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York RTU Gas Heat Quandry

We had an installation that required converting 2 York RTU's from Natural gas to propane. This was done and ran successfully for about a year. They recently got NG on site and I was asked to convert it back. I just reversed the procedure, which was the springs in the valve, the original orifices, and two plates which move the burner assembly out a little bit. New gas pressure is 2 PSI to the regulator, bumped down to 7 inches. Manifold pressure is 3.5 inches on high fire. DC miliamps at 3 while fired. The problem is it runs for a minute and craps out. Does this 4-5 times and locks out. Checked grounds. No luck. Bled line for about a half hour as it is about 200 feet. New sensor, no luck. New unused board, no luck.
These are York ZF090N18R2B6AAA1A2 units. Little over a year old. Any help would be wonderful !!!


  • JUGHNEJUGHNE Posts: 5,731Member
    Have you monitored the manifold pressure the entire time to shut down and lockout?
    I wonder about the gas valve/regulator having the LP spring in it for about a year, could it have permanently stretched the diaphragm?.
    Any high/low gas pressure cut out switches?
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,870Member
    Are you running the 2psi natural gas up near the RTUs before reducing to 7" Just wondering if you are using the old propane pipe which could be undersized for natural gas
  • HVACNUTHVACNUT Posts: 2,272Member
    edited December 2018
    Both units are having the same fault.
    I assume theres no diagnostic codes showing.
  • JackShelleyJackShelley Posts: 12Member
    Manifold pressure stays the same through the burn cycle. 2 psi stays constant too. No high low cutout switches. After about 5 tries, the ignition board blinks a 6. This is a source one 1171-200 board. It always happens at 60 seconds. The sensor runs at 3DC mv. I wonder if this is correct. No York tech support on the weekend.
  • JUGHNEJUGHNE Posts: 5,731Member
    R & W jumped at unit.... most likely?

    Did moving the burner assembly change the flame sensor location?
  • JackShelleyJackShelley Posts: 12Member
    I just set the thermostats to 80, no jumper. The flame sensor only goes in one position, no adjustment possible. This new gas line is about 200 feet to building. Not sure if gas company did much bleeding. I bled line for probably 20 minutes total.
  • JUGHNEJUGHNE Posts: 5,731Member
    The jumper would eliminate any control problem outside of the t-stat?

    Exactly 60 seconds to shut down each time?
    If so something is controlling that to that accuracy.

    How much new underground gas line did the company replace?
    Is this the end of a new line or other customers connected after you without problems?
  • rick in Alaskarick in Alaska Posts: 862Member
    The least time I had a problem like this, which coincidentally was also on a York, was because the customer decided he had to have a Nest thermostat on the system, and the control board, for whatever reason, did not like it. Try bypassing the thermostat and see what happens.
  • JackShelleyJackShelley Posts: 12Member
    Thanks for all the tips, I changed out the gas valves with two new ones out of desperation. Problem fixed ! Not sure what was going on here, but the idea that JUGHNE had about the diaphragm being deformed sounds plausible. Again, thanks for the help.
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