HI,
I have a furnace that won't stay in. The blower keeps blowing, but the heat cuts out because the flames get shut off every two minutes or so.

Over the summer, we remodeled and streamlined the duct work. We eliminated lots of big runs that absorbed all the heat. Now, though, I think we do not have enough. We have one small duct run blowing directly into an open auditorium (its for a church. We don't keep the heat on constantly, but instead need to be able to heat the room quickly when the building is used. Could the problem be from this--not enough returns or vents?

Secondly, I'm not sure if the gas is open all the way. We shut it off this summer, and there is an old valve. The flow through the lines is weak. Someone said that is how gas lines are supposed to be. Someone else said it should have more pressure. Would that cause it to keep turning off the heat?

The unit did survive a flood. Don't know if that could have blown something either. Thanks for your help

Sounds like you have a collection of problems. The short-cycling may be due to overheating, typically the result of inadequate airflow. The high-limit control will sense over-temp conditions and interrupt heating until things cool down a bit. The gas pressure issue should be investigated. Typically, natural gas inlet pressure should be maintained between 5"-14" W.C. under full operating conditions. A static test of gas pressure won't reveal problems that may occur under demand. Additionally, I'm sure the "bath" didn't help.

If you are heating a large room like a Church and the inside air is very cool it is possible that the fan switch is cutting itself off because of the cool air being pulled into the return air grill. If you know how you can reset the fan stitch to a Higher OFF setting and also raise the differintial between the on and off. This will make the Air coming out of the furnace hotter and maybe heat the Church faster.

Thanks for the responses. I noticed tonight that it is blinking four times, which says "open main limit/ aux limit" on our furnace. I guess that probably means there is not enough ducting?

Based on your description of the situation, that is likely the case. You mentioned "streamlining" the duct system. Can you be more specific?