# Sizing expansion vessel

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
In message , Bob Minchin writes
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?
Probably not...
I run 21 rads + cylinder (so probably 100 to 150L of primary water) on the internal expansion vessel on my boiler, and with the initial pressure set at 1 to 1.3 bar, it will never rise above 2 bar.
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
I dont recall ANY expansion vessel on my UFH!
Other than what was in the boiler anyway
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.
No. Unless you start blowng the pressure relief valve.
In message , The Natural Philosopher writes
I am reassured:-)
In message , John Rumm writes
OK. That's clear enough.
In message , Roger Mills writes
Is one still allowed to run the relief valve to an outside wall?
In message , Bob Minchin writes
Thanks Bob. Coincides with other posts but I still say there is something awry with the PV sites.
Often found the wonders of undrfloor heating a bit odd. I can recall many years ago a small flat for some relatives had it and it looked to me limke the system was under a kind of mesh floor with heat sink like vaned pipes underneath it on top of some lagging. Certainly a lot more complex than has been discussed here. Brian
Yes, Apart from physical space required, get a guestimate then go for the next size up. From my own experience the estimated size of expansion vessel based on "at the time" UFH water volume was something like a litre more (or less) than the closest size so I went to the next size up. Fortunately it meant that when I re-fitted radiators in the bedrooms (not previously in the system) I could fit bigger rads to work at UFH flow temperatures and still have spare capacity. Followed by previously un-planned front extension with another 300m or so (3 zones on 4 circuits) of UFH pipework and still within capacity of expansion vessel. I still have another UFH zone to add in the bathroom so again this wasn't originally factored into the volume calcs but will hopefully be met with the existing vessel.
So bigger is better than being accurately calculated to meet "current" system requirement.
:)
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)
UFH for suspended floors is a tad more complicated that those buried in a screed.
Until the internal vessel fails. Then you will probably be glad. I had to put in an external vessel about 20 years ago when the internal one failed after five years. I've just replaced the external one. (But I am on my third boiler now).
In message , John Rumm writes
OK. I was thinking a discharge under the worktop via some tundish arrangement might not be noticed.
For the boiler primary circuit relief valve there is no actual requirement for a tundish (there is no mains water contamination risk), and you also have other indications that the system has lost water (i.e. a loss of sealed system pressure)

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