Help with York intermittent failure to light

Fredrick

So I'm in Idaho and as you can imagine it gets quite cold here. I have a York gas furnace model GF8100C16MU11A. It has been well maintained by me for the last 8 years after many years of neglect.
I have had to replace a failed limit switch 7years ago and the control board 6 years ago. No problems until now. I am getting code 4 (red) and 3. The replacement boards new codes lists these as failed limit switch and failed pressure switch. The furnace will run for 6 to 9 hours and then suddenly blow cold air with the same codes each time. I can turn it off at the thermostat for a minute and it will restart and then fail hours later.

I had a new pressure switch and replaced that although the old one did switch with vacuum applied. I obtained a new limit switch and installed that as well. The filters are new and the same ones I have used for 8 years. The main fan is immaculately clean and the draft inducer motor runs strong with no blockage to the roof. I have cleaned the ignition sensor rod but not tested it yet. The flames couldn't look any better; a beautiful forceful blue with no yellow.

Could this be another odd board failure? Can a spill switch or ignition sensor fail intermittently? Any help is MUCH appreciated.

Forgot to add that I have 24.6 volts at the gas valve when the furnace is running. I have checked the temp at the location of the limit switch and it never rises higher that 122 even after 20minutes of running. I have also cleaned the inducer suction port and hose-all clear. Same situation today. Ran well all night and day then suddenly blowing cold air now. Reset at thermostat and starts okay again!!!??? I'm stumped.

Big Ed_4

You have a few limit switches on a gas furnace wired in series . Some have reset buttons .You will need an electrical meter to find which one is open . Check the wiring scamatic .

Check the condition of the pressure switch hose and the nipple of the draft inducer if it is clear . The pressure switch is a safety , if the venting or chimney is blocked it prevents the burner to fire ....

mattmia2

What are the return and supply temps? How do they compare to the ratings plate?

Is some form of power vent? Did you look to see if the exhaust and intake are clear?

If it is natural draft that is trickier.

109A_5

Hello @Fredrick,

Fredrick said:

I am getting code 4 (red) and 3. The replacement boards new codes lists these as failed limit switch and failed pressure switch. The furnace will run for 6 to 9 hours and then suddenly blow cold air with the same codes each time.

Not sure if this is from the correct manual, but it may be close enough. Those two limit switches share a common wire going back to the connector on the control board. I would try re-seating the 12 pin connector, check for loose connections or poor pin fitment. Also the solder joint on pin 7 of that board connector may be failing and be heat and / or vibration sensitive.

Fredrick

Thanks to all. @Big Ed_4 : Only reset-able switches are the roll out switches and they have never tripped. The pressure switch hose and the nipple of the draft inducer are clear and the nipple cleaned. Vent is clear as well.
@mattmia2 : I have not measured the return temp but as i mentioned the supply is 122F. I'll do the return but I imagine it's fine. The mfg states 30-60 deg F as spec. Vent is clear.

@109A_5 : Good call. Those switches do share the same #7 pin on my wiring diagram. I'll turn it off and clean contacts using some contact cleaner and re-seat.

My wife just reminded me that this all started the morning after I replaced the batteries in the Lux TX9600TS thermostat. It was flashing low battery. The instructions are VERY specific to use ONLY Duracell or Energizer batteries. We were out of those so I put some Harbor Freight ones in . I tested them first and they had 1.55v so I put them in. Can't imagine that's the problem but I will replace them this evening.

EdTheHeaterMan

There has been no mention of the condition of the flame sensor. There are two things to check there.
1. Is the flame sensor clean
2. Is there a good ground path for the flame sensing circuit..

Sometimes when the flame sensor signal is weak, it will operate most of the time, then it will fail. all it takes is one failure if the control is set up for LP gas. You could be burning natural gas (that allows for multiple retries), but if the control is a LP control, some of them only allow for 1 try. If it fails to prove ignition, then you must reset by cycling the power switch off then on.

So the flame rectification circuit comes from the #2 pin on the control board 12 pin plug. It goes to the electrode that should be clean. Then it passes thru the flame to the burner. The metal burner is grounded to the heater chassis. The control must also have a good ground connection to the heater chassis to complete the flame sensing circuit.

In the past, when a furnace or boiler is located in a damp basement, and all the metal screws that holds the heater together start to show signs of rust and corrosion, I have experienced this type of failure. To solve the possibility that there may not be a good chassis ground anymore, I would connect a copper wire with a round screw terminal connected to each end. One end goes at the pilot burner mounting screw. The other end goes the the control where it is screws to the chassis. One of those screws usually has a bare metal bushing or some way to make sure the control is grounded. Look at the control to see if one or all of the mounting screws are ground screws.

I like green insulation THHN strand wire for this but any conductor will do.
I think it is an environmental thing. Even though electric is colorblind, ground electric feels better going green

HomerJSmith

Normally, I put a pressure measuring device ( my combustion analyzer) in between the Inducer Draft Motor and the Neg Pressure Switch and with the furnace running and measure the negative pressure. I want to make sure that the negative pressure is high enough. Some Neg Press Switches operate as Positive Pressure Switches and some are both.

IDMs do fail and sometimes don't rotate the fan to the proper RPM. This can be because of combustion debris and worn bearings. The IDM may be marginal, working sometimes and not at other times. On some IDMs you can see the motor cooling fan and turn it with a pencil. Is yours that kind? If there is any hesitancy in revolving it? Then replace it.
Some board have an electrical plug that goes to the IDM which is relay operated. I have found solder breaks on the back of the board where the plug receptical is attached to the board which leads to an intermittent operation.

Just spitballing here.

Fredrick

It's FIXED!!!

So.....Here was today's fun and HVAC games.
@EdTheHeaterMan : I had cleaned the flame sensor rod. I had refered to it as the ignition sensor rod above. This control board has a dedicated ground wire on pin 8 of the multi connector it's in great shape and tight.

@HomerJSmith : I wish I had a photo electric tach to check the speed of the inducer as there is a rpm rating for it and I was also wishing for a low reading vacuum meter. The only one I have is a high reading one for auto work.

I did have a new flame sensor rod so in desperation I just put it in. No change but I didn't think there would be. I bypassed the thermostat at the control board and no change there either. It would still run for 20 minutes at this point and I was near the end of my rope!

I had ordered a new control board that came on Friday and finally bit the bullet and spent the whopping 15 minutes to change it out. Here is what i found.



Slight discoloration of the heat terminal and on the backside a burn mark on that relay. I'm ASSsuming that's the culprit and the new board has been working great for several hours. If it doesn't "I'll be back!" Thanks for all the help thus far.

neilc

that solder joint looks sketchy

EdTheHeaterMan

It could be the problem, Anything that is not perfect could cause a problem. However that particular solder joint would only account for the heating speed tap on the blower motor that circulates the air thru the duct work. That terminal would cause the blower motor to possibly not operate. If the fan does not operate, then the limit would open and cause the flame to stop. An error code of OPEN LIMIT would be displayed on the board.

If that happens several times, perhaps the control goes into a manual reset. At that point the cooling fan terminal would be activated to purge excess heat from the heat exchanger. That would blow cold air until the control was manually reset by cycling the power OFF and ON.

If you are up to it, Try putting a drop of solder on that defective solder connection. You will then have a spare control board.