On Sat, 16 Dec 2017 20:23:34 +0000, Roger Mills wrote:

On 16/12/2017 19:55, John Rumm wrote:
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.

The Stelrad Group installation and maintenance guide the CH installers
left with me certainly shows the use of the extra ports to create a
gravity HW and a pumped CH system. It doesn't offer much detail about
what happens to the extra ports when plumbed as a fully pumped CH & HW
system though. I guess the guide is aimed at any visiting engineers who
may be tasked with repair or maintenance work rather than the DIY minded
householder.


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.

As a matter of fact, that was the only component *not* supplied by the CH
installers since it had already been installed by the previous owners,
complete with the H/E coil already fitted but unused. The engineer
checked it over and was quite happy to plumb it into the system.

Judging by the fact that it's still going strong some 35 years on, I'm
guessing it must have been installed not long before the previous owners
put the house on the market. The connecting pipework is in 22mm, possibly
matching the heating coil diameter (just a guess on my part).



Fair comment, if it's the original cylinder. I don't think the OP said
he's ever replaced it, but he may have forgotten. If it *is* original,
it's lasted pretty well. [I'm on my third cylinder in 40 years]. Not
only will the coil surface area be a lot lower than more modern
cylinders, but the heat transfer capacity will likely have been reduced
further by scale build-up - unless it's in a soft water area.

Not a soft water area but Istr seeing a report on the quality of our
water supply. I can't recall the details other than it wasn't
particularly poor in regard of water hardness which seems to be borne out
by the slow rate at which the 3KW electric jug kettle has only partially
furred up its H/E base plate during the past year or so of hard use.


If all that is the case, maybe there is some point in heating the HW and
CH concurrently, with the HW side throttled back a bit - but I still
don't see any need for the by-pass.

TBH, it's the bypass between the pump outlet pipe (*before* it hits the
mid position valve) and the DHW H/E coil return that's the most puzzling
arrangement in view of the fact there has always been one radiator, now a
towel rail, without a TRV to act as a bypass in the event that all the
other 12 rads are shut down by their TRVs.

I've just taken another quick look at the airing cupboard plumbing (I
needed to take a **** anyway) and it occurs to me that it just might
possibly be installed for the same reason that the motorised valve has a
manual lever to lock it in the mid position when powered off to
facilitate drain down / refill maintenance procedures.

Could this be a more likely possibility? I've just taken a look at the
valve settings and I appear to have closed off the 'pump bypass shunt'
and left the DHW H/E feed backed off a quarter of a turn from fully open
some time ago, probably when I was sorting out the defective mid position
valve last year. It looks like I may have already reached this
conclusion, rightly or wrongly a year ago, and simply forgotten all about
it.

--
Johnny B Good