flame short cycling

drhvac

Hi, going nuts on this Bryant furnace I'm working on. Flame comes on and goes off in about 5 - 10 seconds. I get a .4 microamp signal, then it goes to

- 3.10 and goes out. I don't know why it says minus 3.10? I cleaned the sensor, then replaced it after I got the same symptoms. I took the burners apart and cleaned, again same problem. The short time the burner runs I do get 3.5 in gas pressure. I thought it might be a ground issue so I ran a copper wire from the burner to a watewr pipe. same thing. I did notice where the molex plugs in to the control board, the flame sensor wire is a little corroded. I tried to clean but same thing. however, if that corrosion was a problem, I'm still not getting a good signal directly off of the flame sensor?

As I'm writing this and thinking of the corrosion on the molex, last year we had a hurricane in this area and there were alot of floods. I wonder if this board was under water, and now the effects of it are coming out and driving me crazy? When I go back I will ask the homeowner. But again, why low signal out of the flame sensor? thank you

JStar

Flame

The internal flame sensor circuit in the board could be damaged. The flame sensor may be sending the right signal to the board, but the board can't read it correctly.

drhvac

sensor

But if I disconnect the sensor wire from the sensor and check micro amp signal, I should get a good signal regardless of if the board is bad, right? What would happen if the ground wire on the molex plug to the board is bad, or if there was something wrong with the stud on the board where the ground wire was plugged in to? would it effect my micro amp reading?

drhvac

flame sensor

Could anybody refresh me on how exactly a flame sensor works. I believe the board sends a low ac voltage to the sensor before the flame lights, then once flame lights, it converts it to a micro-amp signal? does anybody know exactly?

John S_2

it's DC

The board sends a low voltage DC current to ground through the flame. You're measuring the conductivity of the flame as the current passes through the flame to ground. Sometimes it helps to clean the burner surface with emery cloth to increase it's conductivity. It's also why flame sensors can be cleaned rather than replaced. Anything that interferes with (or insulates against) conductivity will mess with the signal. The cleaner the path to ground the higher the micro-amps(DC). 

drhvac

got it

Thanks, I found out something I didn't know from the carrier tech I called. Before the flame is lit, the board sends ac voltage to the sensor. I was getting about 12 volts to the flame sensor because the board was bad as i suspected. The new board sent about 95 volts ac to the sensor. I still don't understand completely how that ac volts turns into millamps when the flame is lit. But it was the board that was bad.

John S_2

this doesn't

explain it all that well either



Here’s the way it works. The controller applies alternating voltage

between the flame sensing rod and the base of the flame (ground). The

ions in the flame provide a high resistance current path between the

two. Because the surface of the base flame is larger than the sensing

flame rod, more electrons flow in one direction than the other. This

results in a very small DC offset current. If there is a flame present,

the DC offset is detected by the controller, which tells the gas valve

to remain open. If there is no current flow, the controller will close

the gas valve and the system will purge itself of any remnant gas before

trying to re-ignite or lock-out. The DC offset is small, only in micro

amps.



by Adolfo Wurtis (quoted from HVACwebtech.com)



I'll try to find you a more complete answer (mostly to satisfy my own curiosity).