I have just been informed that our furnace is continually blowing cold air. The fan continues to run even when the thermostat it shut off. I had my wife flip of the “light switch” on the side of the furnace and that did shut off the fan. The thermostat is one of those cheap round mechanical style and the furnace is a high efficiency unit that is at least 20 years old. Of course I am in the process of flying back from Denver so I can’t view it 1st hand until this evening. I can do basic tests and repairs but obviously if this is serious it will require an expert. I’m just not sure whether to just have my wife call a technician now or hold off until I can mess with it a bit tonight. Any thoughts or suggestions (other than move)? Thanks in advance.

If the heat comes on when you turn up the stat it may be that the thermostat is sending an improper signal to the furnace to run. You can test this with a multimeter. If you have voltage between the g and the c terminal 24 volts it's probably a thermostat thing.
If the heat won't come on but the fan stays on it's likely an open limit. Either a main limit or possibly an auxiliary limit in a blower compartment. If you put your meter on both sides of the limit and it reads 0 its closed and not likely the cause. If you read 24 volts across a limit its open and that's the problem.
If it's a high efficiency it should have an error code should there be a reason other than the thermostat that's causing the issue. Before killing power to the furnace see if its throwing a code. You can update this thread with a make and model and I'll try to help.

I have just been informed that our furnace is continually blowing cold air.

I think that's called an air conditioner.

If the heat comes on when you turn up the stat it may be that the thermostat is sending an improper signal to the furnace to run. You can test this with a multimeter. If you have voltage between the g and the c terminal 24 volts it's probably a thermostat thing.
If the heat won't come on but the fan stays on it's likely an open limit. Either a main limit or possibly an auxiliary limit in a blower compartment. If you put your meter on both sides of the limit and it reads 0 its closed and not likely the cause. If you read 24 volts across a limit its open and that's the problem.
If it's a high efficiency it should have an error code should there be a reason other than the thermostat that's causing the issue. Before killing power to the furnace see if its throwing a code. You can update this thread with a make and model and I'll try to help.

So I had to reset the flame rollout switch. That seems to have at least temporarily solved the problem. The furnace has been running for about 1/2 an hour and the house is finally nearing “room temperature “. I also pulled the limit switch to make sure it wasn’t stuck and that solved the fan running problem. The question now is: is this a bigger concern even if it doesn’t happen again?

Thanks again btw.

Rollout switches indicate either a lack of combustion air or a bad heat exchanger typically.
So... you can take the intake pipe off the furnace and see if that helps. It could be a clogged intake pipe.
If it's a bad or rusted out secondary heat exchanger you would confirm with a combustion analysis. High co and low o2 is likely what you'd find.
One way to test an intake pipe is to drop a golf ball into the intake from outside. The pipes should be pitched toward the furnace so if the ball makes it back to the furnace that confirms the pitch is correct and the pipe is clear. Exhaust should be the same way.
Also I've seen clogged condensate traps cause this. If water is backing up into the secondary it could be enough to cause a rollout to trip but typically the pressure switch won't close before that happens but I've seen it before so I wouldn't rule it out entirely.

To check intake just run the furnace without the cover on the burner. Might have to tape down a door switch.

I'm out working.

Also.... is this a carrier?

Aorrybit took so long to get back to you I'm out working.

at 11:30PM?

This will likely cause other problems. Best practice is to cut the intake pipe a foot or so away from the furnace move the pipe ends apart and leave the combustion door in place. The pipe can be put back together easily enough with a fernco or glue in a coupling.

I think that's called an air conditioner.

what other problems? not arguing. i only know shitty residential furnaces through theory not a whole lot of experience

"HVAC Question for Darkside" can be sung to the tune of "Love That Chicken from Popeye's".

Have water leaking out from furnace so I called someone. The furnace has a humidifier on it and it was completely caked with calcium and crud. Guy took it off and did his best to clean it. He didn’t have the right size filter to replace it. He checked a few more things and then left. I bought the filter and let it run. Still leaking. I called back, the gal puts me on hold and then comes back. She talked to the tech and he said it’s probably a corroded heat exchanger and you will likely need a new furnace. I couldn’t believe what I was hearing. Quite a turn of events. They are sending another tech this afternoon to diagnose. Even if I do need a new furnace I’m not buying it from these guys. Frustrating.

Have water leaking out from furnace so I called someone. The furnace has a humidifier on it and it was completely caked with calcium and crud. Guy took it off and did his best to clean it. He didn’t have the right size filter to replace it. He checked a few more things and then left. I bought the filter and let it run. Still leaking. I called back, the gal puts me on hold and then comes back. She talked to the tech and he said it’s probably a corroded heat exchanger and you will likely need a new furnace. I couldn’t believe what I was hearing. Quite a turn of events. They are sending another tech this afternoon to diagnose. Even if I do need a new furnace I’m not buying it from these guys. Frustrating.

Did you have a home inspection or anything done before you bought the place? Not sure if there is anything you can do seeing as how you've been there just a few months, yeah?