Follow a few simple steps to keep your motorhome warm in winter.
By Gary Bunzer
An RV forced-air furnace is a sealed combustion system, so it is relatively easy for coach owners to perform a few maintenance procedures to ensure that warm air will circulate when needed. This is not to say the need for professional attention is eliminated altogether. As with any propane-burning appliance, it is still vital that the delivery line pressure be measured and adjusted, the regulator be tested, and a deeper cleaning be performed periodically. Also, it is imperative that the complete propane piping system be tested for leaks at least once per camping season.
But there are a few simple steps all motorhome owners can follow to avoid shop labor costs. I’ve always maintained that the four crucial areas regarding the RV forced-air furnace are as follows:
- Cleanliness of the furnace
- Proper routing of the ductwork
- Return airflow
- DC voltage input
Everyone should carry a decent digital multimeter (DMM), a few hand tools, and standard cleaning supplies so these four areas can be addressed before the onslaught of winter.
An important safety note: Coach owners should never remove any component on the sealed system that is held in place with a gasket. Breaking the seal on any gasket potentially can cause a dangerous carbon monoxide leak. Some of the aforementioned specific procedures are taught to professional service technicians and require special diagnostic equipment not normally carried by the average RV owner.
Vacuum and wipe down all portions of the furnace you can easily access. Though all forced-air furnaces are quite similar in their sequences of operation, they may be assembled into many different configurations. But simply keeping the unit free of accumulated dust and dirt will help ensure proper operation.
Vacuum in and around all areas of the furnace compartment, including the floor ducts. In some cases, it may be possible to remove a front panel to gain access to the interior portions of the furnace enclosure. In other cases, access may be gained through a panel from outside the motorhome. Use a soft, damp cloth to wipe down all metal components.
Take time to fully inspect the intake and exhaust vents on the exterior of the motorhome. The fireside stories you’ve heard of wasps, spiders, mud daubers, and birds possibly building a nest inside that intake/exhaust assembly are true. Obstructions in the intake tube will cause an overly rich mixture at the burner, resulting in incomplete combustion. Obstructions in the exhaust tube can be a fire hazard. Neither situation is good for you, the furnace, or the motorhome.
Much of this section pertains to coaches equipped with flexible runs of ducting rather than motorhomes built with rigid in-floor ductwork. Most flex ducts are routed above the floor, inside cabinets, under sofas, etc. Inspect the routing of every run, since they’re usually easily accessible. Look for extended runs and collapsed or pinched ducts. Longer-than-necessary branch ducts sometimes can be shortened to eliminate heat buildup. Excessive heat can cause the furnace to short cycle. As noted above, try to vacuum or wipe away dirt or dust inside each duct as far as you can reach.
Inspect all adjustable heat outlet registers, making sure most of them are fully open. The number of ducts routed throughout the coach is determined by the Btu rating of the furnace. In order for the furnace to operate properly, a certain number of ducts must be connected to its plenum or manifold. If too many of those ducts are closed (or not enough ducts are installed), the chance of a short-cycling sequence increases.
For proper operation, all forced-air furnaces require that a specific amount of fresh air be returned to the furnace enclosure. This is accomplished through a grille or a set of louvers mounted somewhere within the motorhome’s living space. Do not block or modify this grille in any manner. Some coach owners have unwittingly installed a filter of some sort within this grille. Doing so will cause furnace problems. Be sure the return grille and the area immediately behind it are free and clear of obstacles.
Likewise, the casing of the furnace also contains stamped cutouts in its shell that correspond with the minimum return-air requirements, and these openings also must remain clear of obstructions. Instances of faulty furnace operation have been caused by something simply falling out of a drawer or being stuffed under a cabinet and onto those vent openings, thereby blocking the flow of return air to the furnace. Inspect under the cabinet for anything that may have fallen on the furnace enclosure mounted below.
DC Voltage Supply
Of all the maintenance items a motorhome owner can perform, maintaining a healthy battery system is high on the list. The proper amount of DC voltage is crucial to the safe and efficient operation of the forced-air furnace.
Most modern furnaces operate at a voltage input between 10.5 and 13.5 volts DC. Of course, 10.5 volts is basically a dead battery (100 percent depth of discharge), so more than likely, the conscientious RVer will keep the battery bank at a higher level of charge. I stress the 50 percent rule; never discharge to below 50 percent of capacity, unless necessary. But it’s not uncommon for a somewhat-depleted battery to provide less-than-sufficient voltage, especially during a dry camping endeavor. Low voltage causes the blower wheel to spin slower, perhaps too slow to properly close the sail switch. The sail switch initiates the heating cycle by powering the circuit board, which in turn opens the gas valve and creates the spark that ignites the main burner.
On the flip side, too-high voltage coming into the furnace also can cause problems. Never power the furnace directly from a battery-charging device, converter, or off-line power supply. The incoming voltage must be “filtered” somewhat. This is accomplished by connecting the furnace to a battery circuit within the motorhome. Maintaining a clean, properly charged battery bank is essential for optimum operation of the furnace.
Propane Fuel Supply
During winter RVing, always keep the propane container as full as possible. Here’s why. The liquid fuel in the propane container is warmed by its contact with the external walls of the container, which in turn is warmed by the ambient air around it. That means the container’s interior wall surface must be sufficiently “wetted” by its contact with the liquefied propane to vaporize at a rate greater than what the appliances need at any given time. Remember, propane is stored as a liquid but burns as a gas. The total rate of vaporization is determined by the temperature of the liquefied propane coupled with the amount of surface area wetted by the propane. This is not an issue during the warm summer months, but it can cause consternation during winter travel.
In very cold climates, it’s entirely possible that the motorhome appliances (especially the fuel-hungry furnace) will demand and outrun the rate of vaporization as provided by the propane container. The result is that some appliances simply shut down suddenly, just as though the container were out of fuel. Some ASME tanks are more prone to this simply because of their physical design, so keep that propane fuel topped off during the cold winter months.
Serious and seasoned coach owners understand the importance of proper preventive maintenance. Following these few quick and easy procedures will give your heating system an advantage in the cooler months. But I must again offer a reminder about the importance of a thorough cleaning of the burner orifices (to remove carbon deposits); correct gap spacing of the electrode assembly; and complete testing of the pressure regulator and the entire piping system. And for those tasks, rely on certified RV service technicians.
Remember, RVing is more than a hobby; it’s a lifestyle!
Furnace Operating Sequence
Understanding a furnace’s sequence of operation can be helpful when troubleshooting. The following information pertains to a Suburban Manufacturing Company 12-volt furnace:
The wall thermostat controls the operation of the furnace by reacting to room temperature. This allows current to flow through the ON/OFF switch to the module board.
The module board constantly checks for a minimum of 9.5 volts. If there is not 9.5 volts, the module board will go into a standby mode until adequate power is supplied. It will then resume normal operation.
Upon a call from the thermostat, the module board thermostat circuit will go active. The sail switch circuit is verified as being open. The blower output is energized. The blower motor starts.
The module board will then verify that the sail switch circuit is closed and the motor is up to speed. If this circuit remains open for 30 seconds after the blower motor starts, the module board will go into lockout mode and shut down the blower motor.
The module board checks that the gas valve relay contacts (which are located on the module board) are open before the ignition sequence starts.
The board has a pre-purge timing circuit of approximately 15 seconds. This allows the combustion chamber to purge.
The module board will energize the gas valve and enable the high-voltage spark output to the electrode for seven seconds of ignition time.
The module board will then check for flame sense to verify successful lighting of the main burner flame. Sparking of the igniter will then be terminated and the gas valve and blower outputs will remain energized.
If ignition is successful, the module board will monitor the flame sense, sail switch and limit switch circuits, and the thermostat inputs during the heating period.
The flame is sensed through the spark wire and electrode. Therefore, it is essential that the electrode is properly positioned in the burner flame.
If the flame is not sensed after seven seconds, a second 15-second purge cycle will begin, followed by a second trial-for-ignition sequence. After three trial-for-ignition attempts with no ignition of the main burner, the module board will de-energize the gas valve immediately and the blower will run for three minutes and then shut down in lockdown mode.
If during the heating cycle, the limit switch circuit opens and remains open for five minutes, the module board will go into lockout and shut down the blower motor. If this occurs, the thermostat will need to be reset for the furnace to operate.
When the thermostat has reached its set point and the demand for heat ends, the gas valve will be de-energized and the flame will go out. The post-purge period of 90 seconds begins. When it times out, the blower motor output is removed, and the blower stops.