Sump pump question

[fredhargis]

I just put in a new Zoeller high capacity sump pump. It replaces a Zoeller that was about 1/2 of what the new one is. The new one has a requirement that I drill a 3/16" vent hole into the discharge pipe, even with the top of the pump, to prevent an air lock. The directions go on to say do this when using a Uni Chek valve (Zoeller's name for their own check valves). Anyway, the old pipes did not have this vent hole and everything worked fine (with a Legend brand check valve). I put the vent hole in the new pipes but can't help but wonder if there is something unique about check valves in general that make this a requirement. Anyone have a guess as to whether I really needed the vent hole?

[R Clark]

I drilled one when I replaced a Zoeller last fall.  The instructions with the new pump said to drill that hole, so I did.  I looked on line at some YouTube vids and they did same.

[Woodenfish]

The hole is to act as a drain to empty the pipe from the pump to the bottom of the check valve. This is to allow the pump easy to startup by preventing cavitation and in turn hopefully last longer. Not doing so puts a unnecessary added strain on the pump and equipment.

[fredhargis]

Thank you. (filler)

[TDKPE]

No drain hole can allow air to lock at the rotor if the rotor pumps itself dry. This used to happen all the time on a bilge pump in a sailboat I used to have. If heeling or the pump running dry (like with the manual ‘on’ override switch) allows the rotor to get air, it can have difficulty getting that air out of the scroll chamber and developing enough pressure to start the column moving with the weight of water on that check valve.

I drilled a bleed hole below the check valve, and all was right in the world. As the bilge water rose, air could escape and water would flood the chamber, allowing instant start and pumping against that fluid column.

When I posted the problem and solution on a sailing forum, I got lots of howls of protest that it’s not “the right way” and “Never use a check valve on a bilge pump.” Stuff like that. But due to the small but deep shape of the bilge and the nature of sailing, and the fact that lack of a check valve would drain back and restart the pump repeatedly, it wasn’t simple to work another solution with common float switches to keep the rotor flooded without short-cycling, but a small vent hole did the trick while keeping the hose to the thru hull filled so that water wouldn’t drain back (outlet has to be high in a sailboat, so hose is long).

Point being, if the rotor gets air into it, for any reason, the lack of a bleed hole may prevent it from getting a good prime and pushing the resulting bubble of air up through the check valve, which means it won’t pump water. Just make sure it doesn’t squirt somewhere you don’t want it squirting.

[Woodenfish]

I usually angle that drilled hole down.