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Old 05-02-2016, 18:28   #1
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Holding Tank Discharge Plumbing

I'm in the process of designing and installing a holding tank system. My question relates to the discharge side of the tank. I'd like to have a single discharge port on the tank which will lead to both a deck fitting and also to a diaphragm pump for overboard discharge offshore. My question is whether or not I need a Y-valve to redirect the output, or if a simple T-fitting will suffice. I'd like to go with a T-fitting for simplicity but I'm not sure if the check valves in the pump will be sufficient to prevent back flow when pumping from the deck. Another alternative would be to close the sea cock when pumping from the deck but that requires remembering to open it when pumping overboard.

Any thoughts or suggestions? Thanks in advance.
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Old 05-02-2016, 18:51   #2
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Re: Holding Tank Discharge Plumbing

[QUOTE Another alternative would be to close the sea cock when pumping from the deck but that requires remembering to open it when pumping overboard.

Any thoughts or suggestions? Thanks in advance.[/QUOTE]


I think you answered your own question, this is how most I have been on operate


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Old 05-02-2016, 19:00   #3
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Re: Holding Tank Discharge Plumbing

You should keep that seacock closed all the time anyway unless your using it. So I vote for the T


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Old 05-02-2016, 19:23   #4
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Re: Holding Tank Discharge Plumbing

I use a simple tee. Like Sailmonkey said, the seacock should be closed anyway.
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Old 05-02-2016, 19:51   #5
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Re: Holding Tank Discharge Plumbing

I'd like to have a single discharge port on the tank which will lead to both a deck fitting and also to a diaphragm pump for overboard discharge offshore.

Why? Why would you want to complicate a system that can be so easily simplified by simply adding a second discharge port to the tank?

A second discharge port would elimininate the need for any tees or y-valves....one line out of the tank (preferably out of the top of the tank) directly to the deck pumpout fitting...a second line out of the tank that goes directly to an overboard discharge pump and then overboard. I dunno how it could be any simpler.

If adding the second discharge port is what's clouding your thinking, it couldn't be easier, thanks to a li'l gizmo called the Uniseal UNISEAL Or, if you havn't bought the tank yet, check out Ronco Plastics Ronco Plastics Marine Catalog They make top quality tanks for a very reasonable price and have 400+ shapes/sizes...and they install fittings in the quantities, sizes and locations specified by the customer when they make the tank.

It you have a problem figuring out how to do all this, give me a shout via email...I'm glad to help.
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Old 05-02-2016, 22:08   #6
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Re: Holding Tank Discharge Plumbing

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Originally Posted by peghall View Post
I'd like to have a single discharge port on the tank which will lead to both a deck fitting and also to a diaphragm pump for overboard discharge offshore.

Why? Why would you want to complicate a system that can be so easily simplified by simply adding a second discharge port to the tank?

A second discharge port would elimininate the need for any tees or y-valves....one line out of the tank (preferably out of the top of the tank) directly to the deck pumpout fitting...a second line out of the tank that goes directly to an overboard discharge pump and then overboard. I dunno how it could be any simpler.
A second discharge port from the top of the tank is an interesting thought. Wouldn't that require a dip tube inside the tank, sort of like a fuel tank pickup? Can Ronco supply a tank with a dip tube under the port?
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Old 05-02-2016, 23:07   #7
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Re: Holding Tank Discharge Plumbing

Mine came with the tank from the Marine Sanitation store.

SeaLand Holding Tank Diptube Kit
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Old 05-02-2016, 23:10   #8
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Re: Holding Tank Discharge Plumbing

Yep, as long as you specify it when you order the tank. Why not put both discharge fittings on the top of the tank with dip tubes...no standing waste in any hose.

Dip tubes aren't actually installed in the tank...they're cut to the right length and cemented to a threaded tank fitting, and shipped with the tank--for a LOT lower price tag than the SeaLand kit that you have to drill a hole and install yourself. You just insert it, and thread in the fitting like any other tank fitting. It's a good idea to do that before you bring the tank into the boat and put it in place...inserting an 18" tube into a tank that has only 3" of clearance above it ican be a challenge you don't need.

Btw...whether you put your discharges on the top OR at the bottom, put ALL the fittings including the vent fitting inboard (away from the hull) on the tank, or on the centerline if the tank will be centered in the boat. That prevents tank contents from spilling out the vent or running back into the toilet discharge line when the boat is heeled.
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Old 06-02-2016, 11:30   #9
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Re: Holding Tank Discharge Plumbing

Quote:
Originally Posted by wayne.b View Post
I'm in the process of designing and installing a holding tank system. My question relates to the discharge side of the tank. I'd like to have a single discharge port on the tank which will lead to both a deck fitting and also to a diaphragm pump for overboard discharge offshore. My question is whether or not I need a Y-valve to redirect the output, or if a simple T-fitting will suffice. I'd like to go with a T-fitting for simplicity but I'm not sure if the check valves in the pump will be sufficient to prevent back flow when pumping from the deck. Another alternative would be to close the sea cock when pumping from the deck but that requires remembering to open it when pumping overboard.

Any thoughts or suggestions? Thanks in advance.
I believe you need to check what is legal?
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Old 06-02-2016, 11:48   #10
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Re: Holding Tank Discharge Plumbing

I just redid my waste system and used a ball valve on the leg going to a macerator, that pumps overboard. Thank allows maintainance of the pump without completely emptying the tank.
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Old 06-02-2016, 11:59   #11
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Re: Holding Tank Discharge Plumbing

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Originally Posted by peghall View Post
...Why would you want to complicate a system that can be so easily simplified by simply adding a second discharge port to the tank?...
Most holding tanks have ONE bottom outlet. Adding yet another outlet actually complicates the tank and doubles the likelihood of a leak.

A standpipe installation insures that you can never fully empty the tank and so reduces the usable volume. Also the standpipe is vulnerable to damage from the movement of solid waste inside the tank.

If necessary to protect a diaphragm pump, you can install a simple shutoff valve between the tee and the pump. Creating a "trap", lower than the bottom of the tank, helps to insure complete evacuation.
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Old 06-02-2016, 12:28   #12
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Re: Holding Tank Discharge Plumbing

I must be dumb? A diaphragm pump for discharge of a holding tank. I would think a macerator pump makes more sense?
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Old 06-02-2016, 12:45   #13
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Re: Holding Tank Discharge Plumbing

Adding yet another outlet actually complicates the tank and doubles the likelihood of a leak.

I disagree...two separate discharge lines is a lot simpler that one with a y-valve that may be used so seldom that it freezes...or the handle breaks. And if one port doesn't leak, two aren't likely to either. Otoh, if one does the other prob'ly will too...which only points to the competency of the person installing it.

A standpipe installation insures that you can never fully empty the tank and so reduces the usable volume. Also the standpipe is vulnerable to damage from the movement of solid waste inside the tank.

Not true at all. A discharge port at the bottom of a tank has to be far enough above the bottom to allow for the spinweld or flange...typically putting the bottom of fitting at least an inch from the bottom. A standpipe, otoh, can go all the way to the bottom, then be cut at 20-25 degree angle--no more is needed to prevent sludge from clogging...which leaves only about 1/2" in the tank. And if solid waste from a human can damage a 1.5" sched 40 PVC pipe immersed in liquid, that human is in serious need of a change in diet!

You may a like a single bottom discharge with a p-trap that'll fill up with waste--and any sludge--with the first use after pumpout...a discharge hose in which waste will always be standing to the level in the tank and a y-valve...it's YOUR boat, whatever floats it for you is fine with me.

However, IMO two discharge parts on the top of the tank with standpipe inside the tank, no y-valves and no standing waste in hoses to permeate 'em and stink is a much simpler, better, way to do it when possible....it isn't always possible though.
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Old 06-02-2016, 12:53   #14
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Re: Holding Tank Discharge Plumbing

Macerator pumps require electricity...manual diaphragm pumps don't.

Macerator pumps are impeller pumps that can only run dry for a VERY short time before dry friction heat "fries" the impeller. Electric diaphragm pumps can run dry almost indefinitely without harm.

If just pushing a button is all you want to do to dump a tank, SeaLand/Dometic, and Jabsco and Raritan make electric overboard discharge diaphragm pumps. They cost a good bit more than a macerator pump, but replacing only 2-3 impellers and then the whole pump within just a few years vs. a pump that'll last decades makes the electric diaphragm pump a lot cheaper in the long run.
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Old 06-02-2016, 12:56   #15
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Re: Holding Tank Discharge Plumbing

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...A discharge port at the bottom of a tank has to be far enough above the bottom to allow for the spinweld or flange...typically putting the bottom of fitting at least an inch from the bottom...
You should know better than to make a statement like this.

Typically the spin-welded fittings are right at the bottom. And the tank is installed so that it pitches down to this fitting, which allows much more complete evacuation than any standpipe.
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