# Furnace turns off even if thermostat doesn't!



## lizatlanta (Feb 1, 2010)

Our furnace turns off (mainly at night when the temperature drops) and the thermostat will still read that the heat is on with a temperature below the set temp.  It will then begin to blow cool air to I guess "cool itself off."  We've changed the sensor on the furnace several times but this will only last for a couple days before it happens again.  Also, if we turn everything off and reboot the furnace, it will work for a couple days until I guess it "over works itself" and does it again.  This is beginning to be a pain- especially with a new baby on the way it's very cold at night!!! PLEASE HELP!!!


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## Wuzzat? (Feb 1, 2010)

Check that the 'stat voltage between the "R" terminal and the "RH" terminal stays at 0vac as long as the 'stat is asking for heat.
These terminal designations may be different for different 'stats.


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## maveric7383 (Feb 1, 2010)

Sounds like your furnace is hitting its high temp limit. I bet you have a lack of airflow going through your furnace for one reason or another. 
Things to check:
Air filter - make sure its clean
Blower wheel / squirrel cage - make sure its clean alot of dust and hair can get caught up in the fan blades.
Evaporator coil (if you have ac) - make sure its clean.
Return Duct - make sure there are no restrictions or damaged such as being crushed or flat. 
Supply ducts - make sure they are not damaged or crushed, cable guys love walking and sitting on these things. 

I dont think you have gotten multiple bad sensors. I bet your unplug your furnace when you replace the sensor causing your furnace to reset and delete the high limit code.

If you can find the Make and model # that would be alot of help.


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## lizatlanta (Feb 2, 2010)

I think the make is Train? Where do you find the high limit code and how do you change it?  Are there normal settings for this? Since I posted this, the furnace went out again! We do have dual thermostats for different temps on our upstairs and downstairs.  We dont run the downstairs really at all- would that affect it?


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## maveric7383 (Feb 2, 2010)

If two thermostats are wired correctly there should not be a problem. The model # will be near the burners. Did you look at everything I wrote down? If you have a newer furnace, there should be a light (led) red that will flash, count the flashes and it will tell you what is going on. In order for me to help you I need all the info. How many square feet is your house?


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## lizatlanta (Feb 3, 2010)

When we do the red light flash test thing, it always comes up that it is the pressure sensor.  We keep changing these sensors and it will work just fine for a couple days and then the same thing happens.  We looked at everything you wrote down and it was all fine.  Our house i believe is around 1300 or 1400 sq ft.


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## maveric7383 (Feb 3, 2010)

#1 You gotta find the model # for me. Me helping you is like the blind helping the deaf without that #. This is going to get a little more technical that I expected. Im glad you looked at everything I wrote down. Im going to guess and say that the "pressure switch" is your induce draft motor vacuum switch, is it a round disc with a hose attached to it? make sure there is no corrosion inside the tube going to the vacuum switch and inside the switch itself. If you get a little chunk of something it will jam it up. Hunch #2 I'm going to take a wild guess and say you have a 90+ furnace (90% or better efficiency). Does the furnace have any 1.5" - 2" PVC tubes coming out of it? This is only a guess but I bet at night when the outside temperature is low and the humidity is up the induce draft motor is struggling to keep the vacuum switch closed because the air is so dense with moisture. If i am correct your system pulls air into your burner compartment with the "induce draft motor" and pushes it out of the house a similar way that it came in. Now as a safety measure Trane installed a vacuum switch on your suction side of the induce draft motor. this will kick the furnace off if the induce draft motor fails. is quite common. If you dont have 2 pvc pipes coming from your furnace, you need to look at the seal around your induce draft motor, its RED RTV silicone same stuff that you can but at the auto parts store. Also look at the vent for the furnace. If there are too many tight bends its going to be hard for the inducer motor to push the air out. If you can find your owners manual and the install instruction it will explain how many bends within so many feet your system is allowed to have.


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## maveric7383 (Feb 3, 2010)

take some pics of your furnace, with and without the covers on.


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## lizatlanta (Feb 3, 2010)

I'll try all of this tonight and get you the model number. I really, really appreciate your help!!!!


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## maveric7383 (Feb 3, 2010)

not a problem.


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