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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