In message , The Natural Philosopher
writes
On 13/03/18 10:01, 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?

I dont recall ANY expansion vessel on my UFH!

Other than what was in the boiler anyway

I think I had 600m+

I am sure a standard unit will cope - its a relatively easy thing to
retrofit more if needs be

Coefficient of expansion of water is 0.000214/deg C, so even if you max
out at 60 degrees (your house wont go below 10C internally) the worst
case expansion is 0.000214 x 50 x 85 litrs = 0.909 liters

That is something and in-boiler expansion vessel is well able to cope with.

OK

I am reassured:-)

--
Tim Lamb