sorry for the long post, but I've been stumped trying to figure out what has been going on with my furnace lately:

I had a service tech at my house about 2 weeks ago after my furnace started acting up and he didn't find anything wrong with it - cleaned the flame sensor, burners, made sure the drain was open, and billed me an hour of mostly head scratching. Since the furnace was cold that day it ran just fine whenever he tried...

The details:

1986 Trane XL90 gas furnace, updated about 10 years ago with new heat exchanger and some other parts, new igniters, etc. Controlled by programmable thermostat. It usually runs just fine for early and mid winter, but over the last few years it has had the tendency to stop heating short of reaching the thermostat setting once we hit the late winter early spring heating season. Not often, but sometimes. It didn't do it at all last year, but this year it started early and as of this week it now does it all the time.

Details: The furnace runs great when the house is cool for the entire day (set to 55 degrees). It kicks in at 5:45pm, runs without a hickup until the house reaches about 62-65 degrees, which is when the flame stops, the exhaust fan keeps going, and it makes no attempt to refire. There's no LED light blinking, and it will sit there spinning the exhaust fan for hours unless I turn down the thermostat and turn it back up, or simply cycle the power on the furnace.

Now, once I do that, and the furnace is still warm, it will light the burners, but they usually will shut off within 10-60 seconds, putting the furnace back into the state it was in before, exhaust fan spinning. I think once in a while I have heard it actually try to refire, many many minutes after it cut the flame, but usually it doesn't.

So then I turn the furnace way down, let the house and everything else cool for an hour or two. After that, the furnace will fire up and start heating, again for quite some time, but once it hits 63-65, it'll shut off again. On a sunny day, when the house is already at about 65, I have been able to push it to 67...

Is the furnace overheating internally? I have checked if the AC coil (on top) is dirty, but I cannot access the business side of it. The top of it is spotless. It has been a while since that has been cleaned, but I don't think that's the problem. Air flow is significant, and removing the regular high efficiency filter to increase air flow didn't change a thing. Also - why will it "overheat" even after idling for an hour, burning for 10 seconds and then shutting off again, when the furnce is pretty much cooled down and doesn't even get a chance to warm up, while it can run up to 67 degrees house temp on other days?

I'd like to hear some suggestions as to what to do next to determine what's possibly broken, so I can determine if it's time to buy a new unit or pay for whatever it takes to make the thing work again. I'd like to avoid getting a new furnace for a year or two since I want to add on to the house and I may need to get a different size furnace at that stage anyway.

Ideas?

Peter