Technical Details About RV LP-Gas Systems

Understanding the workings of your propane system and how to keep it operating properly can help you to enjoy trouble-free performance from your motorhome appliances.

By Bill Hendrix, F761S
January 2003

The vast majority of the liquefied petroleum gas (LP-gas) sold to the motorhome industry is propane (C3H8); however, I am told that butane (C4H10) is still available in certain parts of the extreme 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 tank in those areas.

Below is a chart that illustrates some of the differences between propane and butane.

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 Btus per hour, and since each pound of propane has 21,591 Btus, 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 8 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 Btus = 12,000 Btus per day. This would give us more than a month from a 20-pound cylinder of propane. In hot weather we could expect roughly twice as much fuel consumption, 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 the 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 Btus per hour when full at 70 degrees Fahrenheit can provide only 64,000 Btus per hour when 50 percent full at 0 degrees Fahrenheit. 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 Tetons. 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 body, and it also affects the air-fuel ratio of the coach 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 may also note that if the mixture gets any richer than 9.6 percent fuel, it will not ignite either. Imagine a bucket full of propane and air that is 10 percent to 15 percent fuel. Throw a match into the bucket, and it will put out the match! P.S. Don’t try this, since I don’t know of a practical way to tell how much propane you have poured into the bucket. Also, propane vapor will pour, since it is half again heavier than air (specific gravity of 1.51 relative to air; would that be at sea level?).

We have all 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 water column pressure. 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, that 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. Generally a manual gas shutoff valve is available there just in case you need to quickly shut off the fuel flow.

To check the manifold pressure, we need a manometer — 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, very easy to make, and easy to use if you are somewhat mechanically inclined. Following are instructions for making and using a U-tube manometer.

Purchase a 6-foot length of 1/4-inch-inside-diameter, 3/8-inch-outside-diameter vinyl tube; a dozen small (1/8-inch) wire ties; two 1/4-inch barb x 1/8-inch male pipe thread (MPT) connectors; and a brass needle valve with 1/8-inch FPT each side. These parts will cost approximately $5. Cut off a yardstick at the 24-inch mark and fasten the tube to the now 24-inch yardstick, starting on the edge at the 1-inch mark and running down to the 24-inch mark. Make a tight loop at the bottom and come back up the other edge with about 20 inches left over. Drill 1/8-inch holes on both edges of the 24-inch yardstick at the 1-inch, 5-inch, 10-inch, 15-inch, and 20-inch marks. Use the wire ties to fasten the vinyl tube to the 24-inch yardstick. (See the accompanying photo.) Drill a 1/4-inch hole near the 1/2-inch mark and fashion a sturdy hook with fairly heavy wire so you may hang the manometer vertically. Color a little distilled water with a few drops of cake coloring so you won’t have trouble seeing the water level. With both ends of the tube open, pour enough of the colored water into the U-tube to bring the water level to somewhere around the 12-inch mark. The amount of water in the tube is not critical but should be within 2 inches of the half-full 12-inch mark. Install one of the 1/4-inch barb x 1/8-inch MPT connectors into the needle valve, and push the barb end into the short end of the U-tube. Turn the stem of the needle valve clockwise until snug to close the valve. The reason for the needle valve is safety. This gives you control of the gas flow to the U-tube and another safety shutoff if needed.

Turn off the propane at the tank. Remove the 1/8-inch plug from the refrigerator test port. Install the second 1/4-inch barb x 1/8-inch MPT connector into the test port and connect the long end of the U-tube to the barb fitting. 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 but is blocked on the other end of the tube by the needle valve. Slowly open the needle valve about one turn; this will allow the water levels to stabilize and indicate the gas manifold pressure. The valve must remain partly open during the pressure test and any adjustments of the regulator. The correct gas pressure will push the water down 5-1/2 inches on the near side and up 5-1/2 inches on the far side, giving a 11-inch differential in the two water levels — hence the term 11 inches water column pressure.

Since you have 12 inches of the yardstick left over, cut it down to 11 inches and clip it to the manometer via a clothespin. Use this extra length to measure the difference in the two water levels, and you won’t have to count the inch marks on the 24-inch U-tube ruler.

If an adjustment of the pressure is necessary, operate half of the LP 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 brand 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. Now you have the manometer in one hand and the connector you just removed from the test port in the other. Screw the connector snugly by hand into the open end of the needle valve, and slip the other end of the tube over the barb. This will close the loop of the U-tube, and you won’t have to go through the cake color and water filling thing the next time.

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.

An up-to-date directory and map of propane suppliers in the United States, is offered at the Web site of the Alternative Fuels Data Center. Visit www.afdc.nrel.gov/refueling.html and click on Alternative Fuel Station Map Site.