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Macerators sometimes require check valves on the inlet for shower waste. I'm thinking the low rate of flow from a shower might not be enough to trigger macerators activated by air switches, so the valve creates lets water from the shower through without letting air out, triggering the pump.

Would that be correct?

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Brand specific/product specific instructions are O/T per the faq. – The Evil Greebo Jan 7 '13 at 13:27
Edited question to eliminate any semblance of product specificity. – jontyc Jan 7 '13 at 13:45

The check-valve prevents waste from flowing backwards after the macerator pump stops.

Macerator pumps usually can't stop backflow by themselves. The pump has to let some solids through so the pump mechanism isn't water-tight. If it was, the solids would wear the those water-tight edges down quickly and the pump would no longer be water-right.

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This isn't really something we can answer specifically as it's brand specific. Contact the manufacturer or use the manual.

Why would you think we know better than the manufacturer?

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Air switches and check valves are common concepts in fluid dynamics. Manufacturer unresponsive. Last manual I had for a ceiling fan was blatantly incorrect. Many SE sites I visit have engineers on board who understand the underlying principles and how such machines work, regardless of brand. – jontyc Jan 7 '13 at 14:02

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