Causes Of Furnace Lock Out

A modern furnace is integrated with a sensor system. This continuously monitors the furnace about a safe operation. If any unsafe condition is detected the sensory system sends signals immediately to the furnace controller. The furnace controller will stop power to the furnace to prevent mishap or damage to the unit. You should call for a qualified service technician even if the furnace seems alright to know the source of the trouble that caused the lock-out. Let’s know the probable causes of furnace lock out-

Faulty Ignitor

A furnace will go to a lock out phase if the igniter becomes defective. When you power on the furnace, the sensor also gets activated to check the action of the igniter. The job of an igniter is to light up the fuel that has reached the burner through the opening of the valves. If the furnace does not activate within a short while, the sensor will intimate the controller about it. The furnace controller stops the ignition sequence after trying it a few times.

Defective Flame sensor

The flame sensor plays a vital role in a furnace. It is a very small device located in the burner assembly. It looks like a thin, bent, metallic rod that is present in front of the flame stream. Its job is to confirm to the system that there is indeed fire after the valve opened to release fuel. If the flame sensor doesn’t report the presence of flame within few seconds, the furnace stops the ignition sequence. The controller in this case will also try to start the ignition a few times. In case it does not work, the ignition sequence will be stopped and furnace will be locked out.

Low fuel pressure and overheating heat exchanger

There are limit switches in the furnace. These monitor the safe functioning of a furnace. They monitor facts such as fuel pressure and the internal temperatures of the heat exchanger. If fuel pressure is too low, or if the temperature in the heat exchanger is too high, the switch closes. The closure of the switch sends signal to the furnace controller to send the system in a lock out mode due to unsafe functioning. If the filter of the furnace has not been replaced it gets clogged with dirt; after a point of time, the heat exchanger limit-switch trips. Furnace may get locked out if there is any problem or blockages in venting. Some modern models have status code lights that can tell you what caused the furnace to lock out.

Resetting the furnace

A furnace can be reset to unlock it. You can shut it off and wait for some time. Power it on again. Some of the furnace models take one or two hours to unlock itself. The oil-burning models have reset buttons on the burner that has to be pressed to restart it. If a gas furnace has a pilot light, then it has to be relighted to unlock the furnace.




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