On gas powered furnaces, the thermostat tells the circuit board
when there is a call for heat.
- The board instantly starts the blower motor.
- A 'sail' switch located in the path of blower air knows when the
blower motor is turning fast enough.
- When the sail switch closes, the board turns on the gas valve and
starts to spark to ignite the burner.
- If the board senses the burner flame has lighted, the gas valve
On OEM boards, if the flame does not light after several tries,
the furnace 'faults' and closes the gas valve, but keeps the blower
motor running so after a short while all we feel is cold air from
The blower will continue to run because the thermostat will not
turn off the furnace because there is no increase in room heat.
Dinosaur brand replacement boards are much lower in cost and will
turn off the blower motor if the furnace faults.
The "click" you hear, may be the gas valve closing, sparking
operation and the sound of the lighted burner.
Barry L. 1/22/08
Using the RV furnace requires adequate
electrical power, sufficient LP-gas supply and thermostat
programming. However, while the furnace appears basic on the
surface, the system is actually an intricate series of components
designed to operate in concert. Becoming familiar with this heating
"chain of command" should assist you in learning how to keep the RV
comfortably heated, and help you troubleshoot any potential problems
that may arise.
A variety of electrical components comprise
the RV furnace system. In addition to the motor and blower, a relay,
circuit breaker, limit switch, sail switches and circuit board are
all parts of the furnace system that require adequate 12-volt DC
power. Whether you are relying on house batteries, the generator, or
shore power, test the source to confirm proper amperage is supplied.
Operating the RV furnace on voltage less than 10.5 can be compared
to sprinting straight up a steep hill without sufficient energy. It
could cause the furnace motor to overheat, damaging the entire
system. Providing the furnace system with amperage exceeding 13.5
volts is like turning around and running quickly down that same
mountain. The excess speed will likely create imbalance and
accelerated wear. So remember to authenticate adequate electrical
power to protect the furnace, as well as the rest of your RV
Engaging the furnace will necessitate
performing a few simple tasks. Begin by turning the LP-gas valve to
the on position. The valve is located at the tank. Tanks equipped
with a remote fill require solenoid power. Once activated, the
solenoid power will produce a click and illuminate the red power
light. Turn on the battery cut off switch at the entry door. The
wall thermostat must be turned on, mode button programmed to the
furnace setting, and the desired temperature set by using the up and
down arrow buttons.
Once interior heat reaches the chosen
temperature, the furnace system will go into a "sleep" mode. When
temperatures fall below your pre-set limit, the furnace will engage
and operate until again reaching the preferred limit.
The moment the furnace is turned on, a path
of power is opened between the thermostat and circuit breaker. The
thermostat controls the operating system by sensing room temperature
to determine when heat is needed. The circuit breaker works to
prevent the motor from drawing excess amperage. A relay sends
current to the motor. Once power is received, the motor engages the
blower to create circulating air. This process occurs in
approximately twenty seconds.
Two safety features are installed within the
furnace system. Sail switches confirm adequate airflow prior to
ignition. A heat sensitive limit switch, similar to the pressure
activated limit switch used to engage lighting on the refrigerator,
bay compartment and oven, disengages the electronic ignition system
to prevent over heating.
The wall thermostat is a crucial component of
the furnace system. The on/off switch at the thermostat sends power
to a circuit breaker. Occasionally, you may find that you have
inadvertently moved that switch and may try to correct the error by
moving the switch again. This quick series of electrical spikes to
the circuit breaker may cause the breaker to trip. If the wall
thermostat is rendered inoperable, include a check of the circuit
breaker while troubleshooting the problem.
Assigned with the critical task of limiting
amperage draw, the circuit breaker is an important part of the
furnace system. Breakers often trip because there is a problem with
the system. Begin by checking the RV electrical source to determine
if adequate voltage is being supplied. Short spikes to the power
will usually not cause a breaker to trip, but too little or too much
power is a common cause of circuit breaker malfunction. Once the
breaker has been reset, thoroughly investigate all amperage draw to
determine whether voltage use is overloaded.
When replacing the circuit breaker of the
furnace system, only use an identical replacement. Mounted
externally, this circuit breaker is rated as "slow Blow" to
withstand excess heat. Additionally, since the circuit breaker
performs the critical job of regulating voltage use, specific
amperage is required.
The thermostat heat sensor regulates the need
for heat. To ensure proper temperature sensing, the thermostat
should be clear of obstructions that may generate excess heat such
as wall hangings or items propped against the wall.
Providing a host of functions to the RV
furnace system is the circuit board. A timing circuit within the
board engages the blower to purge air from the chamber. The circuit
board also sends current to open the gas valve. Some systems include
a manual switch to control this function and that switch must be in
the "on" position for the valve to open. Once the valve is extended,
the circuit board will then send a high voltage spark to the
electrode at the burner. If the circuit board does not sense the
presence of flame in six seconds, the circuit board will retry two
more times. If flame is not achieved in three tries, the circuit
board will lock down for one hour before again attempting ignition.
Heat ducts are used to move air throughout
the RV. Airflow can be maximized by keeping the vent openings clean
and clear of dust and debris. Prevent placing obstructions in front
of heat ducts as this will restrict air movement. In the event
maintenance is performed at or near the heat ducts, carefully double
check the work to confirm that all unattached heat ducts have been
Annual maintenance of the furnace system
involves careful cleaning and inspection. Repairs to the furnace
system should be performed by a qualified service person. When
traveling with pets, or in regions that create excessive dust,
frequently clean the control compartments, burners and circulating
air passageways of the furnace system.
Eliminate dirt and debris from the control
compartment and remove corrosion from the burner using a wire brush,
particularly after storage when insects are likely to have built
nests near the burner tube. Occasionally inspect and clean the air
wheel to prevent damage from obstructions. The exterior air vent is
easy to access for frequent cleaning and removal of debris. Never
store cargo inside the exterior furnace air vent and risk the loss
of necessary air flow to the system. In the event that the
combustion chamber becomes clogged with dirt and debris, have a
qualified technician remove the chamber to flush away obstructions.
Annually inspect all components. Check the
control compartment for loose or disconnected wires. Adjustments to
the burner are rare, and should only be done by a qualified
technician and signs of cracking or holes appearing in the
combustion chamber should not be ignored. They will require prompt
replacement of the chamber itself. Confirm that all gaskets still
have a tight seal and discard any loose gaskets, replacing them with
new ones. View the overall system to ensure the furnace is firmly in
place without sags, cracks or gaps.
Remember that adequate amperage is necessary
for the furnace to operate at full capacity. If you are regulating
the use of electrical power, set the temperature lower to reduce
system cycling. Be attentive to maintenance of the system and you
will discover that the RV furnace is a reliable and efficient
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