Knowledgebase

Gas Furnace - Won't Come On/No Heat

Article ID: 158
Last updated: 29 May, 2024

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

Qty Possible Causes:
11 Bad Glow Plug
2 Auxiliary Limit Tripped
Pressure Switch Fault
No Power (Door Switch)
1 Ignition Control Locked-Out
Bad Inducer Fan
Bad Ignition Control
Bad Thermostat (Thermostat not calling)


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.

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.

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

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Article ID: 158
Last updated: 29 May, 2024
Revision: 50
Access: Public
Views: 120
Comments: 0