Slowing down return

[Mr.Limpet]

I have an eheim 1260 return pump. I need to decrease the gph from my return. I was planning to just close down the gate valve after the pump. My LFS told me that would burn out the pump. Is this true? He told me to just tee off the return line and return some flow to my refugium. Turning down the valve would be easier. Opinions?

[Toddrtrex]

I can't speak for the Eheim, but I have had two Velocity T2s running for 3 and 5 years with a ball valve on them, and they are still running great.

[gabe3d]

I think that is a common misconception people have about pumps. Restricting it doesn't damage the motor, in fact it will probably extend the life of the pump. When you run it restricted the pump will spin slower, use less energy, and generate less heat.

[Icefire]

It's restricting the input that is bad

[chImerAnz]

I think that is a common misconception people have about pumps. Restricting it doesn't damage the motor, in fact it will probably extend the life of the pump. When you run it restricted the pump will spin slower, use less energy, and generate less heat.
I cannot see how this can be true. Restricting the output too much will cause more back pressure on the impellor and place more stress on the motor. Peristaltic pumps work well against pressure, purely because of there design. Your average return pump? Not so sure. Have you ever heard an Iwaki motor when reducing the pipe size on the outlet from 1" to 1/2"??? The sound alone is pretty much a giveaway.

Sell the Eheim and replace it with a smaller one.

[JOSEPHLB]

<a href=showthread.php?s=&postid=9680512#post9680512 target=_blank>Originally posted</a> by chImerAnz
I cannot see how this can be true. Restricting the output too much will cause more back pressure on the impellor and place more stress on the motor. Peristaltic pumps work well against pressure, purely because of there design. Your average return pump? Not so sure. Have you ever heard an Iwaki motor when reducing the pipe size on the outlet from 1" to 1/2"??? The sound alone is pretty much a giveaway.

Sell the Eheim and replace it with a smaller one.

Restricting the output with a ball valve is no different than running that same pump with alot of head.

The pump doesn't see the difference. To the pump, its only resistance; whether you use a ball valve or if you have it plumbed with 6, 7 , 8, 9, 10 ft, etc.. of head. It does not matter.

[gabe3d]

All centrifugal pumps will have the behavior i've mentioned, it's just the way they work. However positive displacement pumps (not an Eheim) works completely differently and have a different set of behaviors.

What you are talking about is non-inertial cavitation, a problem that can come with centrifugal pumps, which will cause the pump to run loud, lose effeciency and possibly wear faster. Generally speaking, installing a ball valve at the outlet will not cause cavitation.

[chImerAnz]

What you are talking about is non-inertial cavitation, a problem that can come with centrifugal pumps, which will cause the pump to run loud, lose effeciency and possibly wear faster.
I thought cavitation was caused by restricting the inlet to the pump?

Generally speaking, installing a ball valve at the outlet will not cause cavitation
When you say "generally speaking", Im assuming you're meaning "usually it won't be a problem, but not under all circumstances"

I am talking reducing the outlet pipe size, not adding a ball valve if that makes a difference? I can see that the velocity of the flow through the outlet (with say 1/2" return pipe on a 1" pump outlet) will simply increase, but there will still be increased back pressure

All centrifugal pumps will have the behavior i've mentioned, it's just the way they work. However positive displacement pumps (not an Eheim) works completely differently and have a different set of behaviors.
Iwaki are centrifugal pumps. BTW, Im not arguing I'm trying to understand the theory behind it!