Gas Furnace
Re: Burners Cycling Off / Burners Won't Stay On
The furnace burners fire and run for several minutes, then the burners shut off and the furnace goes through another ignition sequence.
These furnaces use either a Robertshaw SP735 or White-Rodgers 50E47 hot-surface ignition controller. (A common replacement control is the IGN.02 Universal Hot Surface Ignition controller).
- Once 24VAC is received on "TH" and "TR (Common)", 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.
Diagnostic Light:
- Light On Continuous = Replace Controller
- Flashing = Ignition Lockout
Qty |
Possible Causes: |
4 |
Pressure Switch |
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Limit Switch |
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Losing power from the thermostat |
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Dirty flame sensor |
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Low gas pressure |
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Clogged burners |
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 but 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 when burners kick-off → Step 2. Check for Power at Controller
2. Check for Power at Controller
- White-Rodgers - Check for 24VAC between "TH" and "TR".
- Robertshaw - Check for 24VAC between "24V" and "COM".
- IGN.02 - Check for 24VAC between "TH-W" and "24V (Gnd)".
- Monitor voltage as the furnace operates.
3. Check Safeties
- Pressure Switch
- Monitor 24VAC between "NO" on the pressure switch(es) & "C" on the terminal block.
- Power drops out when burners kick-off = Pressure Switch Trip
- Power present when burners kick-off ⇒ Check Primary Limit
- Primary Limit
- Monitor 24VAC between the "Brown" or "Violet" wire on the Primary Limit to "C" on the terminal block.
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 (2 to 6 mA) but Shuts Off = Bad Ignition Control
- MOVE SENSOR LEADS BACK TO "COM" and "V" BEFORE PROCEEDING!