On 16/12/2017 17:30, Roger Mills wrote:
On 16/12/2017 01:02, Johnny B Good wrote:
On Fri, 15 Dec 2017 22:34:16 +0000, Johnny B Good wrote:

====snip====


So, my questions a

Is my current header tank feed and expansion pipe arrangement
something
to be concerned about?


Having posted the original fulsome query. It's occurred to me that the
point at which the overflow has been teed into the system is on the
suction side of the pump which should make "pump-over" impossible.

Considering that the feed pipe from the header tank is likewise
subjected to the same drop in pressure during pumping, there may be a
small but transient 'draw down' from the header tank during pump startup
with a corresponding small but transient back-flow when the pump shuts
off.

Ignoring evaporation of the header tank water for the moment, the first
draw down event will cause the ballcock to admit a little water to top it
back up, thereafter, the level can vary without further admission of top
up water. Since header tank water does evaporate, the ballcock will
eventually operate to compensate for this loss.

Although the net effect on water consumption remains unchanged, the
same
can't be said for the corrosion inhibitor which will suffer a marginally
higher rate of consumption, particularly of its anti-oxidant component as
it becomes dispersed into the header tank where it can then be consumed
by the dissolved atmospheric oxygen. The 3 or 4 metres of 15mm pipe
between the header tank and its connection into the system plumbing will,
however, act as a buffer zone with an inhibitor concentration gradient
that will reduce the diffusion rate of inhibitor into the header tank.

I suspect I may be "over-thinking" this conundrum. Feel free to respond
to this post rather than the previous one (it'll make quote trimming a
doddle :-) ).


Well yes, if it's still going strong after 30-odd years, I'm not quite
sure what you're worried about.

If it were my system I would try to contrive that, under normal
circumstances, the HW and CH are not being heated at the same time. You
then wouldn't have to worry about HW vs CH balancing and could remove
any restriction on the HW side, resulting in faster recovery. You could
achieve that by using the existing programmer to time the HW, and
inserting a programmable room stat to time the CH. [You really, really
*should* have a room stat]. Then, you could heat a tank of water each
morning before the CH comes on and, with a decently insulated tank, it
should stay hot for a long time.

Judging by the age of your boiler, and the fact that it's got two lots
of connections, it was probably designed with gravity HW and pumped CH
in mind. In that case, the boiler capacity will be such it's own stat
can stop it overheating without requiring pump over-run or a minimum
flow rate. You should certainly be able to do away with a pipe which
by-passes the HW coil.

Worth keeping in mind that a cylinder of that age may be relatively slow
recovery by today's standards. So heating just the cylinder may require
lots of cycling on the boilers stat.

--
Cheers,

John.

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