Anti-siphon loop needed ?

[darrylp]

Hi All, I'm going to be fitting a blackwater holding tank ( above the waterline ) soon, I am wondering as the tank will have a vent - do I need to keep the anti-siphon loop in place ?

As surely the airspace at the top of the tank, and its air vent will provide the same functionality ?

borrowing a picture from Tek-Tanks, they show a similar arrangement to how I would like to plumb this in .... saving the the top anti-siphon loop if possible, means I can place the holding tank higher. I aim to side feed into the holding tank, rather that top fill, the air vent will be on the top however.

Thoughts everyone ?

Darryl

 

[sailorbenji]

We have a holding tank above the waterline and therefore do not have a vented loop..as you say the tank itself and its air vent perform that function. It was fitted this way from the yard new, and has never been an issue.

 

[neilf39]

I fitted mine as per the picture but without the vented loop. If the tank is full and the vent is above the tank inlet hose then you could get a siphon occur so I don't fill the tank.

 

[johnalison]

My holding tank as fitted from new has a diverter valve/seacock and no significant loop. I think that in theory an unvented tank even with air in can create a syphon but you would have to be very unlucky for it to happen. My routine is to either leave the vented tank open or to close the main seacock and I imagine you are planning to do the same.

 

[Tranona]

Hi All, I'm going to be fitting a blackwater holding tank ( above the waterline ) soon, I am wondering as the tank will have a vent - do I need to keep the anti-siphon loop in place ?

As surely the airspace at the top of the tank, and its air vent will provide the same functionality ?

borrowing a picture from Tek-Tanks, they show a similar arrangement to how I would like to plumb this in .... saving the the top anti-siphon loop if possible, means I can place the holding tank higher. I aim to side feed into the holding tank, rather that top fill, the air vent will be on the top however.

Thoughts everyone ?

Darryl

View attachment 160279

No need for an antisyphon valve. The vent is effectively the same thing. Avoid a side entry if you can. Better to have a top entry with a U bend if the pipe is coming up vertically.. If you have a pump out through the deck there will be enough room. 2*35mm plumbers elbows make a compact U.

 

[darrylp]

thanks all, for words of wisdom.

Will be at the boat this weekend so will take a much closer look, and see if the top feed of the "waste" is doable.
I was going to feed in from the side (closest to the centreline of the boat), so the pipe over the top would be the same kind of U bend I think.

I don't intend to let the tank get anywhere near full, and air vents will be open all the time - hoping to keep the smells building up, when the drain seacock is in the closed position.

Darryl