PHU - Burners Ignite & Shut-Off

York Gas Furnace: PHU

Re: Burners Ignite & Shut Off

Code 7 = Failure to Sense Flame

CHECKOUT:

1. Check Diagnostic Indicator

• DO NOT cycle power to the furnace.
• The furnace control board has an on-board LED that will flash to show any active diagnostics. (Killing power clears the diagnostic.)
• Look through the blower door sight glass to see if the LED is flashing.
  • Diagnostic Code Present → Go to Fault Code Article
  • No Diagnostic Code → Step 2. Check Ignition

2. Check Ignition

• Initiate a call for heat from the thermostat.
• Visually inspect the burners during ignition.
  • The burners should light smoothly with little or no delay from the glow plug to the flame sensor.
  • Make sure the flame sensor comes in good contact with the flame.
    • Burners Ignite Properly the Shut-Off → Step 3.  Check Controls
    • Any Delay or Burners Not Igniting = Clogged Burners or Low Gas Pressure → Step 4.  Check Gas Pressure / Valve

3. Check Controls

• Monitor 24VAC from "W" to "C" while the furnace cycles. (It may be necessary to manually close the door switch.)
  • Power Drops-Out when Burners Cycle = Thermostat Short-Cycling ⇒ Check Thermostat & Wiring
  • Power Consistent ⇒ Continue
• Monitor the diagnostic indicator when the burners shut-off.
  • Diagnostic Code Present → Go to Fault Code Article
  • No Diagnostic → Step 5. Check Flame Sensor

4. Check Gas Pressure

• Inlet Gas Pressure
• Check the inlet gas pressure.
• Minimum Pressure = 5" w.c. for Nat. gas, and 11" w.c. for propane.
  • Low Pressure = Gas Supply Issue
• Re-apply power to the furnace and activate a call for heat.
• Make sure the inlet gas pressure stays above minimum when the gas valve energizes.
  • Pressure Drops Below Minimum = Bad Meter/Regulator or Restricted Gas Line
• Manifold Gas Pressure
• Check the manifold gas pressure.
  • Normal Pressure = 3 - 3.5" w.c. for Nat. Gas, 9 - 11" w.c. for LP
  • Low Gas Pressure ⇒ Increase Manifold Gas Pressure
  • Normal Gas Pressure but No or Delayed Ignition = Clogged Burners ⇒ Remove burners and clean spreaders.

5. Check Flame Sensor

• DO NOT clean the flame sensor before testing.
• On the multi-meter:
  • Switch the test leads to "COM" and "mA".
  • Set the meter on DC 200ù.
  • Check the flame reading when the burners ignite.
    • Normal Reading = 2 to 6 mA
    • Minimum acceptable reading = 1 MicroAmp
      • Low Reading = Dirty Flame Sensor ⇒ Clean Flame Sensor & Re-check
  • CAUTION! Move meter leads back to "COM" and "V" before proceeding!