Your furnace is full of sensors, regulators, switches, and detectors of near everything. There are flame sensors, temperature sensors, pressure switches. That’s a lot of stuff that mostly feed into a controller that operates the furnace as long as it sees all the right readings. That one device is an almost single failure point. The only good way to stay safe is with redundancy. In most cases, these are physical, mechanical safeties which will over-ride every other part of a system and force it to stop cold.
Cutting Power on Failure
The Roll Out Switch is in simplest terms, a very fancy fuse. Most furnaces will have several of them spread throughout, each tuned to a particularly temperature. One near the burner or heat exchanger may be designed to trip if the furnace exceeds it’s maximum rated operating temperature. Another near the controller board may be set much lower, perhaps around 90 degrees celsius, just shy of when most silicon chips start to fail. Some may rest near the fuel line and manifold, set to extremely low temperatures, in case a leak and fire occurs away from the burner.
In the event that any of these switches trip, all power to the furnace is cut. There’s no shutdown process, it just loses all electrical power. If things were going wrong or at risk of going wrong, this usually stops the problem dead in its tracks. A shut down furnace cannot get fuel and without an intense flame, it isn’t likely to explode or set the office on fire.
These mechanical switches are often based on the melting point of certain metals and alloys. Once those metals melt, they can’t pass a current. No current is no power. If a switch fails, it’ll cut the power. There’s pretty much no scenario where a switch won’t melt and shutdown at it’s intended temperature. The same cannot be said for the furnace controller or any other piece of hardware, which can have software glitches or short out. Roll Outs simply work where other hardware could fail.
Preventing Fire in a Furnace?
There are a number of disaster scenarios. Perhaps worst among them is the fire in your furnace making it outside the furnace. In order for that to happen, the flame has to melt through a piece of casing or a failure in the fuel manifold, fuel lines, or burner directs an intense flame onto hardware not meant to take those extreme temperatures. In these scenarios, the usual flame sensors and temperature sensors likely wouldn’t detect anything out of the ordinary. Flames on, furnace is hot, the controller says burn on.
Things would roll downhill from there. Maybe solder joints would melt, spewing water all over the basement. Maybe the flames could spread and catch the rest of the facility. The fire could spread into the fuel line and cause an explosion. There’s unknown variables and every path of “uncontained fire in building” leads to tears.
To get a sense of the danger involved and how easily these fires can start, check out the video above. Barbosa Heating & AC posted this case, showing a disaster waiting to happen. The furnace controller would never see this coming. A well placed Roll Out Switch however, solves the problem immediately.