Warming Up to Furnace Facts
By Gary Motley
Master Certified RV Technician
Do you have good DC voltage to the furnace? You must have good voltage to
spin the motor fast enough to close the air flow (sail) switch and open the gas
Do you have plenty of propane and is the valve turned on at the
container? Without getting real technical, be aware that the less Propane you have
in your container and the colder the outside temperature is, the less
efficient your propane system is.
Is your propane monitor turned on and, here again, is your voltage proper?
While I am mentioning voltage, let it suffice to say that anytime your 12V
source in your RV drops below 10.5V you are subject to many problems. This is a
generality. I am always happier to see a good strong 12.6V to 13.2V with 11V
being my threshold of concern.
Many furnaces have a manual shut-off valve at the furnace for the Propane. Is it turned on?
5. Many heaters have a circuit breaker to the fan motor and a switch to the gas valve. These must both be on. Many can be accessed through an access cover on the outside. Some models also have an inner cover and it must also be lowered after removing the access cover. They are then clearly visible.
SEQUENCE OF OPERATION OF DSI MODELS:
As a point of information and because most RVer
want to know what is happening and why, I will go into the Sequence of Operation
and most likely (based on experience) causes of malfunction at each step. The
Sequence of Operations is pretty much the same for both major brands since the
mid 1980’s. About 2005 the printed circuit board on some models began
controlling the fan.
About 2005 the printed circuit board on some models began controlling the fan.
1. The thermostat senses a need for heat and sends 12V to the furnace control circuit. A bad thermostat and broken or cut wires are the most often discovered culprits in this stage if there is a problem.
The furnace control circuit receives the current and starts the fan
motor within 17 to 20 seconds. The time differential can vary slightly based on
the temperature, voltage, and types of control circuit. Broken connections, a bad
control, and low
voltage/bad ground are the most likely culprits here. On newer fan
control boards, this relay is part of the igniter board.
On newer fan control boards, this relay is part of the igniter board.
3. The fan motor turns the blower. One end of the motor shaft is for the air circulating wheel and the other side turns the combustion air wheel. The main problem at this stage is generally a mud dauber nest locks up the wheel. The motor itself can also go bad.
As the room air wheel comes up to speed, it closes the sail switch. This
completes the circuit to the ignition side of the power board (DSI board). The sail switch is placed in
the system as a safety to make certain the air circulation is adequate. The
potential problems with the sail switch are crud buildup on the sail causing it
to be too heavy to “sail.” A mud dauber is not uncommon here. I
have seen bad contacts in the switch.
I have seen bad contacts in the switch.
According to the model and manufacturer, there is a limit switch on one
side or the other of the sail switch. The limit switch is a safety device
that protects the furnace from over heating. The contacts in the limit
switch open at a given temperature setting, shutting off power to the gas valve,
which shuts off the blower. The most common problems here are
bad connections or air flow restrictions causing the area around the limit
switch to become too warm and shut down. I will talk about air flow
restrictions in my third section.
As power is applied to the furnace control circuit board, the system does the following:
A timing circuit allows the blower to purge the chamber (a few seconds).
The board supplies current to the gas valve and causes it to open.
C. As the valve opens, the board sends a high voltage spark to the electrode at the burner. The board detects the presence of a flame. If the flame is not sensed after a few seconds, the board will lock out (three try for ignition, one hour lockout and then three retry), shutting off power to valve. Some of the older boards have only a single try with no one hour retry. Older models have a separate sensor probe, while newer models spark and sense through the same electrode.
D. If the system does not ignite and the thermostat remains closed, the blower will remain on until the thermostat is reset manually. Newer models have a fan control on the board and will shut the fan off.
Some of the most common failures of the
furnace control DSI Board,
which is the heart of the system, are a bad connection at the DSI board plug
itself; an internal failure in the board which calls for replacement; a stuck
gas solenoid valve; a bad connection to the spark or sense electrode; the spark
electrode burned away; and no spark jumping to ground caused by the electrode
being improperly spaced with the ground and crud in the gap stopping the spark.
While the last few items are not really a problem with the DSI board, this is
the best place to mention them as they are controlled by the DSI board.
7. When the thermostat senses the desired room air temperature, power is shut off to the furnace control circuit, removing power from the ignition system and shutting off the gas valve. The blower runs a few seconds to dispel any residual heat.
While we are talking about the thermostat, let me mention that the thermostat in an RV needs to be one made for an RV specifcally.
A few additional items about furnaces that I
feel it is important to mention:
If you have one of the old Duo Therm 9000 Series furnaces, strongly consider a new Furnace as it is generally considered to have some design and safety issues.
Restrictions can cause overheating of the burner
chamber. Some of the undesirable results can be a cracked combustion chamber
allowing carbon monoxide to escape into the duct system and living area.
Carbon monoxide poisoning can cause serious injury or death. A couple of other
potential problems are overheating of the electronic components and safety
devices causing premature failure.
An RV furnace was not designed to have air filters. This too can cut down
on air flow. Those
pretty expanded metal grates that many people put
in the front of the furnace can cut the free air movement by up to one half.
Atwood (Hydroflame) had a recall. It was mostly
on 1993 models. The serial numbers were 699173 through 840685. All these
numbers begin with ULM. This was the 8900 II Series and it involved premature
burner chamber deterioration that created an unsafe carbon monoxide situation.
In order to insure the best possible air flow,
do not block off any ducts or outlets. This reduces air flow, which
can also cause problems.
Those dog and cat bowls or other objects placed
in front of the return air grill can cause many of the crud problems I have
There is a DSI board out on the market, which turns the fan off if the burner does not ignite. It can be retrofitted with some wiring changes on most models. Be sure whoever does this follows the installation instructions.
Those DSI boards can have an intermittent problem
that can be extremely hard to diagnose. Heat and cold are generally what causes
this intermittent failure due to expansion and contraction. When this
happens a new board is generally the best answer.
When this happens a new board is generally the best answer.
I have tried to cover several problem areas to provide some ideas to help RVers enjoy a more safe and reliable furnace. When in doubt, always contact a qualified RV technician. Safety should be of major concern when dealing with propane and carbon monoxide.
Thank you for wading through this.
P.S. I strongly suggest a carbon monoxide detector
as an investment for your personal safety. Be certain it is listed for RV
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