blower fan won't shut off

[popgun2k]

Hi
I have a ComfortMaker forced air gas furnace.Model #GUH075A012CIN.Ever Since it was new,it doesn't shut down after it has gone thru its cycle,that is it continues to blow cold air until u manually shut the furnace off,or turn the thermostat down.This doesn't happen all the time though,which is baffling.It has a troubleshooting code on it which says that the pressure switch is stuck open.I have replaced the pressure switch,put it a new thermostat,cleaned sensors,but problem is still there.Could this be a venting or condensation problem

Any suggestions would be appreciated
Tks

[X Y Scaling]

Terry,

I'm not an expert but I'm going to answer just to see if I'm anywhere close to what the "expert" moderator will tell you. Please wait for his advice and confirmation.
Depending on how old your unit is this is something that I've had to adjust to correct the same problem.
Inside the furnace, behind the removable cover, there is an adjustable device that is called the Fan/Limit Control. This device does three things:
1. Tells the fan when to come on (after gas has been on for several seconds and the furnace get hot)
2. Tells the fan when to turn off (seconds or minutes after the flame has turned off and the unit has cooled down)
3. The "limit" part of the switch is set at the factory as a safety device . It will not let the furnace get hotter than its safety setting......... if it does, I believe, it will shut off the gas flow to the burner(s).

Now then, your problem sounds to me like this, what you need to do is adjust the fan "OFF" control (it's a small lever sticking through a clock like dial) This will turn the fan off at a higher temperature than it is now set. It sounds to me that your "OFF" switch is set too low and therefore will not turn off the fan.
Now lets see if I'm anywhere close.....


Vinyl and Laserdisc Fan

[Fitter597]

Could be a number of things, i think your unit is newer and doesn't use the old fan and limit switch, but i'm not sure, if you have an inducer motor and you said you think it might be a condonsation problem, tells me it is a 90% eff. If so you could have a venting problem, possible condonsate trap plugged up or dirty.you should have two hoses that drain into a trap in the bottom of the furnace, make sure it is clean, also check and make sure the exhaust pipe is pitched back toward the furnace and not bowed and holding water if this has been a problem since it was new it is most likely an installation issue. I would check the venting very good, if you have the installation instructions make sure your termination on the pvc on the outside of the house is per the manufactures specs.

[lonestarheir]

First off, How old is the unit? Is it a 90% furnace? If it is an 80% when the unit was changed did they change the flue pipe? Is it single wall flue pipe or double wall? A couple of things, I have seen it where the flue pipe does not have enough clear space around the termination cap (should have 3 foot clearance all the way around) The newer furnaces are very sensitive to the specs, without enough air they will do what you have described. Also, if by chance there is any sort of blockage or improper sizing of the flue can cause these sort of problems. I really need more details before I can make a better guess.

One other thing, why did you not call the installing company to fix problem if it has been happening since new?

__________________________________________________

Brandon
- Have you changed your filter this month? -

[popgun2k]

Tks for replies guys
The furnace is 10 years old.It is a Super High Efficiency furnace.It was a display home that I bought 10 years ago.The guy that installed it was an old contractor from here were I live(Kenora,Ontario),and hes probably passed on by now.he was old back then.Obviously if it was a venting problem it should never have been passed by the building inspectors,but we know how they work.
Terry