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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 7746
Experience:  Furnace, A/C and heat pump specialist.
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I have a G51 Lennox furnace that is not lighting. No codes,

Customer Question

I have a G51 Lennox furnace that is not lighting. No codes, there is gas supply, does not appear to call for gas as voltage sensed at valve. Red and Green Led flash fast and together, no other led sequence . When there is a ignition attempted, the igniter glows orange but then shuts off after a few seconds. This repeats over and over.
Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

Are you saying you do or do not get 24v at valve?

Customer: replied 1 year ago.
at the point of ignition I am not able to measure any voltage at the valve.
Customer: replied 1 year ago.
Also according to the videos Ive watched, the light up sequence should only repeat 3 times then show status code however in my case it seems to continuously
Customer: replied 1 year ago.
Are there any other sensors that would prevent a call for the valve when the ignitor is on?
Expert:  airheatman replied 1 year ago.

Ok if you do not get voltage we really should get a different flash code at that point. Possibly a bad board. Take a close look at the igmitor and make sure it is not burned and is lighting completely.

Expert:  airheatman replied 1 year ago.

Nothing between the igniter lighting and board sending signal to gas valve.

Customer: replied 1 year ago.
if I see the orange glow, and I don't see any valve voltage it would appear to be the control board? There doesn't appear to be any thing unusual to the igniter, it's glowing orange/red for a number of seconds then shuts off and repeats.
Shouldn't the sequence only try 3 times the flash a code? In my case it keeps retrying, over and over, led's both flashing normal fast and together,
Customer: replied 1 year ago.
Is there any voltage source on board that I can briefly apply to gas valve to hear valve actuation?
Expert:  airheatman replied 1 year ago.

You are correct.The furnace should only attempt to light between 3 and 4 times and then go into lockout.

Customer: replied 1 year ago.
On other thing I was able to rule out were the pressure switches... If I pull off one wire on either pressure switches it deactivates the ignition sequence, to me this suggests that those switches are actually in working state, for as soon as the lines are put back the ignition seq, continues to cycle.
Expert:  airheatman replied 1 year ago.

The only way you could energize valve is o run a wire directly from R and C at board ,but this would be merely for a test and either have gas turned off just to hear it click or make sure that igniter is hot so burners light immediately.THIS IS DEFINITELY for TEST only to verify valve works, but Valve is NOT the problem because we are not getting signal to it.

Expert:  airheatman replied 1 year ago.

And yes you are right. Limit switch or pressure switch are neither one an issue or we would not get to igniter stage

Customer: replied 1 year ago.
you are correct, the valve is not the problem so jumping it out is not really going to tell me anything. I am trained in Micro Computer Electronics and realize that a relay on the control board switches the line to gas valve, would you happen to know which relay it is?
Expert:  airheatman replied 1 year ago.

Honestly I do not.I just know the board should be signaling the gas valve which it is not and if we do not get ignition, board should go into lockout and give us an error code and it is not.

Customer: replied 1 year ago.
I would assume that the transformer that drops down to 24 Volts is fine as controller wouldn't be on at all, correct? There really isn't much on the control board that can other than a micro controller and a bunch of passive components (resistors/caps/diodes) and a few relays. Replacement of the control board is looking like it may be my next option, would you agree?
Expert:  airheatman replied 1 year ago.

Only other thing I can thin of that may cause this is a poor ground. I have seen a poor ground cause erratic issues.

But yes my suggestion once you verify ground is good would be to replace board as there are no other components in circuit between igniter and gas valve opening.

Expert:  airheatman replied 1 year ago.

And you are correct, transformer would not send 24v to allow anything to happen if it were bad.

Customer: replied 1 year ago.
In review of the schematic diagram, I see that there appears to be a S43 Low Pressure gas switch in line with the valve. Where is that switch located?
Expert:  airheatman replied 1 year ago.

I thought the low pressure gas witch was only used with LP to prevent valve from opening if lp pressure was low.(Didnt think to ask if you were on LP) located before the gas valve in the gas line and wired in as shown in schematic.

http://i.imgur.com/ObPGIhy.png

Customer: replied 1 year ago.
I may have miss read the right connection diagram. I took a picture of the control board and on close up examination I see some browning around to higher wattage resistors R49 and R50 indicating over-heating of some kind. Without taking board out I can't be certain that is part of the problem.
Expert:  airheatman replied 1 year ago.

With your expertise in the computer electronics, you can possible repair the board and that is something I admire you for.You might be able to do a Google search or contact White Rodgers or whoever manufacture of board is and find or get them to send you a data sheet showing which resistor, diode,fuse ,transistor etc controls and just repair the one burned out or shorted.As an HVAC contror, we just replace the board.

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