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Brian Horton
Brian Horton,
Category: HVAC
Satisfied Customers: 2
Experience:  Service Technician II at Hobart Corporation
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I have a Evcon, CGH07512E. The fan timer is Honeywell

Customer Question

Hi,
I have a Coleman Evcon, CGH07512E. The fan timer is Honeywell ST9120C 2010 and the gas valve/ignition control is Honeywell SV9501. My problem is on call for heat, the inducer fan comes on, the pilot lights and burns for about 2 seconds and then the ignition control clicks, the flame goes out and the cycle repeats over and over. I measured the voltage at the timer board of the EFT output (blue wire) connected between the board and the SV9501. When the power is turned on I read about 195 mV. When the igniter comes on the power jumps to about 280 mV and then returns to 195 when the pilot goes out. I have no idea if this is the correct voltage or if this might even be a problem and I do not know how to test the flame sensor. I am assuming the problem is that the gas valve is not opening due to a failure in the ignition sequence somewhere after the flame sensor else the valve is faulty. Can someone please tell me how to proceed and if I am even on the right track?
Thanks very much for your help,
***** *****
Submitted: 10 months ago.
Category: HVAC
Expert:  Brian Horton replied 10 months ago.

That pilot is heating a thermopile right? If so your thermopile is bad. Most thermopiles are closer to 400-600mv when fully heated which opens the MV part of the gas valve. The flame sense rod would only come into question when the burner actually lights and then quits.

Customer: replied 10 months ago.
Brian,
This furnace has a hot surface igniter and a flame sensor. According to the manual for the timer and gas valve, the pilot flame is detected by the flame sensing rod. When the pilot flame is sensed, the main gas valve opens. I do not see any kind of thermopile device on the burner assembly, nor do I see any mention of one in any documentation that I can find. The voltage readings I mentioned were measured at the timer circuit board on a wire that connects to the SV9501 at the terminal labeled "EFT Output". I am under the impression that voltage itself doesn't directly open the valve, but I am not sure and that is where I am stuck. As I mentioned in my request, I don't know how to test the flame detector which I believe is the logical next step, so that I can proceed with troubleshooting. If I am completely off base, can you provide a diagram or part number for the thermopile generator please?Many thanks for your help,
David
Expert:  Brian Horton replied 10 months ago.

Have you checked to see that you have 24 volts to the flame sense rod? Is the flame sense rod heated by the the actual pilot flame?

Expert:  Brian Horton replied 10 months ago.

Im not real familiar with this unit. But usually a flame sense rod is in direct contact with the burner flame. Not the pilot. Does the rod that the pilot is heating have wires coming out of it or just a cappilary line?

Expert:  Brian Horton replied 10 months ago.

It would really help also if you could attach a couple of pictures of the pilot, igniter and flame sense rod.

Customer: replied 10 months ago.
Brian,
My problem has been solved. Thanks for your help.David