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Need a good pump

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Posted by john on September 7, 2009, 2:25 pm
 
Started on this project a while back and had to table it for a while,
now jumping back into it.  Summer is over and should have a little more
free time.

I'm building a drain back system and need a good 5 to 6 gpm pump capable
of about 20 feet of head on start up.  (Size may be over kill, the
system only requires 2 gpm through panels, but with 100' of pipe and 20'
of head I thought it should be oversized a bit; the plan is to put flow
restrictors inline.)  Once the system is primed, if I build it
correctly, it should almost gravity flow back; at least that is my
thought.

Any suggestions?

Thanks
John

Posted by Gary on September 11, 2009, 2:44 am
 
wrote:

Hi John,
The usual way to size a pump for a drainback is:

- Pick a pump with a static (maximum) head equal to or greater than
the vertical distance from he surface level of the water in your tank
to the top of the collector.  This is the start up condition -- the
pump has to be able to get the water from the tank up to the top of
the collector to start the flow -- flow rate is very low -- pressure
drop is almost zero.


- For the flow rate you need to supply about 0.04 gpm per sqft of
panel area, add up the pressure drop for all your plumbing plus a bit
more for the collector, and pick a pump that can supply that flow rate
at that pressure drop -- you can get this from the pump curve.
This is the steady running condition -- you just need enough head to
overcome plumbing losses for the flow rate you need.

I don't see any advantage in going to a bigger pump than you really
need -- it will just use more power.

The Grundfos and Taco HVAC circulator pumps are commonly used for
systems that require as much starting head as you do -- pump curves
for these pumps:
http://www.builditsolar.com/References/Pumps/PumpCurves.htm

Gary


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