For the experts, I have a head scratching issue. My Trane unit is acting weird in heat mode. It is a propane dual zone system. It has worked perfectly for ten plus years. Last winter it started acting strange.

One zone would work and heat normally. The burners would ignite and heat would flow. The other zone would ignite, turn off and blow air.
My AC guy diagnosed it as a bad connection in the thermostat in that zone.

Fast forward to this year and now both zones do it. Turning off the breaker would usually reset and fix the problem for a while.
It has reached a point of what to do now.
I am considering changing out both thermostats to see if that fixes the problem.

Advice and suggestions would be greatly appreciated.

Not an expert, but have been battling a similar issue. Bad rollout/high limit switch?

Sounds to me like something in the furnace itself... these things now have circuit boards and so often the solution is to just replace that... but I am by no means and expert....

I hate to also tell you, but a lot of these units are built to last just about 10 years.....

If your thermostat is a two wire model, you could check it pretty easily.

What it does is close a switch at a preadjusted temperature.

Take one of the two wires loose, touch it to the other one. Your heat should go on (it might take a few minutes). If it does, then the thermostat is the problem.

Some systems give both heat or cooling. These need to be in the mode you desire before shorting the wires together.

With what info you've stated and my limited knowledge on residential Trane zoning systems, Sounds like the furnace is locking out on high temperature limit. Have your tech check the temperature rise and confirm the limit is not prematurely tripping. Zone systems must manage the air flow properly, if not, the low air flow will cause limit trips. Have the evaporator coil inspected. May need to be pulled and cleaned. There could be a zone control issue, but it should not cause the heat to lock out directly. Commercial systems do watch the supply air temps to prevent lockouts.

How long do the burners stay lit before going out, a few (3-5) seconds or 30+ seconds or longer?

I would bet against the problem being TWO stats.

quote:

Originally posted by Excam_Man:
How long do the burners stay lit before going out, a few (3-5) seconds or 30+ seconds or longer?

I would bet against the problem being TWO stats.

At the most 3 to 5 seconds if they come on at all.
Last time my guy was over to check the unit, everything worked fine when tested at the unit. We crawled under the house and tested there. Flame sensor was a tiny bit dirty but we cleaned it. That's been about a month ago.
I don't think there is any fire at all now.

Im in agreement that it's probably not two thermostats at the same time.

quote:

Originally posted by fgwilliams1:

At the most 3 to 5 seconds if they come on at all.
Last time my guy was over to check the unit, everything worked fine when tested at the unit. We crawled under the house and tested there. Flame sensor was a tiny bit dirty but we cleaned it. That's been about a month ago.
I don't think there is any fire at all now.

3-5 seconds is a dirty flame sensor.

Not lighting at all now, is a list of several things. Model number?

Off the cuff (not knowing 80/90% model or specs) if the unit is HSI (Hot Surface Ignition), does the igniter glow?

So this is a horizontal crawlspace installation? Probably needs a good cleaning and TLC, which it doesn't get since its in the crawl.

Worked on the unit today. My AC guy instructed. I didn't just go at it myself. Flame sensor had a couple of carbon marks on it. Cleaned it and cycled the unit.
Motors come on, glow thingy glows, and nothing happens. Repeat times three and blower kicks on and cold air flows. No fire.
AC guy said to check voltage on gas valve. Voltage is good when signal is sent. No click of valve opening to allow gas flow.
So, we think it's the valve gone bad.
The plan is to replace the valve and see if that solves the issue.

Replace the flame sensor cleaning doesn’t always solve the problem. Also a possibility that the feeder line for the flame to travel to the last burner may be dirty or blockage and not reaching it

quote:

Originally posted by fgwilliams1:

AC guy said to check voltage on gas valve. Voltage is good when signal is sent. No click of valve opening to allow gas flow.

If you have 24 volts across the valve and its not opening, its bad.

quote:

Originally posted by lee40215:
Replace the flame sensor cleaning doesn’t always solve the problem. Also a possibility that the feeder line for the flame to travel to the last burner may be dirty or blockage and not reaching it

With the information in his last post, it can't be the flame sensor. As it wouldn't keep the flame from lighting. And if the ceramic isn't cracked, there's no reason to replace a FS.