two-speed with solar panels

[mas985]

Oh, I see what you mean. The "suction" from the falling water reduces the pressure at the top, but not quite enough to make it negative (that is, below atmospheric). The term "negative head" confused me. I would think of it more in terms of "head reduction".

However, I am still not sure this works. You're saying that I would use the pump at high speed to fill the pipe, then I can switch to low speed. But you also agree that even with low speed there is higher-than-atmospheric pressure in the pipe at its highest point (where the valve is located). That means that at low speed the pump still has enough power to fill the pipe, and I don't need two speeds, just the low speed.

Anyway, thanks for your help and advice. I will have to do some measurements and calculations and see if I can get by with a smaller pump.

Not quite. Static head, which is basically the rise in elevation, does not depend upon the rate of water flow (GPM). Dynamic head (pipe friction loss) does depend on the rate of water flow. A pump at high speed has a pump head curve such that it can take care of both the static and dynamic head together (i.e. high head curve) quite easily. See here:

http://www.poolplaza.com/WhisperFlo-techspecs.shtml

The pump at low speed, which is basically half speed and half flow, only has enough power to handle the lower dynamic head (notice the head curves are much lower). So at low speed, the flow rate is 1/2 of the high speed so the dynamic head is 1/4 of that at high speed. Since the pipe is primed with full speed, at half speed there is no longer any static head so it will have no problem with the head loss that remains.

Here is what would happen in a typical pool.

No Solar with full speed filter PSI = 15, head loss about 45 feet
With Solar at full speed filter PSI = 20, head loss about 52 feet
With Solar at low speed filter PSI = 5, head loss about 13 feet

To ease your mind, people with pools and two speed pumps have been operating on low speed with solar for years so no worries.


Here are a few more links you can learn about pumps and hydraulics.

http://www.powermat.com/pools/index.html
http://poolplaza.com/pool-pump-sizing-2.shtml
http://www.zoeller.com/zep/techbrief/JF1article.htm
http://www.eere.energy.gov/industry/.../pumpintro.pdf

and I have much more technical version when you are ready.