Furnace shuts off at 70 degrees on the dot?

[rationull]

For my second new house related "General and Off-Topic" post of the week...

The furnace at my new place seems to work OK for the most part (aside from being 20 years old and having a few minor issues). One thing I've noticed is that no matter how high you set the thermostat, the thing turns off right at 70 degrees.

Is it possible this is just a characteristic of how the thermostat works, or does it point to a heat venting problem throwing the limit switch when the room gets warm enough?

(I am very new at this!)

 

[Jim 5]

I'm no expert but I did troubleshoot a furnace issue last winter.

You can rule out the thermostat by taking it apart and closing the circuit to see if your furnace will switch on even after the room is warm.

If that's not it, then it sounds like the limit switch in the furnace is tripping. There's a few reasons for that, including clogged filter, which would be the most common thing I'm guessing. If that's clear and everything else is clean and flowing properly, the next thing to consider is the only moving part (apart from the thermostat if it's electromechanical); namely, the fan motor.

If the motor is gone, it will soon overheat and stop running (which is what happenend to mine), which then causes the fire box to overheat and trip the limit switch. The tricky thing there is that when the motor cools, it might run again for a while.

You could take the motor out and feel the armature for play in the bearings. Check for a little wobble in the shaft. You could also close the circuit in the thermostat, then the heater should continue running. If the fan turns off before the flame/burners then you'll know it's the motor. Shortly after that, the limit switch will shut the gas off. If the furnace is functioning properly, the burners will shut off, then the fan will run for a minute or two to cool off the fire box before shutting the fan off.

Hope these miscellaneous ramblings could be of some help.

 

[The_Jeff]

Is there a seperate thermostat for the AC? Sometimes there's logic that keeps the heat 10 or 12 degrees from the AC setting. Used to be a simple mechanical interlock but you never know how they may have messed it up...

If you were a tenant in a rental property I'd think your landlord has a secret second thermostat locked up and wired in series.

 

[rationull]

eljefino, I think you hit the nail right on the head!

There is only one thermostat, but it's got two "levers", one blue and one red. The blue one dictates how cold it has to get before the heat turns on, and the red how hot it has to get before it turns off. I think I had only tried turning the hot one up, because I thought that was all that needed, and I think the gap between the hot and the cold must've been causing the heater to turn off at some kind of design-limited delta.

Tonight I just put them both at the same place and it heated the place up to 78 degrees or so no problem.

Thanks a bunch!

 

[Nick R]

This is why I like our digital thermostat, heh.

 

[mechtech2]

Jim5 beat me to it.
Bad thermostat or trip relay.
[Or a hidden thermostat over ride as a main control!]