Furnace Safety Devices
Earlier this year, I was called to investigate a fire in an attic of a single-story home. The fire damage was minimal, mostly related to smoke. The residence was occupied at the time of the fire, and the occupants turned the furnace electrical breaker off when they observed smoke coming out of the ducts.
A short time into the furnace examination, I discovered several serious issues. The furnace had not one, but four jumpers installed to bypass four key safety devices. In fact, every single safety device in the furnace had been bypassed!
The picture below shows the jumper (white wires with alligator clips) connected to the terminals of the furnace’s high temperature limit switch. The red wires are original and connect the switch to the safety circuit.
Gas furnaces use several common safety devices to protect against continued operation when certain faults take place. The safety devices are simple electrical switches designed to be closed under normal conditions and open under non-normal conditions. All these safety devices monitor temperatures in the furnace and are all connected in a serial manner, like links in a chain. If the sensor temperatures are within specification, they remain closed, creating a complete electrical circuit. However, if one or more of the devices opens, the continuous chain is broken, and the furnace control system shuts the gas burner off to eliminate hazard.
The high temperature limit switch shown above, is constructed with a metal element that deforms when exposed to rising temperature and is positioned in the airstream inside the furnace. The limit switch is calibrated to open at a specific temperature. Airflow issues, like a dirty filter (minor issue) or like a blower failure (major issue), are some common reasons for a high limit switch to activate. Most high temperature limit switches will automatically reset after a cooling period.
The second safety device is a thermal fuse. This device is similar to the high temperature limit switch in function, but different in that it does not reset when activated. Rather, the thermal fuse must be replaced if it is activated.
The third common safety device is a flame rollout switch. This switch also uses a metal mechanism but is positioned near the base of the gas burner. As the name implies, the flame rollout switch opens if the flame exits the burner compartment. Flue gas venting issues are a common reason for flame rollout switch activation. Flame rollout switches require manual intervention to reset.
The fourth and final safety device bypassed on the furnace was the draft safeguard switch. The switch is located on the inducer fan housing. The inducer fan propels flue gases towards the exhaust. Also a normally closed switch, the purpose of the draft safeguard switch is to monitor the inducer motor housing temperature. High temperatures around the inducer motor can indicate escape of flue gases.
In the case discussed, the blower motor malfunctioned causing a drastic air flow reduction in the furnace. Without air flow, the temperatures inside the furnace rose rapidly. A functioning high limit or thermal fuse would have opened in this circumstance, shutting down the gas burner. However, since both devices were bypassed by jumper wires, the furnace continued to operate resulting in the attic fire.
Jumper wires are common troubleshooting devices used by technicians to isolate issues. However, jumpers should always be used in a supervised manner and should always be removed when troubleshooting is completed. The misapplication of these common troubleshooting devices could have had catastrophic results if not for the vigilant actions of the homeowners.
About the Author
Timothy M. Himes, M.S.E., P.E., CFEI, CVFI is a consulting engineer with our Birmingham Office. Mr. Himes provides technical consultation and analysis of mechanical systems, investigation and assessment of incidents involving machinery and equipment, interpretation of codes and standards, and evaluation of fire and explosion origin and cause. His services also include the failure analysis of fire suppression systems. You may contact Tim for your forensic engineering needs at firstname.lastname@example.org or (205)838-1040.