[Skulemate]

Well, I must say that I am a bit surprised by those results, but they do show almost 50% flow reduction when the large pump dies and only the weak pump remains... hardly next to no resistance. I agree that a check valve meant to prevent back-feed loops in a parallel setup will reduce performance, but they're not that bad in terms of resistance. I'll will however agree with bigben's conclusion that there's really not a whole lot of risk setting the pumps up in series. And while it may be fair to say that series is always better when dealing with mag-drive centrifugal pumps, I don't think that your statement is entirely accurate as is airspirit.