On Tue, 13 Jun 2006 11:21:17 -0300, Derek Broughton
wrote:

PlainBill wrote:

Wow!! A lot of misinformation here!!!

A given pump will be just as efficient if it is pulling water up from
a pond or pushing the water.

No, I'm just not buying that. Look at the edge case - if I have a
sufficiently powerful pump in the pond, I can push water up to any
conceivable height. If I have a pump outside the pond, at the top of a
waterfall pulling the water, it is impossible to pump more than
approximately 32 feet - with _any_ pump. So it seems pretty evident that
any pump placed _above_ the water surface is going to be less efficient
than one below the water level (though not necessarily _in_ the pond).

You're confusing a law of physics with efficiency. A suction pump
cannot lift water more than 32 feet because atmospheric pressure is
14.7 psi. Even that is not strictly true - the motor and impeller of
a jet pump can be more than 32 feet above the surface of the water,
and will work fine provided the jet itself is less that 32 feet above
the water. But that is a special case, and it will show very poor
efficiency compared to a pump close to the surface of the water.

Use this analogy: You have a heavy object, suspended by a rope. The
rope is pulled by a motor. Which is more efficient - mounting the
motor above the load, or mounting the motor on the floor and running
the rope over a large pulley? In the first case, the motor is pulling
up, in the second case, it is pulling down. Neglecting the friction
of the pulley (which will be trivial if ball berrings are used), the
force required is the same.

As I said before, for a given pump, the efficiency is the same pushing
or pulling. One complication is if the pump is more than roughly 5
feet above the surface of the water it may not be self priming, and a
check valve would be required - which impeds the flow of water.

The BIG variable is the pump itself. Then laying out the plumbing
with as few turns as possible.

PlainBill

A rule of thumb is the GPH rating of the pump should be twice the
capacity of the pond.

Odd rule of thumb. Another rule of thumb is 4 changeovers per hour, but it
all depends on the size of the pond. I'd want at least that rate for
anything under 500 gallons. I'd drop it to as little as half a turnover
for something in the 5000 gallon range.