SUburban NT32 furnace cuts off after 30 minutes

[hertfordnc]

This unit worked fine last year. Now it comes on, lights, makes lots of heat for a while and then, before it reaches the set temp the fire goes out and the blower continues to run indefinitely.

Any ideas?

Thanks

Dave

[Chris_Bryant]

The limit switch should cut back in if it were over heating- but the contacts in it get oxidized and cut the voltage down below what is needed by the board.

Another item it could be is thermostat, if it is just for the furnace. Those contacts can get oxidized as well, so when it get close to the set temp, the pressure on the contacts gets lower, again cutting the voltage- the fan relay will close with 10 volts, running the blower, but the circuit board needs 11 at least.

[hertfordnc]

Upper limit switch makes sense. When i warmed the rig up slowly into the range of room temperature then it worked fine for the rest of the day. Outside temp was upper 50's and the furnace cycled on an off throughout the day.

Then at night it got colder and this morning furnace was running cold.

So, does that automatically mean the limit switch or could it be getting too hot and the limit switch is doing its job?

[sclark]

I agree with above posters. Upper limit switch. They are easy to change as long as you can get to the furnace. Good luck/

[Chris_Bryant]

Agree with limit switch or board- limit switch is the most common failure in that model, but the boards are known to fry, because of where they are mounted.
If you do have to replace the board, I would highly recommend the Dinosaur extend-a-life kit (along with a Dino replacement board).

You can do a very rough test of the limit switch by pushing on the metal element in the center of it- snap it a couple of times. If the flame comes back, you have found the problem.

[donn0128]

Upper limit thermostat failure or blocked air ways. Possibly main circuit board failire.