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Cast iron pumps and open systems

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Posted by Gary on July 13, 2006, 12:34 am

I am building a solar space heating system that uses 6 4X10ft collectors that
drain back into a 500 gallon tank when not collecting.  The 500 gallon tank
stores the collected heat.  The fluid is water -- no antifreeze.

The tank will be closed with a tight fitting lid, but will be vented to the
atmosphere via a small vent tube.  There will be an inch or two of air above the
water in the tank.  The water will not be changed, except to add any makeup
water from time to time to keep the tank level constant -- I would think this
makeup water addition would be not much water and not very often.

Can I use a pump with a cast iron casing?
My thought is that there would be oxygen in the water when I first fill the
tank.  This would cause some initial rust, but once this initial oxygen is
consumed, very little more would enter the system over time(?).  So, maybe an
iron pump would rust a bit at first and then stop? Or not?
Is there some way to remove the oxygen from the initial charge of water before
installing the pump?
The bronze casing pumps are about $00 more, so its not a huge issue, but I'd
just as soon not buy one if its not needed.
The pumps I am considering use non-metallic impellers, so its "only" the casing
that would rust.

Do I need to use demineralized water?
If I use ordinary well water, there would be some dissolved minerals in the
initial charge of water.  These might cause some deposits in the collectors(?).
It seems that once the initial charge of minerals deposits out to whatever
extent it wants to, there would be no further mineral deposits, since I am not
adding new water to the system.
I could also use water that has gone through the house water softener to fill
the tank.  The softener exchanges the Sodium for the Magnesium and Calcium in
the water.  The softener mechanism (I think) is that the Mg and Ca leave
deposits, but the Na stays in solution.  I wonder if this would work the same
way for water in the solar storage tank?

The ongoing discussion about pump inlet water head has got me a little nervous.
  I was planning to mount the pump such that the full casing is always below the
level of the water surface in the tank.  Based on Nick's last note from Taco,
this seems OK?   Although Daestrom's comment on keeping prime and eliminating
air in the system also has me a bit nervous.  But, there are a lot of drain back
systems out there that seem to work OK??

What other things am I missing?





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Posted by Solar Flare on July 13, 2006, 1:59 am
The way I understand it. If the water is open to the atmosphere it
absorbs oxygen and your pump will rust. If you use pex tubing it will
absorb oxygen through the tubing and your pump will rust.

The bronze or stainless steel pumps are not that much more, I have

Posted by Bob Devine on July 13, 2006, 5:30 am

I've been planning a similar arrangement of a large tank
of plain water.  Reading enough warnings of rust convinced
me to buy bronze pumps (check ebay, that are occasional
deals that appear).

If you big tank doesn't get above Pasteurization temp,
you might have to add some fungicide (bleach is cheap).

Another thing to consider is how to get rid of any
particulate matter or other stuff that settles out
of the tank.  I don't know if the water needs to be
filtered or occasionally dumped.

Note: all this is still in the planning stage for me;
I don't have any hard experience with it.

Bob Devine

Posted by nicksanspam on July 13, 2006, 12:05 pm

Maybe. I see notes like "don't use it for potable water." I suppose it
will rust a bit. With a little oil on the water, it might last 20 years.

I'd say no.

Yes. Taco specs a minimum _absolute_ inlet pressure to prevent cavitation.
This pressure increases with water temp and pump flow. Do the calc...


Posted by Ecnerwal on July 13, 2006, 1:09 pm

Sure can. Will it rust? Sure will. Will it ultimately be cheaper to
install an initially more expensive bronze or stainless pump? Yes it
will. Do What You Like.

It's a vented system, and that means access to _all_ the oxygen in the
atmosphere. Only sealed systems work this way. In a vented system, some
oxygen in the water is used up, making the water oxygen-poor, which
causes the water to pull more oxygen from the air, which causes more
rust, which makes the water oxygen-poor....you also have galvanic action
between the cast iron and any copper in the system.

Got gutters? Consider rainwater, though you may want to check and
possibly adjust the pH, as well as give it an initial kill (even if you
are not mixing storage water with potable water, best not to have any
little beasties growing in it.) You could also set up a solar still to
make distilled water.

You need to choose a pump that will work for the job. A pure circulator
is a slightly different job than a drainback pump. Check with the pump
supplier or manufacturer and get a pump that suits your application.
Specifically, a pure circulator only needs to overcome the dynamic head
of the system (friction in the pipes) as the up and down sections
balance out. A drainback pump needs to be able to pump enough static
head to fill the system, and then be able to circulate it.

Cats, coffee, chocolate...vices to live by

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