binder

[BINDER]

Just purchased a new circut board for my suburban furnace model nt30m and furnace worked fine on bench. Installed in my AS and it started and hooked all wires and fan came on but would not light.If I crossed thermostate wires at furnace it starts every time and cuts off when limit switch activates.I cross wires at thermostate and furnace fan runs but will not light.Any help would be great.

[wahoonc]

Originally Posted by BINDER

Just purchased a new circut board for my suburban furnace model nt30m and furnace worked fine on bench. Installed in my AS and it started and hooked all wires and fan came on but would not light.If I crossed thermostate wires at furnace it starts every time and cuts off when limit switch activates.I cross wires at thermostate and furnace fan runs but will not light.Any help would be great.

I am not an electrician but it sounds to me like either an intermittent open or a high resistance short. Maybe try stringing a piece of wire from the thermostat to the furnace and it if works you know there is a problem down stream in the wire and will need to fish a new one.

Aaron

[cooperhawk]

Make sure there are no restrictions to the air flow. If the sail switch cannot get enough air flow, it will not allow the furnace to light. If there is any obstruction in the vents that could cause the problem.

[GStephens]

Here are two more possible scenarios. The sail switch has a spring inside of it. If the spring is weak, the sail switch will not function correctly and the furnace will not light. Also, sometimes in checking for current, a probe is placed through the insulation. Occaisionally, a small piece of broken wire will poke out of this test site and short out. The unit will run on the bench because it is not grounding out. When placed back inside, if it touches metal, it shorts.
GStephens

[SafeHarbor]

Originally Posted by BINDER

Just purchased a new circut board for my suburban furnace model nt30m and furnace worked fine on bench. Installed in my AS and it started and hooked all wires and fan came on but would not light.If I crossed thermostate wires at furnace it starts every time and cuts off when limit switch activates.I cross wires at thermostate and furnace fan runs but will not light.Any help would be great.

Hi, Binder,

I'm not sure I understand this. If you short the thermostat wires at the heater, it lights and runs until it trips the overheat switch?

Hmm.... If it has enough airflow out, why would it trip?

Also, do you have a separate thermostat for the heater and A/C?

Lamar