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Do you sometimes get evil smells emanating from your shower drain?  Particularly after driving down some bumpy roads?  It's because the water in the u-trap under your shower pan gets shaken out and then allows grey water fumes to enter your bathroom.  Our standard fix was to pour a cup of water down the drain to re-fill the u-trap.  But what's the fun in that?

This mod solves that problem and also will stop the grey water tank backing up into your shower pan:  Remove the u-trap and replace it with a Hepvo trap.

Warning:  This is far and away the most awkward mod I've made to the trailer.  Access is awful and you can barely see what you're doing.

Anyway this is what it looks like to start with:

Removing the rubber U-trap is easy - just unscrew the fitting from the shower drain and loosen the hose clamp that clamps the utrap to the 90 degree bend.  Water in the U-trap will spill all over.

Next, you need to cut through the (vertical) riser about 3/8" above the floor level.  I cut the horizontal pipe first because the 90 degree bend got in the way of my saw.  Clean up the cut pipe to remove any burrs.

Now you have to assemble the Hepvo trap and associated ABS fittings.  Sorry that I don't have pictures of this bit, but I was a little flustered at the time.  What you need is (in order from the shower to the riser):

1. The right-angle connector that comes with the Hepvo trap
2. The Hepvo trap itself
3. A 5" length of 1.5" white PVC tube (I cut mine from one of these)
4. An 2" ABS compression fitting
5. A 2" ABS street Ell

Start by gluing item 4 to item 5.  Then assemble them all to do a dry fit.  You need to get the length just right (the components slide in and out to make this easy) by pushing item 5 onto the riser and screwing item 1 into the shower drain.  Tighten all the compression fittings and remove.  Now comes the really hairy bit:  Apply ABS cement to the open end of item 5, then reach in and push it onto the riser (which you can't see) while aligning Item 1 with the shower drain (which you can now tighten up).  If all went well, it will now look like this:

and

There are 2 styles of Hepvo trap on the market.  They only differ in the size of pipe that enters the trap.  Since we're not using those bits, you can use either size.