In message l.net,
Dave Liquorice writes
On Tue, 27 Oct 2020 21:20:59 +0000, newshound wrote:

I am considering fitting an additional expansion vessel to our
heating
system. Advice specifies boiler return pipe location. Why?

It's that bit cooler so less wear on the diaphragm.

But it is a dead leg, so not much conduction unless it is very short.

Well it's dead in that it goes nowhere but there is still the
flow/return due to the expansion of the water from cold to hot and
back. I think that expansion is about 1 l per 100 l 10 to 70 C,
guesstimate that's a metre or two of 15 mm pipe.

Umm. My boiler modulates but, due to the characteristics of underfloor
heating, is mostly returning water at reasonably high temperature. I'll
do some checks.
There is an 8L vessel in the boiler anyway.
A question for another day is how to adapt the *weather compensation
provision* to run the system at a lower temperature when there is no
call for re-heating the DHW.

The actual issue is that the diverter valve momentarily blocks the pump
flow leading to water hammer which opens the boiler relief valve thus
losing system water:-)
An alternative solution is to fit a bypass valve: to maintain flow
during the transition. Soldering access is a pig!


--
Tim Lamb