Lennox Gas Furnace: G20Q

Re: Burners Won't Ignite

These furnaces use a Honeywell spark ignition control and universal electronic ignition (3-wire) gas valve.

• When the ignition control receives 24VAC on "TH" it should start sparking through the ignition cable and send power out "PV" to open the Pilot Valve of the gas valve.
• Once pilot flame is sensed, the ignition control should send power out "MV" to open the Main Valve of the gas valve and ignite the burners.

► The ignition control only senses pilot flame, so any ignition faults will be related to the pilot flame only.

CHECKOUT

1. Check Power

• Check for 24VAC between "R" and "T" on the low voltage wiring.
  • Power Present → Step 2. Check Ignition Sequence
  • No Power ⇒ Check the Limit Switch, Blower Door Switch and High Voltage Power to Furnace

2. Check Ignition Sequence

• Set the thermostat to call for Heat
• Check for 24VAC between "W" and "T" on the low voltage wiring.
  • No Power = No Heat Call ⇒ Check Thermostat
• Check for 24VAC on "W" output at the main control board.
  • No Power = Open Limit or Bad Control Board ⇒ Check Limit Switch
• Check for 24VAC at the Ignition Control between "TH" and "TR".
  • Power Present → Step 4. Check Ignition Control
  • No Power or Power Drops-Out → Step 3. Check Safeties

3. Check Safeties

• With the thermostat calling for heat, check each of the following safeties:
• Damper End Switch
• Check for 24VAC between each side of the damper end switch and "T".
  • Power on Both Sides = Damper Switch Closed ⇒ Check Roll-Out Limit
  • Power on One Side Only = Damper Switch Open ⇒ Check damper/motor
• Roll-Out Limit
• Check for 24VAC between each side of the roll-out limit switch and "T".
  • Power on Both Sides = Limit Switch Closed ⇒ Check Primary Limit
  • Power on One Side Only = Limit Switch Open ⇒ Check Heat Exchanger
• Vent Safety Limit
• Check for 24VAC between each side of the vent limit switch and "T".
  • Power on Both Sides = Limit Switch Closed 
  • Power on One Side Only = Limit Switch Open ⇒ Check for restricted vent.

4. Check Ignition Control

• Once 24VAC is present on  "TH" of the ignition control, the pilot should try to light.
  • Sparks + Does Not Light Pilot → Step 5. Check Gas Valve/Pilot
  • Pilot Lights + No Main Gas → Step 6. Check Gas Valve/Main
  • No Spark = Bad Ignition Control or Control Locked-Out ⇒ Cycle power then re-check.

5. Check Gas Valve/Pilot

• At the Ignition Control, check for 24VAC between "PV" and "MV/PV".
  • Power on "TH" + No Power on "PV" = Bad Ignition Control or Control Locked-Out ⇒ Cycle power then re-check.
  • Power Present ⇒ Continue
• Turn the gas valve to the "Off" position.
• Remove the pilot line from the gas valve and turn the valve back "On".
• Check for pilot gas leaving the gas valve.
  • Power on "PV" + No Pilot Gas = No Gas Supply or Bad Gas Valve
  • Pilot Gas Present ⇒ Re-attach pilot line, clean the pilot orifice and re-check.

6. Check Gas Valve/Main

• Make sure the pilot flame makes good contact with the flame sensor.
  • Little or No Contact ⇒ Increase Pilot Pressure
  • Good Contact + No Main Gas ⇒ Check for 24VAC between "M" and "P/M" at the gas valve.
    • No Power = Bad Ignition Control
    • Power + No Ignition = No or Low Gas Supply or Bad Gas Valve ⇒ Check Gas Pressure