A furnace flame rollout switch is a safety device that shuts off the furnace's gas supply when it detects higher than normal temperatures outside the combustion chamber. This switch is normally closed, however as the flame moves away from the heat exchanger chamber (this is referred to as a roll out) the flames are then pushed back which will trip the rollout switch.  Listed below are a few scenarios that will cause the flames to roll out:

• Cracked Heat Exchanger

 A cracked heat exchanger can cause a furnace flame to roll out of the combustion chamber. This can happen when air from the furnace blower interferes with the flame, or when unburned gas projects into the combustion chamber before it has enough oxygen to ignite. 

• Clogged Vent Pipe or Improper Combustion

A clogged vent pipe can cause a furnace flame to roll out. The exhaust vent, also known as the flue, carries harmful gases like carbon monoxide away from the furnace and out of the home. When the vent is clogged, these gases can build up in the furnace and cause the flame to roll out of the burners and into the combustion chamber. This can be dangerous because the flame could ignite flammable materials in the combustion chamber and potentially start a fire.

• Clogged Chambers in the Heat Exchanger

Over time, the chambers in the heat exchanger can become restricted due to soot buildup, rust, or corrosion flakes, preventing combustion gases from escaping.

• Low Air Flow

Low airflow can cause the furnace to overheat, which can lead to flame rollout.

The Rollout switch is generally connected to the low voltage circuit that is also wired in series with the main limit switch. Most system have at least (2) switches in the circuit. The switches are usually resettable with a push button. Sometimes the switch will not reset due to corrosion, over heating, and or broken circuit.

This is where the Furnace Bypass Switch comes in play. Simply remove the wires from the the rollout switch and connect them to the leads from the bypass switch. Toggle the switch to the on position to bypass the circuit. If low voltage is present in the circuit the LED will illuminate. The Furnace Bypass Switch will allow you to run the furnace while you further diagnostics regarding flame rollout.  After the diagnostic is completed remove the bypass switch. Do not leave the switch in place, this may result in fatal injuries.