Gas and Oil Home Heating Furnaces - Newbie question: "Open Limit Switch" on Trane XV90

Howdy. I have a 3-year old Trane XV90 furnace. No hot air coming out of the vents, though the blower is on. Air intake from outside is clear, and gas seems to be coming into the house, since the stove and hot water heater are going fine.

I get a 4-flash red error code on the circuit board, which I think indicates "open limit switch." If I am not mistaken, there should be a reset button somewhere to try, but I've looked the thing over for 30 minutes and I cannot find it.

Anyone able to steer me toward it, or suggest something else to try?

Thank you.

Cycle power to the furnace (leave it off for at least 10 seconds). That will reset an auto-reset switch. If you still get four flashes it is probably a manual-reset switch. Is this a downflow or horizontal furnace or one that can be installed in several positions? If so look for something that looks like this in the blower compartment or on the blower housing.
http://www.qcsupply.com/ProductImages/20620LG.jpg

The button in the middle of the switch (red in this picture) is the reset button. Push on it (with power off) and if it has tripped you should hear a light "click" when it resets.

There may also one or more similar switches on or near the burners.

IF YOU HAVE TO RESET A MANUAL LIMIT SWITCH SOMETHING IS PROBABLY WRONG WITH YOUR FURNACE. IF YOU HAVE TO RESET IT AGAIN SOMETHING IS DANGEROUSLY WRONG WITH YOUR FURNACE. You need to find out what is causing switch to trip. Let us know which switch is tripping.

xpogi:

Thank you for responding.

I turned off power to the furnace. I looked at the printed circuit diagram and saw that a yellow wire #9 from a 12-pin connector at the bottom of the circuit board was labelled ""high limit output."

I followed that wire all the way up to a connector at the bottom of the metal box (with the view port to see the burner).

Feeling around, I found a thin red tab sticking out between the connectors, which I depressed (it didn't look like the picture you posted). They sure don't make it easy to find the button.

Turning the power back on, the furnace seems to have started putting out heat, and the 4-red flash has stopped, and there is now the regular green flash.

Should this have fixed the problem? Is it risky to go to sleep or should I stay up a while and watch it? Is this the sort of situation where it is mandatory to have a pro check it out?

Once again, thanks very much for your input.

You might want to take a look at the air filter. A dirty air filter will cause limit trouble.

xpogi:

...Is it risky to go to sleep or should I stay up a while and watch it
Watch it for a few minutes and if it doesn't trip again it is safe to go to bed. If it trips right away you have problem that needs to be addressed, probably by a pro. If it never trips aging it must have just been a fluke and you are good to go. I am a bit of a pessimist but it probably will happen again sometime. You really ought to have it checked out in that case. And yes, make sure your filter is clean and also make sure all your registers are open and your return register isn't blocked.

Skip4661: Hi. Yes, I checked the furnace filter and it's clean. Thank you.

xpogi: I'll stay up an hour or two and watch. As a chemist, I get paranoid sometimes about CO poisoning. We have had temps less than 20 below here the last couple days, so maybe the furnace has just been overworked or something. Registers/returns are all clear so far as I can tell.

Thanks again.

Have a good night. :)
If I have a chemistry question I will be sure to ask. BTW I was a Chem major in college until I ran into quantitative analysis. That was before calculators. I am sure you understand!