On Mon, 24 May 2004 15:16:08 GMT, "BigWallop"
wrote:


"Tony Bryer" wrote in message
...
In article ,
BigWallop wrote:
A normal central heating pump can be fitted in the outlet of the hot
water tank to increase the flow.

Surely you need a bronze pump not a normal £30 one??



To Andy and Tony.

I was only trying to help with the question the OP (Harry) posed, and the best
solution I thought of was a standard central heating pump with a flow switch
control arrangement. These standard pumps have lasted on our own system for
anything up to 10 years, so I was not under the impression that a materials
change to said pumps would create any massive increase to their performance or
longevity in these circumstances.

My thoughts are that the standard pump cost is small and the length of time it
lasts in this type of environment outweighs the costs of installing a more
expensive pump which gives such a small increase in the life span of the unit.
My research into the subject has shown me that the standard central heating pump
has a life span of up to 10 years, while the bronze units are rated as having a
life span of 12 years.

The conditions are different though. The CH pump is in iron/steel
because it can be. The water is essentially anaerobic after a few
days - i.e. dissolved air is driven out. With the addition of
inhibitor, rusting can be essentially eliminated.


To me, this small increase in the longevity does not
justify the greater increase in cost of installing this type of pump on to a
system which will not have a continual flow of aerated water, but will only
supply a small amount of water over very short periods of time.

It will always be in contact with water containing quite a lot of
dissolved air, even if the water has stood in a roof tank for a few
hours, so it will deteriorate in a much smaller time period than 10
years.

The manufacturers make pumps for DHW and potable water use in bronze,
brass or plastic (or even stainless steel) for a reason......



The standard pumps are fine as long as they are installed in a position which
keeps them internally flooded. The water flow through the pump from a storage
cylinder is not interrupted by massive increases in aerated water, so the
impellor unit on a standard pump is more than capable of withstanding these
small deviations and will work quite happily for many years in this situation.

We're not talking about the water being grossly aerated because the
pump innards are partly out of water, though. Just having it in
contact will cause rusting quite nicely.



So, in summary, although I disagree with the suggestions you've made on the type
and construction materials of the pump required in the situation that the OP
(Harry) has, I will leave it to his own research into the matter to prove
otherwise.

The just of this posting is similar to a letter I have to compose to an
architect. Does it sound OK ?


Yes it does. What do you want him to do for you though?



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