Troubleshooting the
RV DSI Furnace The following descriptions are based on the Hydro Flame
8500 series furnaces, but the operating principles are similar for other
manufacturers.
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.
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. More information on this
here.
The exhaust venting must be clear of all obstructions for proper furnace
operation. Inspect the vents for insect or bird nests or other debris.
Please see the manometer page for more information
and a simple manometer you can build yourself!
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 dissipate before the blower stops.
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.
Next in the ignition sequence is the blower motor. It drives two squirrel 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.
Specific motors are used in each model and BTU size of furnace and the correct
replacement motor must be used.
The sail 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 malfunction are: low battery, restricted return air
inflow, restricted or inadequate outlet vents, restricted combustion air inlet or
exhaust, faulty sail switch.
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
"limiting".
The circuit board will not receive 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 gases.
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.
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 unit is removed as an assembly consisting of the burner head, the gas valve
and the electrode
Note that different furnace models use a different burner head. The correct
replacement part must be used.
The electrode is the device that allows high current from the circuit board to
jump the gap between the electrode probes causing an ignition spark.
The electrode gap should be 1/8th inch as shown.
The electrode also serves as a sensing device to tell the circuit board that the
flame is present.
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.
Digital thermostat upgrade
Gas Valve - a White Rogers dual solenoid gas valve is used on these furnaces.
The solenoid coils can be replaced individually if they become defective. Each coil
should measure about 30 to 50 ohms.
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