RV DSI Furnace
The following descriptions are based on the Hydro Flame 8500
series furnaces, but the operating principles are similar for other manufacturers.
Low Battery Voltage
The furnace is designed to operate at voltages between 10.5 to
13.5 VDC. Low voltage will not run the blower motor at the proper speed to commence the ignition sequence.
This is a safety feature that ensures the blower will be able to provide enough combustion air to operate the
burner at it's designed efficiency.
Obstructed return air intake
Return air is the air that flows in to replace the heated air
that the blower pushes out through the ducting. This air is pulled in by the furnace through louvered
openings in the side of the furnace cabinet, and any items stored near these openings could interfere with
this air flow.
The exhaust venting must be clear of all obstructions for proper
furnace operation. Inspect the vents for insect or bird nests or other debris.
Low gas pressure
Low gas pressure will affect a burner in several ways. When a
malfunctioning burner is encountered, the first step is to make sure the gas pressure is correct.
Gas pressure to a propane fueled appliance should be 10.5 to 11
inches of water column pressure. This is controlled by the main system regulator.
Time delay relay
This relay performs two separate jobs - one to handle the
relatively high current needed to run the blower motor - and two, to allow the blower to run for 45 to 90
seconds after the thermostat is satisfied. This allows excess heat in the chamber to disipate before the
The relay is normally open and should always have power from the
circuit breaker. Only when power from the thermostat is present does the relay close after a 20 second delay.
Power then flows to the blower motor. Newer models have a relay built into the circuit board and do not use
Next in the ignition sequence is the blower motor. It drives two
squirel cage fans to provide separate air flow for the combustion process and for distributing the heated air
to the coach. A heat exchanger is used to separate the heat from the burning gas while preventing exhaust
gases from entering the living space.
The sail switch (air prover switch) is an on/off device.
(normally in the off position) It gets it's name from the "sail" or paddle that is attached to the switch
mechanism. As the blower comes up to speed, it blows air onto the sail with enough force to push the switch
closed, thus allowing electrical current to flow to the next component in line.
It's job is to determine if there is adequate air flow for
proper combustion to take place. If the battery voltage is low or the fan does not come up to 75 per cent of
it's design speed, the sail switch will not close.
Possible causes of this malfuntion are: low battery, restricted
return air inflow, restricted or inadequate outlet vents, restricted combustion air inlet or exhaust, faulty
sail switch or faulty blower motor.
The limit switch is a simple temperature controlled switch. It's
function is to monitor the combustion chamber heat level. If the temperature of the combustion chamber
exceeds the preset limit, the switch will open and disrupt the flow of current to the circuit board, in
effect, shutting down the main burner. Once the chamber temperature cools sufficiently, the limit switch
resets. This initializes the ignition sequence and starts another cycle. This condition is referred to as
The circuit board will not recieve power until the sail switch
is closed by adequate air flow from the blower. The power must also flow through the limit switch.
When the circuit board is triggered by this current, it delays
ignition for about 15 seconds to allow the blower to purge the combustion chamber of any unburned
The circuit board then sends high voltage pulses to the
electrode assembly, providing a series of sparks to ignite the gas/air mixture.
At the same time, the circuit board sends power to open the gas
valves allowing the fuel to flow to the burner.
If everything is okay, the main burner ignites.
The circuit board monitors the burn cycle through a sensor that
detects the presence of the flame. If the sensor does not detect a satisfactory flame within about 10
seconds, the board then shuts off the gas valves and discontinues the ignition spark.
Depending on the board design, it will try for ignition up to
three times. Beyond that point, it goes into lockout mode, will not retry for ignition until reset and the
blower will continue to run.
This illustration shows the burner assembly for the Hydroflame
8500 series furnace.
Many older furnaces used a very different burner configuation,
but the principles remain the same.
The burner needs an ignition source (pilot or electronic spark),
an appropriate combustion air supply, and proper fuel metering (orifice) to operate at it's designed
Electrodes are designed to provide an ignition spark - to
initiate the burn - and to provide feedback to the circuit board when the flame is established. The propane
flame creates a tiny electrical current which is sensed though the electrode assembly. This current is sent
back to the circuit board, which in turn, decides if the flame is adequate. If not, the circuit board will
shut down the burner.
The electrode assembly may have two or three probes, depending
on the furnace make and model. Some systems used the extra probe for a ground circuit, others incorporated
the ground circuit into the second probe. In any case, the gap between the spark probe and the ground (or
other probe), should be 1/8 inch.
The electrodes must be clean and free of carbon or other
deposits. Do not file or sand the surfaces as you will damage the protective coating and shorten the
electrode life. Simply wipe off the contacts with a cloth and solvent, and replace them if needed.
The thermostat is an on/off switch controlled by a bi-metal coil that senses the
changes in temperature and opens or closes an electrical contact. This signal is sent to the furnace to start
or stop the heating cycle.