NULK - Burners Cycling On & Off

ICP/Heli Gas Furnace: NULK

Re: Burners Cycling Off / Burners Won't Stay On

The furnace burners fire and runs, then the burners shut off and the furnace goes through another ignition sequence.

These furnaces use either a  hot-surface ignition controller. (A common replacement control is the IGN.02 Universal Hot Surface Ignition controller).

• Once 24VAC is received on "TH", the control will start an ignition sequence by powering the on-board igniter relay (which breaks one leg of 120V to the glow plug).
• After 45 - 60 seconds of pre-heating the glow plug, the board powers the main gas valve "MV".
• The board will then have 6 seconds to receive a flame signal.
• The gas valve will stay energized until 24VAC is removed from "TH" or the flame signal is lost.

CHECKOUT:

1. Check for Power at LTB

• 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 transformer.
• Monitor 24VAC power between "W" and "C" on the low voltage terminal block.
  • No Power = No Heat Call from Thermostat ⇒ Recheck Stat
  • Power drops out when burners kick-off = Thermostat Cycling
  • Power Present + Burners Kick-Off → Step 2. Check for Power at Controller

2. Check for Power at Controller

• Check for 24VAC between "TH" at the ignition control and "C".
• Monitor voltage as the furnace operates.
  • Power drops out when burners kick-off = Safety Trip → Step 3. Check Safeties
  • Power Present + Burners Kick-Off = Loss of Flame Signal → Step 4. Check Flame Sensor

3. Check Safeties

• Primary Limit
  • Monitor 24VAC between the "Red" wire on terminal 6 of the System Relay and "C" on the terminal block.
    • Power drops out when burners kick-off = Limit Switch Trip
    • Power Present + Burners Cycle Off ⇒ Continue
• Pressure Switch
  • Monitor 24VAC between the "TH" on the ignition control & "C" on the terminal block.
    • Power drops out when burners kick-off = Pressure Switch Open
    • Power Present + Burners Kick-Off = Loss of Flame Signal or Bad Ignition Control → Step 4. Check Flame Sensor

4. Check Flame Sensor

• On the multi-meter:
  • Switch the test leads to "COM" and "mA".
  • Set the meter on DC 200ù.
  • Disconnect the wire from the flame sensor.
  • Place one meter lead on the flame sensor terminal and the other on the flame sensor wire.
  • Check the reading when the burners ignite.
    • Minimum acceptable reading = 1 MicroAmp
      • Low Reading ⇒ Clean Flame Sensor & Re-check
      • Normal Reading (1 to 3 mA) + Burners Shut-Off = Bad Ignition Control
  • MOVE SENSOR LEADS BACK TO "COM" and "V" BEFORE PROCEEDING!