The DSI RV Furnace - sequence of operation.
- The thermostat is turned on and calls for heat.
- Electrical current is sent from the thermostat to the time
delay relay. The relay closes after a delay of 10 to 20 seconds and energizes the blower
- The blower motor must come up to 75% of it's rated speed to
continue the ignition sequence. If it doesn't - the motor is faulty, voltage is too low, the return inlet
air is blocked, or there is a blockage in the exhaust.
Once the motor comes up to speed, the sail switch is
physically moved by the air flow and closes a micro-switch to continue the circuit. It must have a
certain force to close it and if the motor is not running fast enough, or there is a blockage of some
kind, or a "sticky" switch, then the current is interupted.
Note: This circuit is totally independent of the
thermostat or motor circuit and only comes into play when the fan blow "hard enough" to close the
Locate the two wires that come and go to the sail switch - with your test light, probe each one - you
should have power at both wires, with the furnace fan operating. If not, replace the sail switch.
- The sail switch closes and sends the power to the limit switch. The limit switch is normally closed, but
will open at a preset tempurature to prevent furnace overheating. With the fan running, probe each terminal of
the limit switch - you should have power at each terminal. If not replace the limit switch.
- The limit switch then sends power to the circuit board - the circuit board delays ignition for 10 to 20
seconds to allow the blower to purge any gas from the combustion chamber.
- After this delay, the circuit board opens the gas valve and sends a high voltage current to the electrode.
This ignition TRIAL will last for about 10 seconds. If ignition occurs, the electrode senses the flame and
returns this information to the circuit board. The circuit board then stops the high voltage spark but keeps
the gas valve open. If ignition fails, the board will go into lockout (new boards will try three times before
going to lockout).
- Lockout can be caused by a faulty circuit board, faulty gas valve, a faulty electrode or any of the above
- If the ignition sequence is interupted at any point, there will be no ignition. Safety is utmost!
Troubleshooting the RV DSI Furnace