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Thise Grundfos pumps...poor head stats

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Posted by Solar Flare on July 11, 2006, 11:41 pm
Well, after hooking up my UP15-18SU puymp for the test trial of how
far I could push the head spec beyond the stated limit (7.5 feet), I
have discovered their specs are quite a scam.

After some very diaspointing results with head capabilties less than
two feet in height I get out the tiny pamphlet that comes with the
pump and read very closely again.

Seems written into their spec they have a "minimum inlet pressure"
specification that must be matched based on various temperatures.
Below 140 F it seems you need about three feet of head pressure on the
inlet to the pump. What they conveniently forget to tell you is that
this really makes ther pump spec only about 7.5 - 3 = 4.5 feet of
head. Hardly enough to drive a closed loops circulation system and
probably making the pumps a complete waste of money and time. I
purchased two of them based on their long term reputation for

I had this pump connected via 3/4 inch copper each side to reducers.
PEX-AL-PEX tubing into a garbage can full of water on the inlet, pump
laying on the ground with about 26 inches of head.  1/2 inch copper
pipe as a home-built long manifold with 3/8 tubing faucet valves every
24 inches that I can open to check for flow. This manifold was laying
on an angle against the back of my truck so that it's peak outlet was
about six feet in the air.

I expected if I waited long enough the water would flow from the top
of the 6 foot level...nothing. One by one I opened the valves all the
way down until I found about a 2 foot above the supply head capabilty.

I substituted my other pump (same model) and found the same results.

If this is the same for all Grundfos circulation pumps this means they
have no models capable of a draindown solar system as none of them can
pump water to your roof reliably.

Anybody else have these problems? What pump do I buy next to do this?
Taco 0009 specs Ok for this

Posted by Gary on July 12, 2006, 12:21 am
You might send an email to Taco support -- in my experience they provide good
engineering answers.
I'd layout exactly what you want to do in the email, and see what the recommend.
Please let us know what you find out.


Solar Flare wrote:

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Posted by Solar Flare on July 12, 2006, 2:30 am
 It is a Grundfos. I may have to attempt that if I can actually find an
email for them and not just dealers.

Posted by daestrom on July 12, 2006, 12:52 am

Can't say about your tiny little pump there, but when reading the specs of
an industrial pump, the 'head' is the difference between suction and
discharge period.  The minimum head required on the suction is a different
matter all together.

So if the minimum suction head required for cool water is 3 ft, you should
be able to get 10.5 ft head at the discharge (3 ft suction + 7.5 ft pump

Your experiments below show this pump does *not* have 7.5 ft of discharge
head.  More like only 2 ft.  Call 'em up and complain.

That's certainly true.  If it can only develop about 2 ft of head, you
certainly can't refill a drain-down system.


P.S. One last check, were you running it on the proper voltage?  Developed
head varies with the square of the pump speed.  Run it at half speed, you
get 1/4 the head.

Posted by Solar Flare on July 12, 2006, 2:29 am
 LOL. Well I did panic and reread the nameplate at 115v. I did think of
that one. I am also glad I had a second one to prove one of the clues.
I had a 100 foot extension on it but that little pump, 1/25 HP, should
drop much voltage.

I went through some thought on the size of the outlet pipe etc. and
the density of the liquid not being three feet under vs pumping
efficiency etc. and came to the conclusion it was nonsense and the
pump just couldn't do it. If a tight restriction impeded flow it would
just take longer to get the same head. I removed all outlet piping and
the water seemed to thrash and have a lot of turbulence coming out.
Not what I expected at all.

As far as I can make out the pump rating is a lie. They claim 7.5 feet
but require 3 feet of input head. This wasn't disclosed anywhere on
their site or info that I could find. This is disclosed in small print
only on the included flyer in small print. This is generic flyer for
the whole family of pumps too.

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