Check Vents

[Pete Masterson]

The pressure differential between the tank interior and the interior
of the coach can be strong enough to pull (unwanted) sewer gas into
the interior by bubbling through the water in the trap. The trap will
remain wet, but the strength of the pressure will eliminate its
effectiveness. Plugging the drain will reduce this possibility by
increasing the air pressure required to defeat the p-trap.

Think of a kid blowing through a straw into a glass of liquid. The
bubbles pass through. Put a tight lid on the glass, and the kid will
have to blow harder to overcome the pressure. (Kid probably can blow
the lid off, but the pressure is probably more than the natural
effect of the open windows.)

When you open a front facing vent (if your coach has one), the ram-
air effect will slightly increase the interior pressure, so no sewer
gas is drawn into the vehicle. Most automobiles position the air
intake at the bottom of the windshield because there is a natural
high pressure point there. Some older cars placed vents on the front
of the vehicle -- a 1957 Mercedes had vent intakes to the left and
right of the grill. Some models of 55-57 Chevys had vent openings
just above the headlights (if I recall correctly). These vehicles
tended to have fairly high ram air effect to the air intake. My '68
VW had rear windows that would open outward... with a strong negative
effect on interior air pressure. Those open windows would cause the
incoming air through the dash-vents to be quite powerful.

You can put a Bernoulli-type cap on the sewer vent pipe that
decreases the pressure in the tank. I note that my coach has a rather
simple version of such. The "T" on the top of the vent has a larger
diameter pipe facing forward and a smaller diameter pipe facing
toward the rear. Since the change in size forces the air to
accelerate, it reduces the pressure inside the fitting and it is made
up by pulling down the pressure in the tank vent line attached at the
bottom of the "T".

I've seen commercially available caps that utilize this principle or
you can simply fit a larger diameter opening on the front side of the
T (or install a T with two pipe sizes, if necessary) to increase the
effect. Or, keep the windows closed.

Pete Masterson
'95 Blue Bird Wanderlodge WBDA 42
aeonix1@...
On the road at Lockhart Texas



On May 27, 2007, at 12:43 PM, Leroy Eckert wrote:

> There is a sort of vacumn formed inside when the front window is
> opened and it acts like the vent fans. It sucks inside air out. I
> had that happen once and have since closed all the drains with
> their respective plugs before traveling and have not had the
> problem since. I have yet to determine how the smell gets through
> the water traps???I would understand if a trap was dry. Perhaps
> the vacumn opens the check vents somehow?????
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