Posted by harry on July 13, 2006, 7:06 pm
Cast iron pumps. There is bound to be some corrosion. If the pump is
not run for some time it could sieze up (how long depends on your
water) If it was run regularly it wouldn't sieze but corrosion would
gradually cause the pump to lose pressure/head.
Have you thought about an all plastic pump? These are cheaper than
cast iron and no corrosion . You find them commonly in garden centres
for waterfalls/fountains/fishponds etc. Or a cheap submersible pump in
Demineralised water will increase corrosion as it will leach out
whatever is available in your system. Ideally you want a ph of around
7.5 or 8, ie slightly alkaline, for your water. Add a little washing
soda or baking powder.
Watch out for dissimilar metals which could cause a electrolytic
I wouldn't worry about scaling, as you are recirculating the same
water. You could use rainwater as someone says, but it will be acid, be
sure to adjust the ph as mentioned above.
Posted by Jeff on July 17, 2006, 6:36 am
Can you elaborate a bit on how higher ph's effect corrosion/rust? Or is
that for scaling?
Posted by nicksanspam on July 17, 2006, 8:51 am
Gary's shed system won't expose the water to air in the collectors, except
during draindown, so a thin layer of oil on the water should help reduce
the dissolved oxygen. A 2x4 or a small nylon bag full of sawdust or something
in the water might also help by creating a "biological oxygen demand" as
it slowly decomposes.
Posted by Gary on July 17, 2006, 2:32 pm
Thanks for all the good info.
I found a "free" cast iron Grundfos that has about the right head and flow rate,
so I think I will give that a try on the first go. When I actually have to buy
a pump (which probably won't be long), I guess I'll look for a bronze case one.
I like the idea of a plastic housing pump, but was not able to find one with
sufficient head or flow rate. I need a starting head of about 12 ft, and a
minimum flow rate around 12 gpm, but with a fairly small head loss.
Like the oil and sawdust -- sounds like a naturalist approach :)
What I had in mind about when I mentioned some way to get rid of the oxygen in
the water before installing the pump was something like putting a lump of some
type of sacrifical metal (Iron? Mag?) in the tank that would rust and use up the
oxygen? Soemthing that would oxidize faster than the iron pump an protect it?
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Posted by Harry on July 18, 2006, 8:24 pm
Scaling is caused by dissolved solids precipitating out of the water
(at the heat source) when it's heated. Typically calcium or magnesium
BICARBONATE (Soluble) which when heated turns into calcium or magnesium
CARBONATE (not soluble) and carbon dioxide.
This is "scale". which can be white or sometimes not if there are
additional elements present (eg manganese). This can "insulate" your
heat exchanger making it less effective. The above is sometimes called
"temporary hardness". Dissolved sulphates are called "permanent
hardness" as they are not affected by heating.
Corrosion (rust) is caused by oxygen combining with the metal parts of
your system. This happens a lot more easily when the water is acid
(usually due to the presence of dissolved carbon dioxide, see above.)
This is a simplification as to what goes on.
This is best got round by using all plastic parts but failing that,
ensuring the water is slightly alkaline. You can get inhibitors
intended for central heating systems that prevent corrosion but they
need to be renewed from time to time and are quite expensive. Your
local plumbing centre or builders merchant will have them.
Also think about freezing.
BTW re what I said about pumps, check that they are rated for hot
water, pond pumps may not be.