FMCA Tech Tip: Purging RV Propane Containers

When one or more of a motorhome’s gas-burning appliances exhibit strange, random, or difficult-to-diagnose problems, it may signal the need to purge the propane container. Brand-new propane containers also must be purged properly before being filled for the first time. So what is purging? Does it apply to all RV propane containers? How do you know if your container needs such a service? And who can do it?

Read this how-to from FMCA and the “RV Doctor” Gary Bunzer.

About Propane Containers
Before answering those questions, let’s define the term “propane container.” Most motorhomes are equipped with horizontal propane containers bolted to the frame of the coach. Typical motorhome propane containers are regulated by the American Society of Mechanical Engineers (ASME), as documented in its Boiler and Pressure Vessel Code. A horizontal ASME container is called a tank. The capacity of an ASME tank usually is expressed in gallons.

Some smaller Type C coaches and older motorhomes may be outfitted with upright propane containers installed in a side compartment; it’s the same type of container usually found on towable RVs or in home gas barbecue grills. Such containers (which can be vertical or horizontal) are called cylinders, and are governed by the U.S. Department of Transportation (DOT) within the agency’s Hazardous Materials Regulations. Cylinders destined for Canadian provinces are regulated by Transport Canada (TC). The capacity of a DOT/TC cylinder typically is measured in liquid pounds.

So, generally speaking, an ASME tank is bolted to the coach, while a DOT cylinder is portable and replaceable. Both types of containers require purging at some point.

Why Purge?
New propane containers are hydrostatically tested for leaks before shipment. A container that is destined for an RV, whether it’s a tank or a cylinder, usually is filled with compressed air (or sometimes another inert gas) when it is shipped from the supplier. Air contains moisture and other contaminants. Purging removes the air, moisture, and contaminants; if the purge process is not performed before the container is filled with propane the first time, combustion problems or delivery line pressure irregularities may develop later.

Unfortunately, most new motorhome buyers don’t know whether their propane container was properly purged prior to that first filling, since a new motorhome usually is delivered to the owner with a full (or nearly full) container of fuel. I suggest asking that question during the walk-through or before officially taking delivery. Ask to see the pre-delivery inspection (PDI) form for validation. It’s in the best interest of both the RV dealer and the buyer to have a proper purging verified in writing before the retail delivery of an RV.

Purging applies to older propane containers as well. In fact, any time a container has been opened to the atmosphere, for any reason, a new purging must take place before refilling with fresh fuel. Say, for instance, the internal float assembly fails, or the shut-off valve leaks and that component needs to be replaced; the tank must be purged after the replacement, since air has been allowed to enter. And that air contains moisture and perhaps other contaminants.

The Moisture Problem
Even a little water lying at the bottom of a propane container can be a problem. Any moisture present from the container to the inlet of the pressure regulator can cause the regulator to freeze up. There are two types of regulator freeze-up, both with negative outcomes. 

One type of regulator freeze-up occurs as the fuel in the container becomes vaporized on its pressurized path to the regulator inlet. The pressure regulator has individual sections where the fluctuating container pressure is reduced in two steps to the delivery line pressure required by the appliances. The propane vapor must pass through small orifices in these two stages in order for the pressure to be reduced sequentially and smoothly. Propane’s “boiling point” is minus 44 degrees Fahrenheit. Below that, it exists in liquid form; above that, it vaporizes. So, any remnant of water will freeze in those small internal orifices and block the flow of propane vapor through the regulator.

The second type of regulator freeze-up happens externally during the winter months when mud or water or slush splashes up from the roadway, freezes, and blocks the second-stage atmospheric vent opening on the regulator. If the diaphragm inside the regulator cannot breathe, it cannot regulate, and it simply locks up. That’s why it’s important for the vent portion to be positioned downward and the regulator to be covered at all times during operation.

The result of either type of regulator freeze-up is no propane flow, and the appliances will shut down just as though the container were out of fuel.

When To Purge
A container that has been in use for a while may need to be purged if more than one appliance has operational issues. For instance, the furnace doesn’t ignite right away; the water heater burner sputters; the cooktop burner flame lifts off the burner; or a burner flame is abnormally large or small, or more orange than blue in color. Any operational issue with an appliance is a cause for concern and should be investigated. It simply may be an out-of-adjustment regulator or an appliance in need of cleaning, but it could also be caused by moisture inside the container.

Who To Call
Certified and master certified RV service technicians are trained to properly purge a propane container. As an alternative, most propane retailers also can do the procedure.

As mentioned, new tanks and cylinders typically are filled with air when shipped, but containers already in service will contain propane. In the case of the latter, before purging, it will be necessary to remove or burn off the container’s remaining propane.

What Owners Can Do
Since safety is paramount, FMCA recommends an annual test of the pressure regulator, as well as the entire propane gas distribution system, by a certified RV service technician. The condition of the container also should be checked. In addition, the RV owner should test the propane leak detector periodically.

If the propane’s odorant is ever detected, immediately turn off the container’s service valve and schedule the RV for tests.

And RVers should always keep a close eye on how well the RV’s appliances are operating. This can be your first sign of a problem.

FMCA RV Club brings you this monthly tip to Enhance Your RV Lifestyle. FMCA delivers RV know-how to its members. Join today and save $10 — a special offer for RV Trader readers. Learn more at

This information is for educational purposes. FMCA shall not be responsible nor retain liability for RVer’s use of the provided information. Prior to making any RV service decision, you are advised to consult with an RV professional.
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