On 13/03/2018 14:28, Tim Lamb wrote:
In message , Roger Mills
writes
On 13/03/2018 10:23, Bob Minchin wrote:
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

Agreed. But just in case anyone interprets that to mean that a 4 litre
expansion vessel can accommodate 4 litres of expansion - it can't!
Well, not without the pressure going through the roof[1] when you
squeeze its air down to nothingness. In the case quoted, the air
volume will reduce from 4 litres to 2.5 litres - with the pressure
rising from (say) 1 bar to 1.6 bar - which is ok.

[1] And operating the pressure relief valve

Is one still allowed to run the relief valve to an outside wall?

Not only allowed, but required usually. (and positioned / angled in such
a way as there is a low rick of injuring a bystander should it let go).
So normally a pipe taken down to "low ish" on the outside of the wall,
and then turned back to point at the wall a bit (or it could discharge
into a gully)


--
Cheers,

John.

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