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Phil, Mechanical Engineer
Category: HVAC
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Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have 2 @ 5 month old york tg9 single stage furnaces. One

Customer Question

I have 2 @ 5 month old york tg9 single stage furnaces. One will not light, giving me the 6 red flash code (current failure on modulating gas valve). I swapped coils between the two furnaces just to see if the problem followed. It did not. I tested the voltage to the coil on the non firing furnace and get 13 volts to the 24 vac coil. Obviously, not enough voltage, right? So, I go to the working furnace and test for voltage to compare, and get the same voltage as the non firing furnace, 13 volts ac. My screwdriver attracts to the working furnaces coil when induced, and will not attract to the non working furnace's coil. What is the minimum voltage required on these gas valves to function? Why is one woking and the other not? I assume a board problem, but why is the other furnace even operating if that's the case?
Submitted: 1 year ago.
Category: HVAC
Expert:  Heath.M replied 1 year ago.

Hello my name is ***** ***** will be my pleasure to assist you. The voltage is okay on both but the problem is the coil is open on the bad valve. It is an electro magnet that opens the valve when voltage is applied. But if the coil is open then there will be no magnetic current to the valve so the valve will not open. The voltage is low from what I can tell. You would need to check the transformer to see if the transformer is putting out 24 volts to the furnace. If you go from the R terminal to the C terminal of the circuit board you should have 24 volts.

Expert:  Heath.M replied 1 year ago.

if the furnace is only 5 months old the gas valve should be under warranty. I would get a hold of the installer and have them change it under warranty.

Expert:  Heath.M replied 1 year ago.

The voltage will go up and down at the gas valve thus creating the modulation or increase and decrease in gas to the burners because it is a modulating furnace the control board is what controls the voltage out to the valve increasing and decrease the voltage opens and closes the gas valve

Customer: replied 1 year ago.
you'd consider the gas valve eh? Yeah, that's what I was thinking, but the 13 volts to a 24 volt coil makes me wonder. I tested the transformer and have a steady 26 volt supply, but the output voltage from the board is always 13 volts to the gas valve coil, and 26 volts everywhere else. I rapped on the gas valve and it started, but would only run a minute's worth before the code would appear and it flamed out. It did that a few times before failing to further ignite. Do you know if 13 volts is correct as the voltage to the coil of a York brand? The rapping and running does indeed suggest a bad valve. Thank you for taking the time to point me in a solid direction! Good advice!
Expert:  Heath.M replied 1 year ago.

Yes the voltage is correct. As I said in my earlier post the 13 volts will fluctuate to modulate the valve more voltage the more gas flow and thus more btu's the lower the voltage the lower the gas flow and lower btu's. The gas valve is definitely the issue.

Customer: replied 1 year ago.
Ah, yes. It just threw me that 13 volts would be enough to energize the coil, but if it modulates, that would be correct then! Thank you again for your time sir! Very good advice!
Expert:  Heath.M replied 1 year ago.

you are very welcome!

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