In message , Bob Minchin
writes
Tim Lamb wrote:
Confused....

Either my known to be suspect mathematics is wrong or there is something
amiss with sites recommending pressure vessel sizes.

Under my new floors I have carefully:-) laid 820m of 16mm pex-al-pex
piping.

Sadly absent from the mainly American sites offering pipe volume
calculations. My vernier gives a bore of 11.5mm and Pi x r2 x L comes to
85L. A more agile brain may kindly care to check!

Could that length of pipe bore conceivably squash into an 18 gallon
container? Probably.

The underfloor heating systems suppliers suggest a 4L vessel is adequate
for 1000m of 16mm pipe yet most sites marketing PVs are wildly in excess
of this. Surely they know that the system is run at a much lower
temperature than 85C boiler output!

Any thoughts?


Coefficient of expansion of water is 0.000214/degree c or for your 85
litres is 18ml per degree c
I'd allow for a max temperature assuming your mixing valve had failed
and a minimum of + a few degree assuming the system was filled on a
cold day. But even if you assumed 5 degrees to 85degrees you only get
1.5 litres so I'd reckon a 4l should be more than adequate unless I've
done my sums wrong.
hth
Bob

Well the boiler has an internal expansion vessel of 8L I'm uncertain if
that is total volume (air plus water) or working range.

The flow and return piping about 14L

Heat exchange coil in the thermal store 5.5L

Extraneous pipework a few more say 3.0L

Do I actually need a separate vessel?

--
Tim Lamb