You are correct that a vacuum to suck out the contents. But a liquid is far
denser than air, so a small air vent lets in tons more air than a pump can
suck out. The only way there could be a problem is if the air vent gets
totally plugged and that is rare. I have never heard of a properly installed
and constructed holding tank imploding. Most are made of a strong poly
material, stainless steel, or sometimes fiberglass (stainless is probably
the best, but most expensive, the poly tanks work very well). I should say
there are flexible holding tanks available also and they are very easy to
install. They sometimes don't contain the odor as well as other tanks
though. Speaking of odors, if you have them, they often come from the hoses,
but I digress.
Holding tanks should have an input from the head, an outlet (over the side
where legal) and a tee on the outlet to a deck fitting. This deck fitting is
a standard size, similar to the fuel fitting. You don't need an additional
input to wash the tank out. After pumping just add some fresh water through
the outlet deck fitting and pump again.
If you've never done it, it isn't as bad as it sounds. Just get the boat
ready by the pumpout and take out the deck fitting. Then turn on the pump. I
always put it in the pond for a few seconds to see if it is pumping. The
there is usually a valve at the sucking end, close it, put the rubber hose
fitting in the deck fitting and turn the valve on. Most pump outs I have
seen have a clear plastic section so you can see what is coming out (well
actually to see when it stops, though the sound will tell you). Turn off the
valve put the pump out hose in the pond and turn it on for a few seconds.
Now if you wish to wash out the tank, add some water through the deck
fitting you just pumped from, you really don't have to fill it to the brim,
and thenpump it again. When all finished turn off the valve again, remove
the hose, put it in the pond for a few seconds and turn the valve on (this
leaves it a little cleaner for the next guy). And then finish up.
Here is a link that talks about the installation of a simple system, with a
schematic at the bottom that is pretty clear.
One other thing, I always carry a couple boxes of plastic or latex
disposable gloves for the obvious reason - though they really aren't all
that necessary.
From:
AlbinVega@yahoogroups.com [mailto:
AlbinVega@yahoogroups.com] On Behalf
Of groundhog
Sent: Sunday, June 17, 2007 8:33 AM
To:
AlbinVega@yahoogroups.com
Subject: [AlbinVega] RE: Holding tank vent..
Looking for a holding tank vent through-hull or through-deck piece.
I have searched and the only thing out there is fuel tank vents and they
seem to have very small openings. Will that be ok?
When the holding tank sucking machine starts up, won't the vent be the only
place that the air can come into the system? Creating a strong vacuum?
What part did others use?
Thanks,
Roy
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