Gas Furnaces

Re: Furnace Won't Come On / No Heat

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

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

These furnaces come with either a Robertshaw SP735 or White-Rodgers 50E47 hot-surface ignition controller. A typical replacement is the IGN.02 Universal Hot Surface Ignition control.

• Once 24VAC is received on "TH" and "TR (Common)", the control should start an ignition sequence.

Diagnostic Light:

• Light On Continuous = Replace Controller
• Flashing - Ignition Lockout

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 for 24VAC between "W" and "C" on the terminal block.
  • No Power = No Heat Call, check the thermostat.
• Ensure that the inducer motor starts.
  • Call for Heat + No Inducer = Bad Inducer or Bad Heat Relay
  • Inducer Running ⇒ Check for 24VAC between "TH" or "24V" 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

• Pressure Switch
  • Check for 24VAC between "C" on the pressure switch & "C" on the terminal block.
    • No Power = Bad Heat Relay
  • Check for 24VAC between "NO" on the pressure switch and "C" on the terminal block.
    • No Power = Pressure Switch Not Closed
• Primary Limit / Fan & Limit
  • Check for 24VAC between each side of the limit to "C" on the terminal block.
    • Power on Both Side = Limit Closed (Ok)
    • Power on One Side Only = Bad Limit or Limit Tripped
• Auxiliary Limit
  • On counter-flow or horizontal furnaces, there is a manual-reset, auxiliary limit mounted in the blower compartment.
    • This auxiliary limit is designed to protect against a motor failure.
  • Check for 24VAC between both wires of the limit to "C" on the terminal block.
    • Power on Both Side = Limit Closed (Ok)
    • Power on One Side Only = Limit Tripped ⇒ Suspect a blower motor problem.
• Roll-Out Limits
  • Mounted above the burners (Typically a manual-reset limit).
    • Check for 24VAC between both sides of the limit(s) and "C" on the terminal block.
      • Power on Both Side = Limit Closed (Ok)
      • Power on One Side Onlt = 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 Glow plug should begin glowing within 30 seconds of receiving power on "TH" or "24V".
• 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