flame sensors

[theapprentice]

What is the best way to test a flame sensor to deturmine if it needs replacing? Also the same on the old electronic ignition model of furnaces?

[noheat]

take a microamp reading in line on the sensor wire.many ways to do this. Most units, the board, require a different amount of current to proove. Anyshere fron 2 to 6, after cleaning.

[mark beiser]

Originally posted by theapprentice
What is the best way to test a flame sensor to deturmine if it needs replacing?

The only time they ever need replacing is if the insulator is broken, or if someone has used incorrect methods to clean it in the past and has scratched up the rod.

Use only steel wool or other type of abrasive that will not scratch the rod, or leave any kind of residue behind.

NEVER use sandpaper or emery cloth to clean a flame sensor. It scratches up the rod, causing it to get dirty again faster, and leaves silica particles imbeded in the rod, wich act as an insulator.

[MikeJ]

If you don't have a meter that reads micro amps, better get one. Some of the less expensive new meters have this feature. Keep it just for sensor readings.

Hook it up in series and before you do anything, take a reading. Move the sensor a little and retighten it and see how the reading changes. Then clean it as described above. Some lennox units seem to fail on flame sense because the crossovers are dirty or the flame moves away from the sensor enough to drop the signal strength below the rated voltage and the unit shuts down. Then restarts and shuts down and goes into lockout.

[theapprentice]

do flame sensors rust?
what gives the difference in micro amps other than the fact that it is dirty, the length meby? what kind of reading can
you expect from a longer one than from shorter ones?

[MikeJ]

Originally posted by theapprentice
do flame sensors rust?
what gives the difference in micro amps other than the fact that it is dirty, the length meby? what kind of reading can
you expect from a longer one than from shorter ones?

1. no, they spend most of their life in fire so they like carbon up. But if someone uses sandpaper or whatever and scratches them, the metal now exposed can rust from moisture in the air.

2. heat. Heat drives the signal strength.

3. Depends on where the sensor is in the flame. A longer one may have the end too far from the flame. Manufacturers design the length, you should not change that.


As I said, best to experiment with a meter on the sensor wire. Most tech manuals tell about the sensor and what reading to expect.

[docholiday]

2 is wrong, sorry to say. The heat doesnt drive it. A/C voltage drives it and the flame is the rectifier to DC amperage provided it is proper. The proportional size difference from the face of the burner and the amount of flame rod in the flame determines the signal therfore as you said, the amount of rod in the flame is preset, you dont want to mess with the gap or amount of rod in the flame.

To answer the OP's question. Rarely should you have to replace a sensor, it is basiclya piece of metal insulated with porcelen at the mounting. As long as the rod is not bent, broken or burnt off, or the ceramic is not broken, the flame rod shouldn need replaced.

A low microamp reading is most often the result of no flame, dirty burner, low or high gas pressure, dirty or blocked crossover tubes or a dirty sensor as has been mentioned. You should also be able to read (with the burner off and power on to furnace a respectible voltage from the flame rod to ground (70-110v ac). If you dont then the connections or wire or board is the problem.

I agree with the others also that you should clean with steel wool or believe it or not, a dollar bill works well.

[countryhick]

Yes..... a new dollar bill works great.....takes a person back to the days of cleaning the points of your distributor in your vehicle.....some things never change

[Jim Davis]

The metal in flame rods can actually break down in the heat of the flame. Had this problem with Johnson Ignition systems back in the 80's. Hopefully that was a certain type of metal that is no longer used. Over time you can also lose ground. The AC signal is transmitted through the metal of the pilot tubing,(ground), rectified through the flame and brought back through the sensor. Because of oxidation this ground can be lost. Running a wire from the ground of the control to the bracket that holds the sensor will create a stronger circuit. This has fixed thousands of flame sensing problems over the years.