Check Vents

[Leroy Eckert]

Thanks again Pete:
I looked that guy Bernoulli up and had a look at his equations. I cannot
complete them. lol I see ole man
Euler was on to the pressure differential phenomenon also. I wonder what they
did for fun?

Leroy Eckert
1990 WB-40"Smoke N Mirrors"
Niceville, FL
----- Original Message -----
From: Pete Masterson
To: WanderlodgeForum@yahoogroups.com
Sent: Sunday, May 27, 2007 12:22 PM
Subject: Re: [WanderlodgeForum] Check Vents


One of the 'cautions' I received from friends and others with more RV
experience than I was to avoid driving down the road with windows or
vents open as it causes a reduced pressure (due to the Bernoulli
effect) inside the coach. Under those circumstances, it's possible to
pull sewer gasses past the water in the p-traps, resulting in
unpleasant odors in the coach.

You might check your black and gray tank vent pipes to ensure that
they're not blocked. They should provide sufficient air flow to
preclude odors from entering the coach during a tank dump. (I haven't
experienced any odor in the coach during/after a dump. I have gotten
some slight odor (most noticeable in the bathroom) when I've
forgotten to close the center roof (Kool-matic) vent. I've not run on
the road with windows open, except for the driver's window for a
short period as I approach a toll booth, etc.

I do get occasional odor from the Microphor toilet -- sometimes, it
seems the chamber below the bowl doesn't (fully) clear after a flush
cycle and some odor can result in the next flush cycle. Fortunately,
it's only mildly annoying, not a serious problem. (I plan to spend
some time studying the Microphor documents after we return home from
this trip, so I better understand how the toilet works and if it is
working properly.)

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

On May 27, 2007, at 12:06 PM, Rob Robinson wrote:

> Thanks for that Pete. I think I might check out these vents to see
> if I have
> a stuck one. I do sometimes get a smell when dumping the tanks and
> always if
> I leave a window open as I'm going down the road.
<snip>




[Non-text portions of this message have been removed]

[Pete Masterson]

Might simply be a sticky vent or a weak spring that allows it to stay
open for a time ... then some other movement through the pipes (say,
in the bathroom) forces some foul air out (eventually closing the
valve). Might simply try a replacement -- a new valve might either
not stick or have a stronger spring (or whatever else may ail the
valve).

It actually doesn't take much air to break the vacuum. I worked in a
very old building some years ago and we had a drinking fountain that
often failed to drain. After taking some standard steps to clear the
drain (that didn't work), the maintenance guy drilled a 1/16" hole in
the top of the drain pipe near the p-trap. Problem solved. We never
got any bad odors, but then there may have been other, more effective
vent lines elsewhere that took care of that. Apparently, the drain
line to the fountain simply didn't have enough venting to allow it to
drain properly.

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



On May 27, 2007, at 12:35 PM, Leroy Eckert wrote:

> Thanks, Pete:
> Your post make perfect sense to me. Apparently these vents open
> very quickly and minutely when the water is draining. Otherwise I
> would think an odor could occur just as if one were stuck open??
> Mine seem to be operating fine as the phenomenon does not occur often.
<snip>

[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?????
<snip>