520820 RV Furnace Circuit Board with Fan Control | Atwood Hydro Flame Replacement

520820 RV Furnace Circuit Board with Fan Control – Atwood Hydro Flame Replacement

The furnace circuit board (also called the ignition control board or module) is the electronic brain of your RV furnace. It manages the entire heating sequence: pre-purge fan operation, spark ignition, flame sensing, gas valve control, fan speed, and safety lockout. When the circuit board fails, your furnace may fail to ignite, cycle on and off, the fan may not work, or the unit may lock out completely.

This 520820 circuit board with fan control is designed as a direct replacement for Atwood Hydro Flame RV furnaces. It features built-in fan motor control (high/low speed), spark ignition with flame rectification sensing, 3-trial ignition with 30-minute lockout, and plug-and-play installation with no programming required.

Compatibility

• Atwood Hydro Flame 85-IV series: 8516-IV, 8520-IV, 8525-IV, 8531-IV, 8535-IV
• Atwood Hydro Flame 89-III series: 8916-III, 8920-III, 8925-III, 8930-III, 8935-III
• Atwood AFMD series: AFMD20111, AFMD25111, AFMD30111, AFMD35111
• Atwood 7900-II series: 7912-II, 7916-II, 7920-II, 7924-II, 7932-II
• Dometic DFMD series (rebranded Atwood): DFMD20121, DFMD25121, DFMD30121, DFMD35121
• Replaces board numbers: 520820, 520814, 31501, 33488, 33727, 521099
• Most 16,000-40,000 BTU Atwood/Dometic Hydro Flame furnaces with fan control

Specifications

Ignition Sequence

1. Call for heat: Thermostat closes, sending 12V signal to the board’s thermostat input.
2. Pre-purge: Combustion fan (and circulation blower on some models) runs for 15 seconds to clear any residual gas from the heat exchanger.
3. Trial ignition: Spark igniter activates (15kV spark), gas valve opens. Board attempts ignition for up to 8 seconds. Flame is sensed via flame rectification (microamp current through the flame).
4. Flame confirmed: If flame is detected within 8 seconds, spark stops, gas valve stays open, fan continues. Board monitors flame continuously.
5. Retry: If no flame is detected within 8 seconds, gas valve closes, spark stops. Board waits 30 seconds, then retries. Up to 3 attempts total.
6. Lockout: After 3 failed ignition attempts, board enters safety lockout for 30 minutes. Gas valve remains closed, fan may run for post-purge. Manual reset (power cycle) required to clear lockout.
7. Heating cycle: With flame confirmed, furnace continues heating until the thermostat opens (set temperature reached).
8. Post-purge: When thermostat opens, gas valve closes, spark off. Fan continues running for 30-60 seconds to cool the heat exchanger and clear residual combustion gases, then shuts off.

Installation Guide

1. Safety first: Disconnect 12V power at the battery. Turn off propane supply. Allow the furnace to cool completely.
2. Access the board: Remove the furnace exterior access panel. The circuit board is typically mounted near the burner assembly, behind a plastic cover. You may need to remove the furnace from its mounting for easier access.
3. Document connections: Take detailed photos of all wiring connections before disconnecting. Label each wire connector with tape if needed. Note the position of each connector on the old board.
4. Disconnect wires: Unplug all wire harness connectors from the old board. This typically includes: 12V power (positive/negative), thermostat, gas valve, spark igniter, flame sensor, fan motor (high/low), and combustion fan (if equipped).
5. Remove old board: Unscrew the mounting screws (2 or 4 screws) holding the board to the furnace housing or mounting bracket.
6. Install new board: Mount the new 520820 board in the same position. Secure with the mounting screws. Ensure the board is properly seated and all mounting holes align.
7. Reconnect wires: Plug all wire harness connectors into the corresponding positions on the new board (refer to your photos). Ensure all connections are secure and properly seated. Double-check the 12V power connections — red to positive, black/white to negative. Incorrect wiring can damage the new board.
8. Replace cover: Reinstall the plastic protective cover over the board if equipped. Ensure all vents are clear.
9. Test operation: Reconnect 12V power. Turn on propane. Set the thermostat to call for heat. Verify the complete ignition sequence: pre-purge fan → spark → gas valve opens → flame ignites → flame sensed → continues heating. Check for proper flame (blue with yellow tips), consistent fan operation, and that the unit cycles off when the set temperature is reached. Test at least one complete heating cycle. If the unit locks out, check the error code (if available) and verify all wiring connections.

Common Symptoms of a Bad Circuit Board

• Furnace won’t ignite at all — no response to thermostat
• Fan runs but no spark or gas (ignition sequence doesn’t proceed)
• Spark occurs but gas valve doesn’t open (no flame)
• Flame ignites but won’t stay lit (flame sensing failure)
• Furnace cycles on and off frequently (short cycling)
• Fan doesn’t run or only runs on one speed
• Furnace locks out after one ignition attempt (should retry 3 times)
• No spark at the electrode (spark output failure)
• Clicking sounds from the board but no operation
• Burning smell or visible burn marks on the board
• 12V fuse for the furnace blows repeatedly (short circuit on board)
• Intermittent operation — works sometimes, not others
• Error code or flashing LED on the board (check your manual for code meanings)

Frequently Asked Questions

Q: What’s the difference between 520820 and 520814?

A: 520820 and 520814 are similar furnace circuit boards with minor differences. The 520820 includes fan control (high/low speed relay) and is used in furnaces with variable-speed blower motors. The 520814 may not include fan speed control or may have a different connector configuration. Both boards have the same basic functionality: spark ignition, flame sensing, 3-trial ignition, and 30-minute lockout. The 520820 is generally backward-compatible with furnaces that used the 520814, but you should verify the connector type and fan control requirements before installing. If your original board has a fan speed control connector, use the 520820. If not, either board may work, but the 520814 may be a closer match. When in doubt, contact our technical team with your furnace model number.

Q: How do I reset the board after a lockout?

A: After a safety lockout (3 failed ignition attempts), the board will remain locked for 30 minutes. To reset immediately: (1) Turn off the 12V power to the furnace (at the thermostat, fuse, or battery disconnect). (2) Wait at least 10 seconds — this allows the board’s memory to clear. (3) Turn the 12V power back on. (4) The board is now reset and ready for a new ignition attempt. If the furnace continues to lock out after resetting, there is an underlying problem (no gas supply, blocked burner, bad electrode, weak fan, etc.) that needs to be addressed before resetting again. Repeated lockouts without addressing the root cause can damage the board or create a safety hazard. Always check the gas supply, burner, electrode, and fan before resetting.

Q: Can I install this board myself?

A: Circuit board replacement is one of the easier furnace repairs and can be done by most RV owners with basic DIY skills. The board is plug-and-play — no wiring modifications or programming required. The key steps are: (1) Document all wiring connections before disconnecting (take photos!), (2) Ensure all power is disconnected before starting, (3) Connect all wires to the correct terminals on the new board, (4) Test the ignition sequence carefully. The entire process typically takes 20-45 minutes. The most common mistake is misconnecting the wires — always refer to your photos and the furnace wiring diagram. If you’re uncomfortable working with 12V electrical systems or LP gas, we recommend professional installation by a qualified RV technician.

Q: Why does my furnace work on the bench but not in the RV?

A: If your furnace works when tested on the bench (outside the RV) but not when installed, the issue is likely related to the RV’s electrical or gas system, not the furnace itself. Common causes: (1) Low 12V voltage: The furnace may not be getting enough voltage when installed due to corroded wiring, loose connections, or a weak battery. Test the voltage at the furnace terminals — it should be 12.0V or higher. Below 10.5V, the board may not function properly. (2) Gas pressure issues: The propane tank may be low, the regulator may be faulty, or there may be a restriction in the gas line. Test the gas pressure at the furnace inlet — it should be 11″ WC. (3) Thermostat wiring: The thermostat wire may be damaged, shorted, or disconnected. Test the thermostat signal at the furnace board. (4) Grounding issue: A poor ground connection can cause intermittent operation. Ensure the furnace is properly grounded to the RV chassis. (5) Draft issues: When installed, the exhaust vent may create back-pressure or draft issues that affect combustion. Check the exhaust vent for blockages or restrictions.

Related Products

• 520814 Module Ignition Board — Atwood furnace ignition control module (alternative model)
• 31501 RV Furnace Ignition Board — Compatible with Atwood & Dometic Hydro Flame furnaces
• Suburban Ignition Electrode (232286) — Replacement spark probe for furnaces

Need help diagnosing or replacing your Atwood furnace circuit board? Contact our technical team with your furnace model number and symptoms. Free shipping and duties included on all orders.