Sump pump check valve placement

For the past 16 yrs I've used 3 sump pumps in a hole with 10' of head & I've always put the check valve right at the pump, in the sump hole. I'm guessing that I saw that setup when I installed the first one & have just copied it since.
I'm adding another sump hole now, much closer to the cellar drain & with a water-powered backup. All the diagrams I've seen have the check valve up out of the hole & warn to drill a hole in the pipe to avoid air lock. Diagram- http://www.plumbingsupply.com/installhome.html 'Drill the air hole' http://www.zoeller.com/zcopump/TechSupport/faq.htm [and note that the Zoeller FAQ says drill it just above the high water level, but the plumbingsupply diagram shows it below water level]
Is there a reason that the check valve *shouldn't* go right next to the pump? It seems like that would eliminate air lock problems.
Have I just been lucky that I've never had a problem with mine set up that way? My sump pump gets a serious workout every spring and occasionally during the summer.
Jim
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Like you, our sump pumps don't have vent holes, and the check valve is right down on the pump. The problems we've had is when the checkvalve flapper broke off (and plugged the outlet), or, the column pump motor seized.
I suspect airlock is effectively only possible when:
1) there's water above the checkvalve holding it shut 2) the water has receded below the pump inlet, and then risen again. 3) the empty pump and tube acts like an inverted cup pushed under water, and the impeller doesn't contact any water (or, at least the impeller housing isn't mostly full), so the impeller can't get "traction" on enough water to overcome the checkvalve.
I don't think the checkvalve height makes (much) difference at all, except to make airlock _slightly_ less likely the higher the valve is.
I also suspect that whether airlock is even remotely possible depends a lot of the precise geometry of the pump inlet/impeller housing. I don't think it's even remotely possible for our current sump pumps to airlock because of the way the inlet/impeller housing is designed.
All that being said, it's best to follow the instructions. May be specific to the pumps you're using.
--
Chris Lewis, Una confibula non set est
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