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  #1  
Old 05-13-2007
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Fresh water and head

Does anyone know if a traditional marine head can be used with a pressure fresh water system? We are going to be liveaboarding and I want to run the fresh water system off the dock instead of the 12v pump. I also want to be able to use this fresh water for the head to prevent the problems that come from salt water use, such as build up in the hoses and stinky dying marine creatures. When I took the old system apart the hose diameter was reduced by about 1/3 with calcified deposits.
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Old 05-13-2007
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I would not recommend using or connecting the head directly to a pressure freshwater system... or to your freshwater system at all... the risk of contaminating your potable water tanks is not worth risking.

The way my system is setup on my boat allows you to safely use the head sink to flush the head as the drain to the head sink has a diverter valve that allows the sink to drain into the head pump system or out the through-hull. The sink acts as a massive vacuum break and prevents almost any risk of cross contamination.

If you have a pressure water system, you can plumb the head sink this way, and use the freshwater to flush the head. Also, it works quite well for winterizing the head.
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That sounds like a brilliant solution -but don't you find it confuses already confused visitors? Getting used to a marine head can be daunting for noobs, never mind asking them to switch valves under the sink and having to turn on the tap? And after a few beers if they forget to switch it back, won't the head fill up with water when they (supposedly) wash up afterwards?

As for contamination, if bacteria could swim backwards up the bowl, against the current, out through the pump, along several metres of water line, wouldn't that be happening on our land-based homes? I'm not saying you're wrong, just that I don't know what I'm missing.
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Old 05-13-2007
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"if bacteria could swim backwards up the bowl, "
There's no "if", they can and do. In home toilets with a flushometer, there's a considerable pressure head in the system and an air bleed in the valve, so the standpipe isn't full of water after the flush--it's an air gap.
WIth toilet tanks, there's also an air gap, the fill pipe is set a good inch above the water level in the tank, among other precautions, so the incoming water is again isolated from the bowl. Even in sinks and tubs, there are requirements that the sink spiget is located above the level of the overflow drain, so there's always a break between the water source and the basin.

Considering that a marine head isn't set up with the necessary siphon design to "flush" by incoming water and has all those valves and things to much it up...Your simplest solution is probably to set up a day tank in the head with fresh water. Fill it from an overhead spigot (so there's that break again) and plumb that to flush the head with fresh water. Resist the temptation to use pressure water for the head--because that would also fill your holding tank in record time.

When shore water is hooked up to a boat for any reason you also have a risk of flooding and sinking, because that shore pressure is too high and if you put a regulator in line, sometimes they still fail. Better to run the shore supply to a tank, and use a ball valve in the tank inlet to securely close off the shore supply except when filling the tank--unless your whole system was designed for high pressure water.

In NYC last week, there was a water scare over perchlorethane (dry cleaning fluid) found in the domestic city water supply. It took a week but they traced it to a car wash with an illegal connection to the highly pressurized city water supply--which somehow "sucked in" the tainted water being dumped at the car wash, even against the high pressure supply.

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It doesn't really confuse them, since I don't let them futz with the valves. I just tell them to flush the head after washing their hands... and that flushing the head will drain the sink.

The head won't fill up since, the head pump won't let water just flow through it. However, if they are extremely stupid... they can get the head sink to overflow.

As for contamination... one thing you seem to forget is that most marine heads use a dual-action piston type pump. The exception for this is the Lavac head, which uses a diagphragm bilge type pump. The bacteria may cross from the flush side to the intake side of the pump past the inner pump seal. No swimming needed. Then the bacterium can get into the water tank via the water in the intake line, if it is plumbed into the fresh water system directly.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
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Last edited by sailingdog; 05-13-2007 at 07:52 PM.
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Now I know. Thanks for the explanations. I'm new to these systems and learning as I go along. In fact I just realised I made a mistake in putting an anti-siphon arch in the seawater intake of the head. With that one way air valve in there it's gonna take a lot of pumping to bring the water in.
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