[bj101279]

The light was blinking 11 times and according to the troubleshooting section on the furnace it says that the furnaces limit switch is open and blower failure. How do I fix this problem?

[Appzalien]

The limit switch on a furnace is a safety device that measures the temperature during flame on and flame off. Once the heat exchanger reaches a very hot temperature (there is usually a setting on the switch for high and low temps), the fans will come on to blow hot air through the heat exchanger. After the house reaches the thermostat setting, the flames go out but the squirrel cage fan continues to run until the heat exchanger reaches its lower limit (also settable on most limit switches). The term "open" means there is no electrical current running to the fans, just like a light switch is open when the lights are out and closed when they're on. Newer furnaces have different switches than the older ones which were like a box screwed to the furnace wall with a sensing tube sticking out inside to measure the heat (its probably labeled as a limit switch if you look close.

Newer furnaces also have a vacuum switch safety, that will not allow the furnace to come on if there is no fan blowing out the exhaust (carbon monoxide gases out the roof of the house). If there is no vacuum, the limit switch might not open either. The vacuum safety is also a switch that will not close unless there is enough vacuum from the exhaust blower and is often in line in front of the limit switch so it will not get power if the vacuum switch does not close first.

The limit switch is often behind a removable box cover where you can set the limits and unscrew it from the furnace. The vacuum switch is connected to the exhaust blower with a rubber hose that pulls a vacuum when the blower is running, and if the hose leaks, the switch will not close (often you can see cracks near where it connects at either end). Be sure to turn the power off to the furnace before working on it.

[Appzalien]

I thought afterwords this might be helpful in diagnosing the issue:

This is the order of operation:
1. thermostat upstairs tells the furnace the room is cold
2. the exhaust fan comes on first and runs for a while to test the vacuum
3. if there is vacuum the gas valve will open and the main flame will come on and heat the exchanger
4. when the high limit is reached the squirrel cage fan that blows air around the house comes on
5. when the house reaches temperature, the thermostat tells the furnace to stop, but the squirrel cage will continue to run until the limit switches lower (cooler) temperature limit is reached
6. finally the furnace goes quiet until the thermostat starts it all over again