Warming Up to Furnace Facts

By Gary Motley

Master Certified RV Technician

          This time of the year one of the major catastrophes an RVer can have is for the furnace to stop working.  When this happens many RVer rush to a repair facility when the reason for the failure could be simple fix-it-yourself problem.  I will try to address some areas that will help RVer understand the furnace.  Before I go into real detail, I’ll start with some real simple diagnostics you can do yourself. 

1.       Do you have good 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 valve.

2.       Do you have plenty of LP GAS and is the valve turned on at the LPG container?  Without getting real technical, be aware that the less LPG you have in your LPG container and the colder the outside temperature is, the less efficient your LPG system is.

3.       Is your LPG 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.

4.       Many furnaces have a valve at the furnace for the LPG.  Is it turned on?

5.       Most Hydroflame (Atwood) heaters have a CKT breaker to the fan to the fan motor and a switch to the gas valve.  These must both be on.  Hydroflames can be accessed by dropping the heating cover on the outside.  Some models have an inner cover and it must also be dropped.  They are then clearly visible.  Suburbans manufactured after 1977 have a master switch inside the cover. 

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.

1.       The thermostat senses a need for heat and closes the contacts allowing current to flow to the time delay relay.  Dirty contacts in the thermostat and broken or cut wires are the most often discovered culprits in this stage.

2.       The time delay relay receives the current and closes the CKT to the fan motor within 17 to 20 seconds.  The time differential can vary slightly based on the temperature and voltage.  Broken connections, a bad relay, and low voltage/bad ground are the most likely culprits here.

3.       The current then flows to the motor.  The 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.

4.       As the room air wheel comes up to speed, it closes the sail switch.  This completes the CKT to 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.

5.       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 direct spark ignition (DSI) system that controls the gas valve.  The most common problems here are band 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.

6.       As power is applied to the DSI board, the system does the following:

A.     A timing circuit allows the blower to purge the chamber (15-17 seconds).

B.     The board supplies current to the gas valve and causes it to open.

C.     As the valve open, 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 approximately six 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.

D.     If the system does not ignite and the thermostat remains closed, the blower will remain on until the thermostat is reset manually.

Some of the most common failures of the 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, the contacts open,

Removing power from the ignition system and shutting off the gas valve.  The blower runs until the heater in the relay cools and opens the circuit, shutting off current to the motor. 

While we are talking about the thermostat, set me mention that the thermostat is an RV needs to be one made for an RV and not the mercury bubble style found in homes.

   A few additional items about furnaces that I feel it is important to mention:

1.       If you have one of the old Duo Therm 9000 Series furnaces, you should strongly consider a new

Furnace as it is generally considered to have some design safety issues.

2.       You should make certain that the area around your furnace is free from air obstructions.  Air flow

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.  A couple of other potential problems are overheating of the electronic components and safety devices causing premature failure.

3.       Your 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, you should not block off any of your ducts or outlets.  This reduces air flow, which can also cause problems.

Those dog and cat bowls are other objects placed in front of the return air grill can cause many of the crud problems I have previously mentioned.

There is a new DSI board out on the market, which turns the fan off if the burner does not ignite.  It is so new that I have only seen it at the National Show in early December.  It will be available soon.  It can be retrofitted with some wiring changes on most models.

Those DSI boards can have an intermittent problem that can be extremely hard to diagnose.  Heat and cold are generally what causes this (expansion and contraction).

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, you should always contact a qualified RV technician.  Safety should be of major concern when dealing with LPG and carbon monoxide.  

Thank you for wading through this.

P.S. A carbon monoxide detector can be a real good investment towards you personal safety.