RV Plumbing: What Is This?

Over the last few weeks a contributor to this blog, Gary Bunzer, has posted an excellent 5-part article on odor control in your RV. It covers a wide range of topics of concern to us all so that our RV experiences can be as pleasurable as possible.

Gary touched on proper venting of your systems as an important means of accomplishing odor control. The vent pipe from the tank through the roof of your RV allows the escape of gases and odors within the tank and piping systems.

On your grey water system which includes the kitchen sink, bath vanity sinks and the shower, you have “traps”, also called P-traps that retain water to act as a baffle to block any gases and associated odors within the piping system and tank. This trap is directly below the drain opening of each fixture.

It is important that the water remain in the trap at all times and not be allowed to escape due to evaporation or by siphonage. Each trap is required by the plumbing code to have a vent pipe or device connected immediately downstream of the trap. When waste is discharged from another fixture on the same piping system, this flow of water produces a vacuum in the waste pipe system behind the slug of water. It is possible for this vacuum effect to be great enough to allow the water inside another fixture trap to be siphoned away into the waste pipe system, eventually leading to a dry trap and allowing gases and odors to escape through the fixture drain into the living space. The presence of this vent connection allows the introduction of air into the waste piping system and prevents the vacuum from affecting the trap, thereby protecting the integrity of the trap.

For RV’s, as well as in some cases for residential and commercial plumbing systems, it often is not practical to route a separate trap vent pipe within the walls and/or through the roof to allow the introduction of the vacuum relief air. This calls for the use of an air admittance valve, or AAV. See Photo at top.

The AAV is nothing more than an air check valve that opens only in the direction to allow the entry of air into the vent pipe system at the point downstream of the trap. This valve is designed to operate with very minor changes in air pressure within the pipe system, and opens at the slightest amount of vacuum. It remains closed tightly at all other times in the reverse direction to keep any gases and odors from flowing out of the pipe system into the living space.

If you have one of these AAV’s in your system and you detect foul odors that may be emanating from your grey water system, you should check the fixture trap and the AAV. First, try to detect the odor directly above the waste opening of the fixture. If odor is present, perhaps you have a dry trap. If so, pour a couple of cups of water into the fixture drain to fill the trap. Take a look at the trap from below the fixture now that it is full of water and watch for leaks at the joints and/or the piping to see if trap or pipe leakage has been the culprit. If there are no leaks and the odor has gone away after using the fixture for some time you have probably solved the problem.

If you detect the odor while inspecting the trap and there is no indication of leaking water from the trap area, perhaps you have a defective AAV and the odor is originating from the device itself. Unscrew the AAV from the pipe system. You may want to put on disposable gloves for this as the grey water system is contaminated with bacteria. The valve usually has a male pipe thread of 1-1/2” or 2”. It can normally be removed by hand, but you may need a wrench or pliers to remove it. It is made of plastic so take care so as not to damage it. While visually inspecting the AAV, make sure that the air entry vent holes are clear.

Many AAV’s come with foam packaging around the valve for protection during shipping. I have on occasion found this foam to have been left in place during the initial installation which completely blocks the entry of air and renders the valve totally inoperative. If you find any blockage of the air inlet screen, remove it.

Holding the valve upright, shake it rapidly up and down. If you hear a clicking sound on every stroke, it indicates that the valve mechanism is not stuck in the open or closed position. If you hear no click, it is a good indicator that the valve mechanism is probably fouled or stuck and the valve is not operating as designed. You can try washing the internals using warm water and soap and repeated shaking to try to free the valve. If successful, rinse it off and allow it to dry and place it back into service. Use a small amount of “pipe dope” thread sealant or several wraps of Teflon thread tape on the male pipe threads when re-installing, and turn it only hand tight to seal it back in place.

The AAV has no serviceable parts so if you suspect yours is defective, replace it. They usually sell for around $15 at any plumbing supply outlet. These are available from several manufacturers such as Studor or Oatey.

Hopefully this advice can help you to enjoy the sweet smell of success!

yudamann