> My boat then has a separate discharge pump to allow emptying the > holding tank when off-shore. The pickup tube comes out of the top of the tank and the hose > runs down to the pump which is more or less level with the bottom of > the tank. Then the pump outlet runs up hill a bit to the thru hull. > The result is a fluid collection point at the pump and connecting > hoses. Because the pickup tube comes out of the top of the tank and > the pump is level with the bottom of the tank, the antifreeze in the > tank can't drain into the pump... Well, no...but that's why you run the pump after you've put antifreeze into the tank: to pull it through the pump and out the thru-hull to replace any water in that plumbing. > ...and the pump contents can't drain > into the tank. No...but that's not a problem because all that matters is that all the water in the system be removed, and replaced by antifreeze (or nothing) anywhere that water could pool and freeze. It's not necessary for the antifreeze to remain in the plumbing, but it won't hurt anything if it does. You're over-thinking it. Just follow the instructions for making sure antifreeze makes it through ALL the plumbing, valves and pumps and be done with it. However, fwiw, the pump in your system should be above the tank, which would make it a much shorter route from the discharge at the top of the tank TO the pump and also make it a downhill run from the pump to the thru-hull. Pump doesn't have to work as hard--and if an impeller macerator, the shorter time running dry would greatly extend impeller life...anything in the line between the tank and the pump would be able to run back into the tank and everything in the pump would drain out the thru-hull. 'Twould also make winterizing and tank maintenance much easier. An easy mod to do on most boats. -- Peggie ---------- Peggie Hall Specializing in marine sanitation since 1987 Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and Other Sources of Aggravation and Odor" http://shop.sailboatowners.com/prod.php?51996

Peggie wrote: However, fwiw, the pump in your system should be above the tank, which would make it a much shorter route from the discharge at the top of the tank TO the pump and also make it a downhill run from the pump to the thru-hull. Pump doesn't have to work as hard--and if an impeller macerator, the shorter time running dry would greatly extend impeller life...anything in the line between the tank and the pump would be able to run back into the tank and everything in the pump would drain out the thru-hull. 'Twould also make winterizing and tank maintenance much easier. An easy mod to do on most boats. Reply: I have a different view of where to place the pump relative to the tank. If the pump is above the tank you are asking it to suck the contents of the tank all of the time even at start-up. Most pumps do better at pushing than pulling. Also, you are asking for a dry start-up every time. If the pump is at the bottom of the tank it will always be pushing, but you will finish the cycle with the worst of the contents, and it will stay there when the pump is shut off. Essentially, you are filling the pump with the bottom of the tank and leaving it there. My preference is to put the pump in the middle or a little below middle of the tank. The pump will push most of the time (including at start up if the tank is full above the level of the pump) and at the end of its cycle it will drain dry (assuming the tank is empty) when you shut it off. The only winterizing of the pump is to empty the tank because the pump will be dry. Anyway that is how I see it (and it works so far). Have fun pumping... Eric Thoman Abyssinia