If a roll out switch is a master “things have gone horribly wrong, stop the show” switch and thermisters are basically limited to measuring temperature, how do we know the burner is actually making a flame? Sure, the thermister will read heat, but that takes time. Imagine the igniter has failed, it takes maybe 10-40 seconds to register enough heat to confirm a flame. The combustion chamber is now pumped full of a potentially explosive fuel mixture and nothing is happening. We need something much, much faster, we need a flame sensor.
How Not to Detect a Flame
The way a person knows something is on fire is usually the bright flames and the fact that sticking their hand near it becomes really painful. This approach doesn’t quite work for a furnace. We could measure the light output, that requires more processing power to interpret the data, some incredibly sensitive hardware to detect the tiniest start of a flame, and it doesn’t work on every fuel type. There are systems that work this way, but it’s a little more expensive. We could measure the temperature, but we run into challenges with making a sensor you can shove in the heart of a flame for years on end without failure. It has been done, but it’s expensive.
There are however, laws of physics we can exploit to detect a flame without anywhere near so many challenges. We can detect a flame with little more than a volt meter and some careful positioning of electrodes.
Fires Rectify Electricity
AC Current is a current which oscillates between a negative and a positive charge. It goes up and it goes down. The process of converting this to all positive is called Rectification. Flames will rectify AC Current into DC. There are numerous ideas as to why this happens, which are irrelevant here. If we stick an electrode in a flame with an AC-charge, it will conduct across the flame to another electrode which will detect it as DC. It’s an inefficient process, terrible if you actually want to have a usable DC Current, but that’s ok. We just need to know that something is coming across.
This set up ends up being incredibly cheap and accurate. No flame = no signal. Flame = signal. The signal can be measured with any current sensing equipment, which is a couple of cents in silicon these days.
The furnace starts up, tests for the electrical current to pass as it starts the burner, and so long as no other sensors trip, all is good with the world. If it doesn’t detect a flame, it simply shuts down and after a cold shower, someone makes a service call to get it taken care of.
It’s worth noting, there is some maintenance involved in Flame Sensors. Overtime, the electrodes can become coated in carbon and stop conducting electricity altogether. In some cases the electrode can be cleaned and it’ll work perfectly afterwards. In other cases, it cannot be cleaned and the whole sensor needs replacement. This particular part is relatively easy to get to, given that it’s right at the burner. In some furnace designs it sticks through a hole into the combustion chamber. In other designs some parts of the case need to be removed to get to it.
If your flame sensor has failed, the furnace will do something similar to the following:
1. Attempt to start, fail to start, shutdown.
2. Repeat the start up procedure, waiting a few seconds each time.
3. After the 3rd failed start up, enter “lock up” mode, in which the furnace will not attempt to start until it has been power-cycled.
With a failed flame sensor, rebooting the system will not make it start. It’ll do the same 3 failed start ups and return to lock up mode. Numerous other failures can cause these same symptoms. If your furnace enters a lock up and refuses to start, consult an HVAC Professional. It could be something as easy as replacing a sensor or it could be more complex.