Gas furnace

I have a gas furnace made by Inter City Comfort of Lavergne
Tenn. Model # GUH-100AU-16. When the burner starts the blower motor comes on right away. The burner seems to shut down after about 4 minutes but the blower keeps on runningfor about 15 minutes, all this time it is blowing cold air. To get the blower to shut off I have to turn the thermostat all the way up
then all the way down before the blower will shut off.
Any help would be greatly appreciated.

Thank you

 

Since you're having to manipulate the thermostat to get the fan to shut off, I'm going to guess this is a newer furnace with a solid state board to control the fan? The problem will most likely boil down to either a bad board or incompatibility between a digital thermostat and a solid state board. We need to know more about the furnace like age, does it have a solid state board or a temperature sensing switch controling the fan? The thermostat- digital or electromechanical? Brand, model #? Is this a new problem or has it always been this way? Did it start after you installed a new thermostat? With these answers, we should be able to come up with a solution.

 

We did install a new thermostat about 6 weeks ago but the problem just started in the last day or so. I forgot to tell you that it is not a digital thermostat

Thank you

 

How old is the furnace? Within the past 10 years or so? Newer furnaces have either a separate board to control the blower or an integrated furnace control board/module which controls the byrners and the blower all in 1 control. I know that ICP uses White-Rodgers modules a lot. Look inside the furnace and see what you find. If you see a module with numbers like 50A22, 50A50, 50E47, or 50F47, those are White-Rodgers integrated furnace control modules. See if you have a board/module or an older furnace with a temperature sensing fan control. I'm thinking you have a module in which the blower timing circuit has gone haywire. The other possibility is that the furnace is shutting off on the limit control. When that happens, the burners shut down and the blower comes on full time but usually continues running until reset. Look around for dirty filter, closed registers, obstructed return grille or return duct. We recently had one gentleman who found a large chunk of foam in his return duct that had been left behind by a duct cleaning company. If the blower is coming on right away [as soon as the burners fire up], then I think the module/board will be your problem. Normally, the blower comes on about a minute after the burners fire and runs for 90 to 180 seconds after the burners shut off.

 

I just finished installing a White Rogers Intergrated Furnace control # 50A55-143. The furnace seems to be working ok.
Can you tell me what Heat delay to fan on means.
And what heat delay to fan off means

The heat delay to fan on Switch 1 switch 2
15 sec. off off
30 sec. off on
45 sec. on off
60 sec. on on

heat delay to fan off switch 1 switch 2
60 sec. on on
90 sec. off on
120 sec on 0ff
180 sec off off

what does all this mean.
I also want to say thank you for all your help.

 

I figured that it had to be a bad module. The module controls the circulating fan by using timing circuits. The ON delay starts the blower X number of seconds after the gas valve is energized to give the furnace a chance to warm up before blowing air into the ducts. The OFF delay keeps the blower running X number of seconds after the burners go off to dispel the residual heat in the furnace. What you're looking at are dip switches to set the time frames. Normally, th ON delay is 60 seconds and the off delay is 90 or 120 seconds. The dip switches are there to give you some flexibilty. So, for example, to set the ON delay for 60 seconds, both switch 1 & 2 want to be in the on position. To set the OFF delay for 90 seconds, switch 1 is off and switch 2 is on.


You're welcome. I don't mind sharing knowledge and responding on this board keeps my thinking sharp.

 

Your furnace doesn't have a temperature operated fan control. It is set by the installer based on the temperature rise calculations taken at the intake and the plenum. It is basically how long (hot) before the fan starts in the beginning and how long (cool) after the set temperature is met and the flames go out does the fan run. You can do it with two thermometers or by feel at a register.

 

What Big John said.