I have a model GMNT080-4 condensing gas furnace.

When the weather outside is about 25 degrees and higher the heater kicks in and heats to thermostat setting perfectly.

When the outside temp gets colder than about 25 degrees the heater kicks in but will not heat to the thermostat setting. Instead of heating to 74 degrees thermostat setting, the heater will kick in at about 62 degrees and never get the home above 67 degrees.

I want to know how to get this heater to keep our home at thermostat setting when it gets very cold outside.

I had two "professional furnace technicians" look at the furnace: The first one did the standard checks, burner, drain, exhaust, etc. etc. and said it should be working fine now. There goes $100. The next one told me I needed a new control board - that would really fix it. There goes a few hundred bucks more. That did nothing to fix the problem.

So, what do you think is the problem and how do I resolve it?

 

The most common reason for the symptoms you describe are a limit switch opening because of a plugged up warm air distribution system.

Did your repairman identify the reason why the furnace was shutting off? Was it shutting off because the furnace was overheating and the limit switch opening?

Exactly what did the repairman say they FOUND?

 

Here's exactly what he wrote on his invoice: "Diagnose furnace problem on Boodman 90+ furnace. Furnace fails to stay light off running for a period of time. Need to replace PC board B18099-13."

 

That's not the same thing at all. Indeed, he's not stating what he observed, just the conclusion he reached (replacing the circuit board).


If you have a multimeter and understand how to use it, I'd start by checking to see if the limit switch is opening and shutting off the burners. If so, the circuit board is not the problem and replacing it will do no good.

 

Thanks. The circuit board was replaced and did nothing to solve the problem.

Can anyone answer why it is that the outside temperature is the ONLY variable that causes this issue?

I just want to underscore what the ONLY cause is with heater not heating to the correct temp -- the outside temp:

I keep the thermostat at 74 and the heater kicks on faithfully at 71-72 and stops at 74. It runs like this perfectly at outside temps of 25-72. But when the outside temp drops below about 25 the furnace only kicks on when the house/temp/thermostat gets down to 62 degrees and then stops at 67, not 74 where it should. When it gets very cold outside, say 0 to -10, the heater won't go on until the house/temp/thermostat gets down to 59-60, then will stop at about 63-64.

I've been in this house for 2 years and the same exact thing happened both years after new board and thermostat and full furnace inspection.

What is it with the outside temp? Is it atmospheric pressure?

 

I often hear that the outside temp is the propblem when that has nothing to do with what is going on and is just a coincidence. Your vent termination may be improperly done and you are recirculating flue gases that are freezing when the temp gets that cold and this is blocking the air intake. You need to find the LED flash code on the control board while the unit is acting up for us to verify what the problem is. WIth out that you are just guessing at what the problem is without standing in front of the furnace all day waiting for it to fail.

 

Your stat should not drop more than 1 to 1&1/2 degrees before it comes back on. On you furnace
electronic board you have a R and W. post. When the temp has dropped 4 degrees, test for 24 volts
from post C to post W, Then check from post R to C. if you have 24 volts from C to R and you don't have it From C to W then the stat is not working right. This test will check the stat.
Do you turn the stat or power off then back on to start the furnace?
Is this unit nat. or LP gas? what else is on this gas line? Paul

 

The outside temps will be back in the 20s tomorrow. Now, at 34 outside the heater's working fine. I have the code descriptions and will reply back when I have some more solid information. Thanks for your great info.

 

The symptoms you describe, including that the problem exists when temperatures drop, are typical of a furnace that is overheating and shutting off on the limit switch.

It's quite simple, really. The furnace only has a chance to overheat when the burners are operating for a lengthy period of time --- when the outside temperature is cold.

During moderate temperature the furnace doesn't get hot enough to shut off on the limit switch, so all is well.

Your need to verify that this is the problem --- guessing doesn't count.

I asked you in an earlier post to check and see if the limit switch is opening when the burners shut off. You apparently haven't done that check. It's the next thing you need to do to verify that this is the problem.

You don't need to wait for cold temperatures. Just turn turn thermostat up to 80 degrees or so so the burners will stay on and see if the burners shut off after a while. When they do, use a multimeter to see if the limit switch has opened and shut off the burners.

If this is the problem you can replace the thermostat and circuit boards a dozen times and it wont make any difference.

So--- do you want to follow a logical diagnostic process or keep playing the game of guessing and supposing and changing parts and hoping it will solve the problem?

 

Thanks! That's the most solid information I've gotten so far. Ok, I'm very busy today but will try to check the limit switch just as soon as I can. That could be the prob but, yes, it needs be verified. I'll get back in the next couple days. Thanks again for your great advice.

 

Goodman Furnace model #12408A serial # 03F-60021

When the weather outside is about 25 degrees and higher the heater kicks in and heats to thermostat setting perfectly.

When the outside temp gets colder than about 25 degrees the heater kicks in but will not heat to the thermostat setting. Instead of heating to 74 degrees thermostat setting, the heater will kick in at about 62 degrees and never get the home above 67 degrees.

There was icicles hanging from the exhaust, so I wire wrapped it. Didn't help.

I read I should remove the vent cover. I believe the VENTING is the problem.

Look at this:https://www.youtube.com/watch?v=g7n7-PWEq8g