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Thise Grundfos pumps...poor head stats - Page 2

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Posted by daestrom on July 12, 2006, 10:09 pm

Agreed. When operating with no flow, the size/shape of the piping is not
material to how much head it should develop.

I removed all outlet piping and

Again, that 3 ft is *not* supposed to be a part of the 7.5 ft head.  The
'head' of a pump is the *difference* from suction to discharge.  So a 7.5 ft
head pump with a 3 ft minimum suction requirement should be able to pump the
water 7.5 ft *above* the suction head.  So, what it should do is have 3 ft
of water in the supply tank above the pump, and it should be able to pump to
7.5 ft above that tank (for a total of 10.5 ft from the suction line).

Many pumps have a 'minimum suction head' requirement to prevent cavitation
(which is bad for the pump and prevents it from developing the head that
it's supposed to).  If you tried it with very little suction head, it may
have been cavitating.  That would prevent it from developing the correct
head.  Try your test rig once with a 3 ft suction head and see how much head
you can get on the discharge.

Like I said, it should be 7.5 ft more than the suction, but it sounds like
your pump isn't going to do that.


Posted by daestrom on July 12, 2006, 10:12 pm

Oh, and did you make sure to prime it?  If there is any air in the pump
casing or suction line, that can cause a nearly complete loss of head.  Some
casings are easy to 'vent' just by tipping them up/over/around to get the
air out.  Others can be harder to get the air out.

Especially pumps that require a positive suction head (like your data sheet
requires +3 ft of suction head).  They are often *not* the 'self-priming'
type and if not carefully vented and primed, will just spin developing


Posted by Solar Flare on July 12, 2006, 11:25 pm
 I did try to hold the pump vertically and it made no difference. I
thought about the cavitation also. I will try putting the supply tank
on my tailgate to get more head. Just maybe what they aay has some
validity? Imagine that?...LOL

I did prime the pump by sucking water through the end of the
plumbing/copper manifold I created and gotr a lung full of solder
smoke and fumes... as soon as it happenned I realized that was a big
mistake...exhaled as hard as I could to remove as much heavy metal as
I could.

I gave up on plumbing joints with the lead-free solder and wen back to
60/40 lead solder with flux core (electronic stuff) I never could
solder with that lead free crap. One of my joints fell apart. Scares
the hell out of me everytime I see a copper fitting joint on the
plumbing in my house. If I did a joint that looked that bad I am sure
it would be spraying water out everywhere...LOL

I will have to try the larger intake head early next week as I am off
to my off-grid barn again for more construction bliss on the weekend.
I ordered another Grundfos UP26-64SU which is supposed to be good for
6.4m head or about 19 feet...nervously. I also found I can buy them
cheaper right at home than I can from eBay dealers....much
cheaper...since I am considered a plumber here too. doh!

Thanx for some wake-up calls and ideas.

Posted by News on July 15, 2006, 2:35 am

This pump I used recently adds 0.5 bar to 0.75 bar to the inlet pressure.
So, 1 bar in and 1.75 out.  They do say, hidden a bit, that it requires a 2
metre head minimum, to avoid cavitation, etc.  So, I would think what you
said was right.

Grundfos tend to have ceramic bearings which are about the best in small
pumps. Cheaper pumps have carbon fibre type of bearing which are fine but
fail quickly if dirt is in the water.  Grundfos make excellent pumps, about
the best.  Except their customer support could be better - too big for their
own boots. They are the biggest seller in the UK.


The UP15-18SU pumps is for DHW secondary circulation in a closed loop.  To
take hot water from a hot water cylinder and return it with the taps teed
off this loop to give instant water at the taps.  It is not a lifting pump.


Posted by Solar Flare on July 18, 2006, 2:01 am
 The UP15-18SU is also stated for open systems. This is the main reason
for the stainless steel.


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