First of all millivolt heaters dont have an actual flame sensor. flame rods are located in the electronic ignition models. The only "flame sensor" millivolt units have are the pilot generator or thermal pile. it also works as a safety, no pilot flame = no dc current, heater wont run
For just a little more info than what was provided... On digital control heaters, The job of a flame sensor is to ensure and confirm that there "IS A" flame consuming the raw fuel. Hence, the name. If no flame is present, no reading of a millivolt can be achieved (the heat transfers the understanding that there is a flame to consume fuel and sends a DC Milivolt reading back to the electronic circuitry.) Thus allowing the circuit board to confirm this and fuel can then be continued to be provided and/or allow a Main valve to open and have final ignition of main burners to heat pool/spa water. The same type of safety confirmation is achieved on a Milivolt System. Even though there is no Flame Sensor, per se, it works in about the same way. As you hold down a "Pilot" button or a gas valve knob in the Pilot position, you are manually keeping a pilot valve open to provide fuel through a small tube to the pilot burner assembly. At the pilot burner assembly is a Pilot Generator/Thermopile/Thermocouple - all 3 of these are the same thing, just have different names. This thing generates millivolts as it gets hot. When the correct amount of millivolts is achieved, the pilot valve will automatically stay open and you can then release the knob. If it never gets hot, the valve will not stay open, hence a safety feature.
Answers & Comments
First of all millivolt heaters dont have an actual flame sensor. flame rods are located in the electronic ignition models. The only "flame sensor" millivolt units have are the pilot generator or thermal pile. it also works as a safety, no pilot flame = no dc current, heater wont run
For just a little more info than what was provided...
On digital control heaters, The job of a flame sensor is to ensure and confirm that there "IS A" flame consuming the raw fuel. Hence, the name. If no flame is present, no reading of a millivolt can be achieved (the heat transfers the understanding that there is a flame to consume fuel and sends a DC Milivolt reading back to the electronic circuitry.) Thus allowing the circuit board to confirm this and fuel can then be continued to be provided and/or allow a Main valve to open and have final ignition of main burners to heat pool/spa water.
The same type of safety confirmation is achieved on a Milivolt System. Even though there is no Flame Sensor, per se, it works in about the same way. As you hold down a "Pilot" button or a gas valve knob in the Pilot position, you are manually keeping a pilot valve open to provide fuel through a small tube to the pilot burner assembly. At the pilot burner assembly is a
Pilot Generator/Thermopile/Thermocouple - all 3 of these are the same thing, just have different names. This thing generates millivolts as it gets hot. When the correct amount of millivolts is achieved, the pilot valve will automatically stay open and you can then release the knob. If it never gets hot, the valve will not stay open, hence a safety feature.
Good Luck