Mailport: February 2013
Your January 2013 PS Advisor article on holding-tank vent fittings mentions check valves in the caption for the illustration. What check valves do you recommend? I am interested in using them rather than anti-siphon loops.
The illustration we published with that article is based on a schematic produced by Sealand/Dometic (www.dometic.com). In that system, the check valves are internal components in the discharge pump; there is one on either side of the pump inlet and outlet.
These bicuspid “joker” valves are not infallible. They can get stiff and stick in an open position, and they need to be replaced at regular intervals. Some cleaning chemicals can also damage the valves—we’re currently testing this.
The concern, of course, with any overboard discharge pump is seawater back-siphoning into the boat. But with this system, there are two inline valves plus the pump that the water would have to bypass, so it offers a good measure of security; however, we still recommend taking the usual precautions like closing the below-the-waterline through-hull when leaving the boat.
We generally prefer vented loops, because they are cheaper, require no power, and are easier to maintain and visually inspect—although incorporating one usually extends the hose run.
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