Flame sensor questions??

[hvac meister]

I have 2 questions about flame sensors in residential furnaces maybe some out there can help me with. While doing a tune & clean on a furnace this week I hooked my meter in series with the sensor lead to check the micro amp output as I always do but when I disconnected my meter, and therefore the lead to the sensor was not connected, the burners stayed on. I let it run a few minutes, turned it off and on several times and each time the burners remained on. Normally the burners go out right away. I told the customer this was a potentially dangerous problem and the only solution would be to replace the control board. Any other ideas on this? My 2nd question is about the sensor output on some of the newer furnaces. When measuring the output the reading jumps around like crazy, anywhere from 0 to 8.0 or more. Any ideas on this one? Thanks for any input.[/SIZE]

[slctech]

Flame sensor uA vary so much depending on the module, control, or board. In your case, if I had a basic furnace that was not shutting off on flame sense, I would assume the same. I agree that as rare as this is, that I would deem it as very unsafe, also.

[cjpwalker]

What kind of furnace? Some actually sense flame through the carbide ignitor.

[hvac meister]

It was a 10 year old rheem. Pretty standard, HSI on the right, 6 in-shot burners and sensor on the left.

[gravity]

i have this problem with rooftops i work on.

heres another test for ya.

let furnace run and unhook the flame sensor spade terminal and see if it runs.

sometimes when i hook my meter up to it and take 1 lead off it still runs. if i put everything back to normal and pull terminal off board it cuts off.

i notice on honeywell s8910u and johnson controls(blue box) they run in between .1 - 1

a lot of others run from 3-6

[jnsrose]

Does the flame sensor drop out once fame is sensed, or does it remain in the circuit the whole time?

[small change]

Those older rheem ignitors sensed flame and they then added a flame sensor kit
in the field to some of those.

I usually put my finger over the orifice of the burner that the hsi sets in
and have the flame sensor disconnected to see if the flame shuts down

[Chris_Worthington]

i have this problem with rooftops i work on.

heres another test for ya.

let furnace run and unhook the flame sensor spade terminal and see if it runs.

sometimes when i hook my meter up to it and take 1 lead off it still runs. if i put everything back to normal and pull terminal off board it cuts off.

i notice on honeywell s8910u and johnson controls(blue box) they run in between .1 - 1

a lot of others run from 3-6

Its been awhile on this end, but does not the positive go through the carbon in the flame to the negative (Ground) and that amperage is what is measured?

How does leaving a meter in the circuit, even partially, accomplish this task when the sensing end is disconnected?

I realize a meter draws amperage as well, but dag gone that sure is some pretty scary flame rectification on this end.

[socotech]

I think it depends if the circuit is only "looking" at flame signal during ignition. Never came across this.

[Chris_Worthington]

I think it depends if the circuit is only "looking" at flame signal during ignition. Never came across this.

There is Interrupted, Intermittent and continuous Pilots/main flame rectification circuits, but there are none that simply ignore "A" flame/Non Flame before/after ignition.

[pacnw]

Those older rheem ignitors sensed flame and they then added a flame sensor kit
in the field to some of those.

I usually put my finger over the orifice of the burner that the hsi sets in
and have the flame sensor disconnected to see if the flame shuts down

X2

I had this same "diagnosis" on a unit, but found out the flame sensor was no longer used. I removed the wire completely, so the next guy would not have the same diagnosis or bother cleaning the sensor.

I am fuzzy on whether the update was to or from flame sensor, might be both depending on the unit being worked on. Seems I remember having to add one on one unit as well.

[cjpwalker]

Its been awhile on this end, but does not the positive go through the carbon in the flame to the negative (Ground) and that amperage is what is measured?

How does leaving a meter in the circuit, even partially, accomplish this task when the sensing end is disconnected?

I realize a meter draws amperage as well, but dag gone that sure is some pretty scary flame rectification on this end.

The flame sensor is energized with an AC voltage from the board. The flame conducts to ground through the burner.

Because the current can flow better from the 'sensor' to the burner than from the burner back to the sensor, the board sees a pulsating DC current (rectification). If you short the two and the board sees AC it will shut down.

Could the meter be confusing the board (still one lead hooked up)? I am curious if the OP tried again with everything disconnected.

[beenthere]

Older Rheem may have also been sensing the uA through the HSI. Disconnect it also next time.