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.

It makes pump over less likely, but at the enhanced risk of drawing air
into the system on installations which don't have much head above the
point the vent tees off. (by the sounds of it, not something you need to
worry about on yours).

(a good solution in those cases is to combine the F&E and Vent into one
tee into the system, with the vent teed into the F&E pipe a few inches
up from where it leaves the main primary circuit. That pretty much
ensures the only thing that will get sucked in is water from the F&E tank)

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.

Indeed. So long as you have left enough space between the set level in
the F&E tank and its overflow.

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.

This is a problem with any vented system, since even without the effects
of the pump you will see significant flow into the F&E tank as the
system heats (probably around 4L for every 100L of primary water in the
system), and then flow out as it cools. So you are continuously cycling
the system water the tank and exposing it to more atmospheric O2.

I suspect I may be "over-thinking" this conundrum. Feel free to respond

Agreed ;-)

to this post rather than the previous one (it'll make quote trimming a
doddle :-) ).


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/