bought a honeywell T-stat and put this new one in. issue still there. The T-stat is wired correctly because it can cool. what's funy is that even AC is cooling, I can still 'activate' the heating burner. below is how:

this seems to be motion activated. It kicks on when I walk toward the furance which is attic and things shake a bit when people walk towards it. When I have heavy steps walking toward the furnace, then I see flames burning (even with AC turned on and running). generally, it dies after a few seconds. the exhaust pipe gets hot.
I remmember when the AC guy did the installation summer 2009, the heating didn't even come on first in that winter. I called the guy. He came over and hooked it up. I wonder if he did it incorrectly. But my gas bill has been very low in summer.

when I installed EZ-Trap, I had to tighten pipes etc which could have shaked things a bit. I wonder if that was a factor. I noticed this start to happen after yesterday when I put ez-trap. but it could have been there and I may never paid attention.


I begin to wonder if new Trane gas furnace has some sort of protection mechnism. For example, does it detect some wind getting inside and start kicking in?

Or is this a failing control board of some sort?


My system is under 10 year part warranty. but I'm afraid most AC guys may not even know how to diagnose the issue and charge me a bunch of labor cost without achieving anything. that's why I want to make sure.

 

It sounds like you need to pull new stat wire.

I would snap a picture then remove all thermostat wire connections to the furnace and walk around to see if it continues to act up.


This mechanism is not present.
It is more likely that you are walking on the wire or causing enough movement for R to touch W on cut stat wiring.

 

Houston204,
Thank you so much. You are so right. It's the T-stat wire. I believe the issue is 'fixed' for now.
I started down the road to unwire furnace. But I noticed that it's easier to trigger when I walk on certain spots.
then I noticed that T-stat wire and a power supply wire are all pressed between a wood board and frame. The wood board is meant for walking. Basically the wires are 'squeezed' between two pieces of wood.
I put some wood chips around the wires to ensure they are not squeezed when walking on the board.
Now I can't reproduce the issue by walking towards the furnace many times.

I can't believe funks whoever installed the wiring initially would do that. this home was built by a reputable builder. It tells you that homeowner basically needs to understand everything to certain extent.

thank you so much. I hope you have a company in Dallas so that I can use your business for AC needs in order to 'pay back' so much contributions you made to the forum.

 

Since the thermostat wire is low voltage..... no great care is taken in running it.
I see haphazard cable runs all the time.

 

actually in my case, thee is a power cable lying the same spot. I'm grateful that it didn't get shorted.

It's amazing that the house passed inspection when it was built, which may be just a formality.

 

That is great news I'm glad that you found the short.

Low voltage issues can sometimes be tough to locate. It is often faster and more reliable to pull new stat wire then pull up any decking in an attic.

Stat wire is inexpensive should the problem return.

We do not have a location in Dallas.
We are located in Stafford, TX but mainly service the River Oaks and Memorial area.

I remember crawling by an old furnace in River Oaks to see it cycle off and on depending on where you put your weight.

 

Houston204,
they are lucky to have you there. I bet you get lots of repeat customers.

I really appreciate the help and will remember the t-stat wire is damaged if AC acts up again down the road.

 

Thanks,

I recommend getting at least 2 conductors more than you need, for future upgrades, should you decide to buy more stat wire.

 

I hope you will excuse me for asking this, but when you found the behavior remained AFTER removing the thermostat, sub-base and made sure all the wires were separated, why did you go buy a new thermostat? Totally removing the thermostat and yet STILL having the problem proved it was something other than the thermostat that was causing the problem.

 

The only T-stat is 20 year old. It has a mechanical switch. some times, you switch from off to heating, it would trigger fan. I was thinking of replacing it anyways. So before I started to remove the wires for testing, I got a new T-stat from home depot and planned to replace it regardless.

I didn't want to put old one back as I went through all the trouble of unwiring and labeling wires etc.

 

Okay, that makes sense. s-kjgmerwoiq0-