Wednesday, November 29, 2023


This is much better than a P-trap

Hey Doc,
I was at an RV show recently and I noticed that there wasn’t a regular P-trap under the lavatory sink, just a straight fitting or adapter. Any idea what this was? I thought all sinks had to have a water lock to keep the holding tank fumes from getting inside the motorhome. —Craig J.

Hey Craig,
Welcome to a unique type of waterless P-trap. Actually, it’s best described as a waterless sanitary waste valve. I’ve mentioned this device in previous posts and they’re being found on more RVs directly from the factory. It’s called the HepvO valve, and it eliminates the need for a bulky P-trap hanging down below RV sinks or below a shower/tub enclosure.

During manufacture, this maximizes the ceiling height in the shower area so there’s no need to have that bubble skylight in the roof any more since the pan can be installed directly on the floor with the drain plumbing below. In the aftermarket, the HepvO valve will provide owners more under-counter storage space. Additionally, it does not require RV antifreeze during winter prepping, or any maintenance for that matter.

The HepvO’s unique design incorporates a flexible, self-sealing, internal silicone membrane that opens when water is flowing and completely closes off when the flow stops to prevent holding tank odors from escaping up and through the fixtures. Because it is in itself an effective air-admittance device, this valve even eliminates the need for additional anti-siphon trap vent devices (ASTVD) at fixture trap arms – another cost-saving manufacturing advantage.

It can be installed either horizontally or vertically, thereby maximizing installation options. This design also has proven to be hygienically safer since waterborne food particles and debris cannot become trapped inside or given a chance to develop microbiological and bacterial growth.

The HepvO cannot be rendered ineffective by freezing temperatures, siphoning, evaporation, water compression, foaming, capillary action or by any disproportionate physical movement of the motorhome – all possible outcomes common to a standard water lock P-trap. All in all, it’s a neat space-saving method of preventing holding tank odors from entering the RV, even during periods of non-use, winterization and storage.

Already factory-installed by many coach manufacturers, the HepvO waterless valve is gaining more aficionados from the aftermarket all the time. By virtue of its inimitable design and proven track record, it’s unlikely you’ll ever have gray plumbing odor issues again.

Check out this RV Doctor video.


HepvO is available on

(Previously published November 2017.)

Read more from Gary Bunzer at the See Gary’s videos about RV repair and maintenance.




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Mike Griffin (@guest_47088)
4 years ago

You should have mentioned that these traps should NOT be cleaned out by using plastic “Zip-it” devices because they will invert and be permanently damaged. A lot of RVers on social media ask how they can clean out their slow (or stopped-up) drains, and many recommend buying and using these plastic strips with the barbed sides. They are fine for normal p-traps, but should never be used for the waterless p-traps.

John Whitney (@guest_47087)
4 years ago

These valves are similar to valves in vacuum flush toilets. Eventually they will harden up or develop deposits that will require cleaning or replacing. Probably no more maintenance than that required of a p trap. Very few things that open and close constantly requires no maintenance. One should be careful to not create unrealistic expectation of an otherwise great advance.

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