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Solar Pool Heater - Flow Rate

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Posted by Jamie on May 24, 2004, 7:45 pm
I appreciate any help I can get.
I have a 1 HP pump with 2 inch PVC SCH 40 plumbing.
The pool guy estimates the pump is probably pushing 90 gallons per
I am going to install a solar pool heater on my roof of my 2 story
house (23 feet). The solar pool heater has 1 1/2 inch fittings.
Should I use 2 inch plumbing to the roof and then reduce it there or
just reduce to 1 1/2 inch before going up? What would be the
difference in flow?
How much will the flow rate be reduced by reducing the pipe size?
How much will the flow rate be reduced by pumping 23 feet in the air?
I want to have very little pressure (5 psi or less on the pool
heater). How much pressure would be created by the 2 inch to 1 1/2
inch reduction?
Thanks allot for your help.

Posted by Ecnerwal on May 25, 2004, 1:11 am
 jamieledwell@yahoo.com (Jamie) wrote:

None of this can really be answered without reference to a pump curve
for your pump. Look up "total dynamic head" on the web for several
helpful web pages that will help you answer the questions yourself when
you get that information.

Cats, Coffee, Chocolate...vices to live by

Posted by John Stiekema on May 27, 2004, 7:42 am
 Ecnerwal wrote:

OK some quick answers:

1) Find your pump max flowrate Qmax (no inlet and outlet restriction or
substantial head difference flowrate)
2) Find pump max differential pressure (run against a blocked valve,
briefly), and convert that to a head of water Hpumpmax
3) Draw yourself a pump curve: Hpump(Q) = Hpumpmax - (Hmax/Qmax^2)*Q^2
4) Find your system characteristic curve which is of the form
Hsystem(Q) = Hsysmax + K*Q^2   (in your case, Hsystem is the 23ft and
any other fixed height diffs in the system that the pump successfully
5) The intersection of the sysem and the pump characteristic will give
you the flowrate.

The trick of course, is to find the system characteristic K. You could
do this by removing all of height effects of the system (drop your pipe
you want to lift 23ft in the air down to pump level, or drop your pump
curve down by the amount of head you can't remove). Find the flowrate,
make an "X" on your pump curve at that flowrate, and read off the pump
head at "X", then your characteristic K = Hx/Qx^2

In terms of pipe size, for K is reduced (roughly) proportionally with
reduction in diameter, as is it increased proportionally by increase in
length of pipe. Note that the flowrate doesn't have a proportional
relationship, because of the Q^2 relationship to K.

Hope this helps? Since this is something I have wanted to do myself, if
you do the Hmax, Qmax test on your pump and do the "no system head" test
with pump connected to your system (or say how much head remains in
system, from suction to exit) then I'd be prepared to prepare a
spreadsheet for you.


Posted by daestrom on May 28, 2004, 2:15 pm

If you really want to get into the engineering of piping systems, I highly
recommend a rather cheap booklet called 'Crane Technical Paper # 410'
(either the English or metric version).


Lots of formulae, theory, nomographs and 'real-world' examples.  I use it at
work quite a bit.

Nobody has mentioned it so far, but another item to consider are fittings.
A typical elbow can be the equivalent of ~30 pipe-diameters of additional
length.  This and other data is in the book.

Another point about the OP's setup.  It may be 23 feet up to the roof, but
what he does coming back down can be very important.  That 23 feet drop can
be used to siphon the flow and effectively cancel out the 23 foot head once
the plumbing is flooded.  Only need a high shutoff head to pump the water up
for the first flood-up.  Once the return line is flooded as well, the two
vertical runs (up and back down) cancel out.


Posted by LarenCorie on May 25, 2004, 8:12 pm

Hello Jamie;

A 2" pipe will handle roughly 75% more flow than 1-1/2" pipe.
The suggested flow rate for most plumbing (indoor) is 36GPM
for 2", and 20GPM for 1-1/2".  That will give very low back
pressure.  If you push the 2" to 80GPM you will be creating
9.70' of head*.   You only need to push the 1-1/2" to 45GPM
to produce 11.68' of head*. Taking that on up to 90GPM will
probably increase the head exponentially to 47,' which is
totally too much.  On top of the 9.70' for the two inch pipe,
we have to add the actual height, which you say is 23' (to
the top of the panels, I hope) that gives us 31' of head, and
maybe a little more for other things inline, like filters, etc.

* This is for aprox 100' of pipe.

You will have to check the specs for your particular pump.

However, these formulae may help:

GPM x Total Head (Ft.)/(3960 x Pump Efficiency) = Horse Power

Horse Power x 3960 x Pump Efficiency / Total Head = GPM

If the pump is 80% efficient (just a wild guess), then that will
give you 102GPM,  75% 96GPM, 70% 89GPM,  65% 83GPM
With a 1 HP pump and 2" lines, you should be fine.

  Your heater will be on the return side of the collectors and
tall climb to the roof, so it won't be getting the high pressure.
Since the flow rate will, most likely, be somewhat reduced, the
pressure through the heater will actually be less, than it is now.

31' of head is equal to 13.4 PSI, at the pump.

Here is how to plumb it.  Run the 2" up to the roof, at the center
of your collector array.  'T' the 2" at that point, off into the two
1-1/2" panel headers, one to the left, and one to the right.  From
the hot headers, run 1-1/2" pipe back along the top of the panels
to rejoin at another 'T' at the 2" down (return) pipe.  That way
your 1-1/2" panel headers will never restrict the flow to less
than 2".  You could also feed into the panels at the extremities,
and out at the center, if you prefer.

Talk to the company that sells you the panels.
They should design the system for you.
They know their product.......or at least should ;O)

Best Wishes
-Laren Corie-
    "Generate electricity from wood"

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