NULK - No Heat/Won't Ignite

Heil Gas Furnace: NULK

Re: Burners Won't Ignite

These furnaces use a Hot-Surface ignition control for ignition (GHAC replacement = IGN.02).

CHECKOUT:

1. Check Power

• Check for 24VAC between "R" and "C" on the terminal block.
  • No Power ⇒ Check Blower Door Switch and High Voltage Power to Furnace
  • High Voltage + No Low Voltage ⇒ Check the transformer.
• Energize a call for Heat from the thermostat
• Check 24VAC between "W" and "C" on the terminal block.
  • No Power = No Heat Call, check the thermostat.
• Ensure that the inducer motor starts.
  • Power + No Inducer = Bad Inducer or Bad System Relay
  • Inducer Running ⇒ Check for 24VAC between "TH" on the ignition control and "C" on the terminal block.
    • No Power → Step 2. Check Safeties
    • Power Present → Step 3. Check Ignition Control

2. Check Safeties

• Roll-Out Limit
  • Mounted above the burners, Typically a manual-reset limit.
    • Check for 24VAC between both sides of the limit (Red wires) and "C" on the terminal block.
      • Power on One Side Only = Limit Tripped ⇒ Push Reset button & Re-check
• Primary Limit
  • Check for 24VAC between each Red wire of the limit to "C" on the terminal block.
    • Power on One Side Only = Bad Limit or Limit Tripped
      • Open Limit + Cold Heat Exchanger = Bad Limit
      • Open Limit + Hot Heat Exchanger ⇒ Suspect Blower Problem
• System Relay
  • Check for 24VAC between terminals 4 (Blue) and 6 (Red) of the System Relay to "C" on the terminal block.
    • Power on One Side Only = Bad System Relay
• Pressure Switch
  • Check for 24VAC between each wire on the pressure switch (Blue wires) & "C" on the terminal block.
    • No Power = Bad System Relay
    • Power on One Side Only = Pressure Switch Open

3. Check Ignition Control

• Look for a flashing lockout light on the control.
  • Flashing Light = Lockout ⇒ Reset power and retry.
• The Glow plug should begin glowing within 30 seconds of receiving power on "TH".
• Gas Valve should energize within 60 seconds of powering the glow plug.
  • No Glow + Gas Valve Opens →  Step 4. Check Glow Plug
  • No Glow + No Gas Valve = Bad Ignition Control

4. Check Glow Plug

• Look for any visual cracks.
• Use a meter to test resistance of glow plug.
  • No Resistance = Bad Glow Plug
• Check for 120V power to the Glow Plug during an ignition sequence.
  • No Power = Bad Ignition Control
  • Power Present + No Glow = Bad Glow Plug