GCS20 - No Heat/Won't Come On

Lennox Rooftop Unit: GCS20

Re: Won't Come On / No Heat

This condition is generally reported two different ways by the customer:

1. "The unit won't do anything." - Indicating that there is no activity from the furnace on a call for heat.
2. "The unit makes a noise, but never blows-out hot air." - Indicating that the inducer motor runs, but ignition never takes place.

These units use a Johnson Controls "Direct Spark Ignition" controller.

• Once 24VAC is received between "Power" and "Ground", the control should start an ignition sequence.
  • Generates Spark
  • Opens Main Gas Valve

Diagnostic Light:

• Light On Continuous = Normal
• Flashing = Ignition Lockout
• Off = No Power or Faulty Control

CHECKOUT:

1. Check Power

• Check for 24VAC between "R" and "C" on the terminal block.
  • No Power ⇒ Check High Voltage Power to the unit.
  • High Voltage + No Low Voltage ⇒ Check the mini-breaker and 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.
  • Inducer Running ⇒ Check for 24VAC between "Power" and "Ground" on the ignition control.
  • • No Power → Step 2. Check Safeties
    • Power Present → Step 3. Check Ignition Control
  • No Inducer ⇒ Check for 230VAC going to the inducer. 
    • No Power = Bad K13 Inducer Relay
    • Power Present + No Inducer Operation = Bad Inducer Motor

2. Check Safeties

• Primary Limit
  • Check for 24VAC between the "C" and "N/C" wires of the limit to "C" on the terminal block.
    • Power on one wire and not the other = Bad Limit or Limit Tripped
• Auxiliary Limit
  • Check for 24VAC between the "C" and "N/C" wires of the limit to "C" on the terminal block.
    • Power on one wire and not the other = Bad Limit or Limit Tripped
• Pressure Switch
  • Check for 24VAC between both wires on the pressure switch & "C" on the terminal block.
    • Power on one wire and not the other = Open Pressure Switch
• Roll-Out Limit
  • Check for 24VAC between both sides of the limit and "C" on the terminal block.
    • Power on one side and not the other = Limit Tripped ⇒ Push Reset button & Re-check
      • If tripped, suspect a bad heat exchanger.

3. Check Ignition Control

• Look for a flashing lockout light on the control.
  • Flashing Light = Lockout ⇒ Reset power and retry.
• The ignition control should begin sparking and the Gas Valve should energize within 15 seconds of receiving power.
  • Spark + No Gas Valve →  Step 4. Check Gas Valve
  • No Spark + Gas Valve = Bad Ignition Control
  • No Spark + No Gas Valve = Bad Ignition Control

4. Check Gas Valve

• At the point of ignition, check for 24VAC on the two wires feeding the gas valve.
  • No Power = Bad Ignition Control
  • Power + No Ignition ⇒ Check Inlet & Outlet Gas Pressure