[C38] Vented loop for head discharge

[les]

You would never put a vented loop in the head discharge.  Or perhaps I
should say I never would.  The holding tank is vented out the port
stanchion near the forward lower shroud.  

An anti-siphon valve with a loop is often inserted between the hand or
mechanical pump and the bowl flush inlet, to prevent the siphon effect,
and that loop never has anything but raw water (salt or fresh depending
on where your boat is sitting).

If you think about the direction of flow, from the inlet thruhull
through the pump to the bowl, you will realize that the pump valves will
NOT prevent siphon effect, because that is the direction they are
designed to pass water to flush the bowl.  This is especially true if
the valve on the pump is left in or falls to the flush position.
The flush inlet and output valves are in the pump body and I believe are
generally popet type valves. 

The duckbill valve is typically in the head discharge.  Someone can
correct me if I am wrong about all this.  I never rebuilt my pump, but
did have the head replaced once.

Generally flow is as follows:

	Head inlet thru-hull
        hose to pump flush side inlet
        pump
        hose from pump to bowl rim flush inlet
        toilet bowl
        toilet base
        duckbill valve to head flush pump
        pump flush side
        duckbill valve to pump outlet
        hose to y valve
          A side of y valve to overboard
          B side of y valve to holding tank
        holding tank
        vent hose to port stanchion vent
        outlet to pumpout valve
        hose from pumpout valve to pumpout fitting on deck port side
forward.
        At the other side of the TEE from the pumpout valve is the
macerator
        Hose from macerator output to overboard thru hull port side
under galley settee.
        thruhull to raw water (ocean, lake or whatever).

	To pump out, open the pumpout valve, and connect the pumpout hose to
the pumpout fitting (wear a mask.  I once had one do something very
untoward, and it was horrible on my skin!!!)
        Once pumpout is finished, close the pumpout valve (you do not
want the macerator to back up to the pumpout side.

Regards,
Les H

On Mon, 2011-10-31 at 23:43 -0700, Steve Smolinske wrote:
> Question for anyone with a vented loop in their head discharge.
> Looking at it on paper it seems that the pressure of the grey water in
> the section from the head to the vented loop would be enough to leak
> back into the toilet bowl.  I know there is a duckbill valve and a
> flapper valve in the head to stop the reverse flow, but as we all know
> if you leave the discharge seacock open the head will fill up.   Does
> the vented loop work because the head pressure of the grey water in
> the run from head to loop is not as great as that coming from the
> seacock?  Any experiences are greatly appreciated.   Thanks
>  
> Steve
> _______________________________________________
> Listserve mailing list
> Listserve at catalina38.org
> http://catalina38.org/mailman/listinfo/listserve_catalina38.org