Sequence of Operation – DC Models (Standard Units)

The ON/OFF switch allows power to pass to the circuit breaker and the thermostat.

The thermostat controls the operating circuit to the furnace by reacting to room temperature. When room temperature is below the thermostat set point, the contact closes to allow current to flow to the relay (relay can be either external or part of the ignition control board).

The circuit breaker limits amperage draw of motor. 

The relay allows current to pass to the motor by closing a switch within the relay. Voltage from the thermostat activates the relay to turn the fan on. This takes 1 -25 seconds (on units with the relay on the ignition control board there is only a 1 -2 second delay).

Current flows to the motor to operate the blower. One end of the motor shaft is for the circulating air wheel and the other side is for the combustion air wheel.

Circulating air blows against the sail switch and closes the contacts, completing the circuit. The sail switch is a safety device that insures air flow before ignition.

The limit switch is a safety device that protects the furnace from overheating. The contacts in the limit switch open at a given temperature setting, shutting off power to the electronic ignition system that controls the gas valve.

As power is applied to the circuit board, the system closes the following:

  2. A timing circuit allows the blower to purge the chamber (15-17 seconds).
  3. The board supplies current to the gas valve and causes it to open.
  4. As the valve opens, the board sends a high voltage spark to the electrode at the burner. The board detects the presence of a flame. If the flame is not sensed after approximately six seconds, the board will lock out (after three tries for ignition, the control will lockout for one hour unless power is removed, or the thermostat is cycled). Turning off and re-establishing power will restart the cycle.
  5. If the system does not ignite and the thermostat remains closed, the blower will remain on until the thermostat is reset manually on units with an external relay (units with the relay on the ignition control will shut the blower off even if the thermostat contacts remain closed). If the thermostat  has not been satisfied within one hour the system will try the ignition cycle again.

When the thermostat senses the desired room air temperature contact open, removing power from the ignition system and shutting off the gas valve. The blower runs until the relay opens (90 seconds) the circuit shutting off current to the motor.

Furnace Components

This pulls air from the inside of the RV and blows it over the outside of the heat chamber to wipe the heat from the chamber to force into the duct system to distribute it throughout the RV.

Pulls air from the outside of the RV, blows it into the burner chamber to mix the gas and air for combustion, and then blows the combustion air out the exhaust vent.

Circuit Breaker Tripping?

  • High Motor Amperage.
  • Poor Return Airflow.
  • Doubles as a Thermal Switch (Excessive Heat can trip Switch).
  • Voltage drop through the Switch while under a load indicates a weak Switch.
  • Excessive tripping could indicate a weak switch once other items are confirmed not be an issue.

The Sail Switch must proves the fan is running before the burner can ignite. This prevents a malfunctioning furnace from overheating and causing damage.

The sail switch is a normally open switch that closes when at least 75% of airflow is established

Sail Switch common issues

Sail Switch issues will have the possibility to create a few symptoms:

  • Fan comes on but gas valve does not open and electrode does not spark.
    • Sail switch is stuck open. 
  • Thermostat is calling for activation but no response from the furnace.
    • Sail switch is stuck closed.
  • Unit comes on and lights but will not stay light. Keeps lighting and going out.
    • Sail switch is fluttering, keeps opening and closing. 

      *Will attempt ignition for 30 secs and show a 1 flash error code on  control board indicating an air flow / limit fault.

Verifying the Problem

To verify if the sail switch is the issue, jump the sail switch. In cases where the fan motor runs however,  DO NOT  jump the leads prior to the motor starting or you will lock the furnace out. Wait for the motor to start running for a few seconds, then put your jumper across the leads. If the unit lights, and starts running correctly, replace the sail switch.

Water Damaged Circuit Board

The limit switch is  a normally closed, heat sensitive circuit breaker, It can be tested by checking continuity at room temperature or checking power on both terminals

The gas valve solenoid is a normally closed dual solenoid

When testing the coils you should see between 15-25 ohms

if you isolate the coils you should see between 30-50 ohms

Note: AFM / DFM 35K BTU models come with a slow blow fuse that MUST be used. The fans on these units get to speed faster than other models and if the slow blow fuse is not used there is a high potential to trip a standard fuse in the fuse panel upon start up.

 

If you are not hearing the tick of the Electrode, it is possible either the Board or the Electrode is defective. A shorted electrode can cause the gas valve not to open.

 

A simple test would be to disconnect the Electrode wiring from the High Tension located on the Board, if it begins to spark, the Electrode is defective and needs replaced. If there isn’t any spark coming from the High Tension, the Board is defective and will need to be replaced. 

Should have 1/8” gap. If adjustment is needed, bend ground wire as to not damage the electrode.