Heil Gas Furnace: NTGM, GNK, NCGM, GCK

Re: Burners Don't Ignite

These furnaces use a Honeywell Smart Valve for ignition control.

• When the Smart Valve receives 24V from the Fan Timer Board it opens its pilot valve and energizes a 24V glow plug. 
• When pilot flame is sensed, the Smart Valve opens its main valve to ignite the burners and de-energizes the 24V glow plug.

CHECKOUT

1. Check Power

• Check for 24VAC between "R" and "C" on the Fan Timer Board.
  • No Power ⇒ Check Blower Door Switch and High Voltage Power to Furnace
    • High Voltage but No Low Voltage ⇒ Check fuse and transformer.
• Set thermostat to call for Heat
• Check 24VAC between "W" and "C" on the terminal block.
  • No Power = No Heat Call, Check Thermostat
  • Power & No Inducer = Bad Inducer or Fan Timer Board
• Inducer Running → Step 2. Check Smart Valve
• Inducer Not Running = Bad Inducer or Bad Fan Timer Board

2. Check Smart Valve

• Check for 24VAC between Terminal 1 & 3 on Smart Valve square plug.
  • Should have 24VAC power at all times.
    • No Power = Suspect Bad Fan Timer Board
• Check for 24VAC between Terminal 4 & 3 on Smart Valve square plug.
  • Power present = Smart Valve should start ignition sequence.
    • Power Present + No Pilot or Glow Plug = Bad Smart Valve or Bad Glow Plug → Step 4. Check Glow Plug
    • No Power = Safety Trip → Step 3. Check Safeties
  • Glow Plug Glows → Step 5. Check Pilot

3. Check Safeties

• Pressure Switch (up to 3 depending on model)
  • Check for 24VAC between both sides of the pressure switch & "C" on the Fan Timer Board
    • No power on either side ⇒ Check Auxiliary Limits
    • Power on One Side Only = Open Pressure Switch
    • Power on Both Sides = Pressure Switch Closed
• Auxiliary Limit (up to 2 depending on model)
  • Probably mounted in the blower compartment.
  • Typically a manual-reset limit.
  • Check for 24VAC between both sides of each limit to "C" on the terminal block.
    • Power on both sides = Limit OK
    • Power on one side and not the other = Bad Limit or Limit Tripped --> Press reset button on the limit and re-check

1. Check Glow Plug
   • Inspect glow plug for any visible defects.
   • Remove flat 4-wire plug from the Smart Valve.
     • Check resistance of the glow plug across the two blue wires.
       • No resistance = Bad Glow Plug
   • Re-insert the flat 4-wire plug on the Smart Valve.
     • Check for 24VAC between the two blue wires on the plug.
       • No Voltage = Valve Locked-Out or Bad Smart Valve --> Cycle power to the furnace to reset possible lockouts then re-check.
2. Check Pilot
   • If pilot gas is present and the glow plug glows, the pilot should light.
     • Glow plug glows but doesn't light pilot = No Gas, Clogged Pilot Orifice or Bad Smart Valve
   • Make sure the pilot flame makes good contact with the flame sensor.
     • Pilot lights by but never lights main gas = Dirty Flame Sensor or Clogged Pilot Orifice
       • In some cases, it is necessary to bend the pilot hood to better direct pilot flame to sensor.