Understanding the workings of your propane system and how to keep it operating properly can help you enjoy safe, trouble-free performance from your motorhome’s appliances.
By Bill Hendrix, F761S
The vast majority of the liquefied petroleum gas (LP gas or LPG) sold to the motorhome family is propane (C3H8); however, I am told that butane (C4H10) is still available in certain parts of the South and in Mexico. Huge differences exist between these fuels, and motorhome owners should make certain that they are indeed getting propane when filling their tanks in those areas.
The Differences Between Propane And Butane
(This is the temperature where vapor pressure starts)
F 32° F
|Weight, Per Gallon, at 60° F||4.24 lbs||4.81 lbs|
|Btu Per Pound of Liquid (This may vary slightly)||21,591||21,221|
|Air-Fuel Ratio (Peak)
(This tells us the fuels aren’t interchangeable)
|Range of Combustion
(This is the percent of fuel in the air-fuel ratio at which ignition occurs)
|2.4% to 9.6%||1.9% to 8.6%|
|Flame Temperature||3.596° F||3.615° F|
|Cubic Feet of Vapor, at 60° F (Per pound of fuel)||8.5||6.5|
This is pretty neat info! Now we can tell how much fuel each appliance consumes. Look at the data plate on any appliance that has an agency approval, and it will disclose the Btu rating or consumption. The British thermal unit (Btu) represents the amount of heat necessary to raise the temperature of one pound of water one degree Fahrenheit and is the most common measurement of heat.
Let’s take an 8-cubic-foot refrigerator as an example. The data plate says the burner uses 1,500 Btu per hour, and since each pound of propane has 21,591 Btu, simple division shows that this appliance can run for more than 14 hours constantly on one pound of propane. Most of our LP-gas appliances “” such as the refrigerator, furnace, water heater, and oven “” cycle off when the thermostat is satisfied, so we must estimate the duty cycle to arrive at a per-day figure.
Let’s guess the refrigerator burner operates about a third of the time, or eight hours per day, in very mild weather, and about two-thirds of the time, or 16 hours per day, in warm weather. So, in mild weather, one pound of propane would supply almost two days of energy for the refrigerator “” 8 hours x 1,500 Btu = 12,000 Btu per day. This would give us more than a month from a 20-pound cylinder of propane. In hot weather we could expect to use roughly twice as much fuel, because of the longer duty cycle, or about 15 days from a 20-pound cylinder. This math can be applied to any of the propane appliances.
In severely cold weather, butane just doesn’t work very well. From the chart, note that its boiling point is 32 degrees Fahrenheit. At temperatures below this, the fuel cannot give off any vapor “” it won’t evaporate or create a vapor pressure. Remember in the pre-propane days when butane tanks were buried? It wasn’t just because they were ugly; it was to keep the temperature above 32 degrees Fahrenheit so the liquid fuel in the tank could vaporize. Propane will vaporize at temperatures down to -44 degrees Fahrenheit, and this is real temperature, not wind chill factor. But as the fuel becomes colder, the rate of evaporation decreases. This is reflected in LP-gas manuals as a vapor pressure chart. The propane vapor pressure gradually decreases to zero at -44 degrees Fahrenheit.
Another factor is the fullness of the tank. For instance, a 100-pound cylinder that is capable of vaporizing 300,000 Btu per hour at 70 degrees Fahrenheit when full can provide only 64,000 Btu per hour at 0 degrees Fahrenheit when 50 percent full. This tells me to keep my LP-gas tank very full when traveling into climates with severe weather if I want to operate two 30,000-Btu furnaces at low temperatures, and it also tells me to refill the tank before it gets half empty.
It is rather immaterial which fuel is more efficient, propane or butane, since we are more or less locked into propane unless we want to change quite a few parts to convert the appliances, but I’ll go through the exercise, because someone will ask. From the chart, it appears that we can get a lot more vapor from a pound of propane than from a pound of butane, but the fuel ratios are different, and when we do the multiplication (vapor x fuel ratio), no significant difference materializes.
Altitude is another consideration. Data plates and air-fuel ratios are configured for a sea-level environment. Engineer types really like to do this. The amount of oxygen in the atmosphere reduces by about 4 percent per 1,000 feet of elevation. Keep this in mind when you are skiing at Breckenridge or hiking in the Teton Mountains. You may be dealing with altitudes of 9,000 or 10,000 feet, and the oxygen supply in the air has dropped almost 40 percent. This affects your lungs, and it also affects the air-fuel ratio of the coach’s appliances to the extent of the oxygen reduction factor. A 24:1 ratio at sea level would equate to a 40:1 fuel ratio at this altitude, since there is that much less oxygen in the air. This would provide the equivalent of only 2.5 percent fuel in the mixture, putting it right on the ragged edge of being non-combustible. Again, looking at the chart, you note that a propane mixture with less than 2.4 percent fuel won’t ignite. You also may note that if the mixture gets any richer than 9.6 percent fuel, it will not ignite either.
Also, propane vapor will settle to low spots since it is half again heavier than air (specific gravity of 1.51 relative to air). We all have heard that our LP-gas appliances must be operated at 11 inches of water column pressure (11 inches WC), and this is correct. Again, look at the data plates. This is the same as 6 1/3 ounces per square inch, which is a very low pressure. Normal lung power is much greater than this. The pressure regulator reduces the pressure coming out of the tank to a much lower and more steady pressure. Dual-stage regulators first reduce tank pressure to about 10 pounds per square inch gravity (PSIG) and feed this to the second-stage regulator, where it is reduced to 11 inches WC. This becomes the manifold pressure on the pipe system delivering fuel to all the LP-gas appliances. Most RV ranges and furnaces have an additional regulator, frequently built into the gas control valve, which will reduce the pressure one additional inch to 10 inches WC. It is very important when adjusting the regulator that the pressure test is not made downstream of one of these individual regulators, such as at the range burner. I like to use the test port provided on the refrigerator, because it is outdoors, easy to access, and has a standard 1/8-inch pipe thread fitting. A manual gas shutoff valve generally is available there just in case you need to quickly shut off the fuel flow.
To check the manifold pressure, a manometer is needed “” either a dial type or a U-tube. The dial type is a little costly and should be calibrated frequently with a U-tube. The U-tube is always accurate, and easy to use. (To make your own U-tube, see the LPG article titled “Technical Details About RV LP-Gas Systems” in the January 2003 issue.)
If you are somewhat mechanically inclined, here is how you can check and adjust the LPG pressure. Turn off the propane at the tank. Remove the 1/8-inch plug from the refrigerator test port. Install a 1/4-inch barb by 1/8-inch MPT connector into the test port and connect the manometer to the connector. Make sure all connections are tight and that the manometer is secured to the RV. Turn the propane back on, inspect the connections again, and then light the refrigerator burner. When the gas pressure is applied to the burner, it is also applied to the test port and to the manometer. The dial type will give an analog reading in water column inches. The gas pressure to the U-tube will force the water down 5.5 inches and up 5.5 inches, giving a differential of 11 inches of pressure “” hence the term 11 inches of water column pressure.
If an adjustment of the pressure is necessary, operate half of the LPG appliances to give the regulator an average load. Remove the slotted plug from the regulator to expose the adjusting screw. Adjust the pressure to 11 inches WC by turning the regulator adjuster clockwise for more pressure and counterclockwise for less pressure. When the other appliances are turned off (leave the refrigerator burner on), the pressure should not climb to more than 11 1/2 inches WC. If it climbs to 12 inches WC, the regulator is a little off, but I wouldn’t be concerned. If it climbs to 12 1/2 inches WC or more, the regulator is definitely worn and should be replaced. If all appliances are turned off with the manometer on the distribution manifold (not on the refrigerator test port), the pressure will climb to 12 1/2 inches WC, which is the allowed “lockup” pressure, and this is normal. This is another advantage of using the refrigerator test port, so you don’t have to deal with the lockup, as the regulator will not go to lockup while the refrigerator burner is operating.
Words of caution
Always turn off the fuel at the tank when installing any test equipment. Make sure the connections are tight. Reseal connectors with a light coating of thread sealer (pipe dope) or two turns of Teflon-type tape. Check for leaks with a commercial leak detector fluid. Home brew soap solutions will usually corrode the fittings, and if there is ammonia in the soap, the solution will stress and possibly crack brass fittings. NEVER use a flame! If you are not comfortable with propane, or are not mechanically inclined, don’t attempt to do this “” have a technician check and adjust the pressure for you.
When you are through, turn off the propane at the tank, remove the barb connector, and seal and reinstall the plug at the test port. Turn on the propane, relight the burner, and test for leaks.
Propane, the most popular of the liquefied petroleum gases, is a very safe product if handled correctly but can be a dangerous commodity if misunderstood. The combustible range of the air-to-fuel ratio actually is quite narrow. As noted above, if the mixture is too lean (less than 2.4 percent fuel) or too rich (more than 9.6 percent fuel) the gas will not burn, so it has a little safety factor built in.
All recently manufactured RVs bearing the Recreation Vehicle Industry Association (RVIA) seal, if propane-equipped, will have an LP-gas detector installed at the factory. These detectors utilize an electronic device, akin to the ion cascade chamber of a smoke detector, to trip a piercing alarm. If the LP-gas tank is frame-mounted (not portable), the alarm also may shut off the fuel supply at the tank. Unfortunately, the detector cannot distinguish this particular gas from some other chemically similar compounds or products that may be propelled by a hydrocarbon. So, false alarms may occur from time to time, and these false alarms might encourage one to ignore a real one. If you are consistently getting an alarm with some particular item, such as hair spray, try a different brand or purchase the pump type that doesn’t use a propellant. But do whatever is necessary to stop the false alarms, as it is most important that we rely on the alarm being genuine.
Too many of us are traveling without turning off the propane at the tank. I do understand that we like to keep the refrigerator cooling as we travel, but this provides a flame to kindle a fire or an explosion in the event of an accident. Also, many communities have local ordinances that forbid operating a flamed appliance while traveling within their jurisdictions. An accident that causes loss of life or serious injury might invoke not only civil liabilities but also criminal prosecution if you are deemed negligent. And, heaven forbid that such an accident may draw national attention, releasing an avalanche of restrictive legislation upon our industry. The refrigerator should hold acceptable temperatures for a four-hour to six-hour travel day if it is well cooled the night before.
Most, but not all, states have adopted the NFPA-58 regulations, which require portable, upright tanks to be equipped with an overfill protection device (OPD). Some very small and very large tanks are exempt. The OPD will be indicated by the triangular shape of the valve knob. This valve contains a float that will prevent overfilling the tank, thus ensuring that only propane in a vapor state is available from the vapor valve.
When regulations requiring OPDs were developed in 2001, it was recognized that the horizontally oriented tanks installed in RVs and manufactured prior to October 1, 1998, could not be retrofitted, so they were made exempt. Regardless, most ASME (American Society of Mechanical Engineers) tanks of recent years have been voluntarily fitted with an OPD and would have a non-removable triangular-shaped hand-wheel, and be marked as such.
When transporting portable tanks, they should be secured to prevent damage, as propane stations should not refill a damaged, dented, or even rusty tank.
Most propane stations have strict safety regulations that we should recognize and cheerfully comply with, and they are frequently posted in plain view. Here is what Flying J requires of its customers:
- Set Vehicle Emergency Brake
- Stop Vehicle Engine
- Turn Off Generator if Running
- Extinguish All Open Flames, Burners, and Pilots
- Extinguish All Smoking Materials
- All Occupants Must Exit the Vehicle
Since propane, in its natural state, has little to no odor, refiners add a fragrance that smells of rotten eggs. Immediately after a filling, this odor may linger awhile so you may catch a whiff occasionally, but this should dissipate rather quickly. Other than this, if you smell propane, turn off the system at the tank and seek professional help. If you elect to find the leak yourself, never use a flame to do so! Use a leak detector solution.
You, as the vehicle owner, can legally make repairs on your propane system if you elect to do so. However, service for hire, in most states, requires a demonstration of proficiency and a state license. Most RV service centers will have a trained, licensed technician on staff who is qualified to make repairs to LP-gas systems.
If your vehicle has been idle for a few weeks, the LP gas in the manifold loses some of its volatility and may not ignite readily with a spark-style ignition. To flush out the old propane, light a burner on the cookstove by using a kitchen match. This will replace the old propane in the main part of the LP-gas manifold with fresh propane. Make a trial ignition on all the gas appliances so you will be good to go when you travel.
Having a basic understanding of the properties of your LP-gas system, and conducting regular checks, can result in smooth operation of your coach’s propane appliances. Propane is actually your friend and should not be feared, but well respected.