New condensing combi boiler has an integrated expansion vessel of 10 litres..... so whats the maximum system volume before needing to add an additional pressure vessel?

[Stephen H]

As per header...............

The new combi condensing boiler has a 10 litre pressure vessel but does not say what the maximum system volume is before you need to add an additional expansion vessel.....

Also although I know the number of metres of 22mm and 15mm pipe I have used (and hence can calculate the volume), how can I do this for the 14 Rads when the brochure does not state their internal liquid volume?

My pipe volume is 50% higher as I have two heating zones, upstairs and downstairs so am using more 22mm tube than usual.

I have a tactic for measuring the system volume when I finally fill the system... read the water meter before and after, and subtract... :-)

Regards,

Stephen.

[Doctor Drivel]

The header is too long - a silly thing to do. Try again.

[John Rumm]

There is a formula you can use...

If V is the expansion volume in litres, p1 is the absolute starting pressure, p2 the maximum pressure, and e is an expansion factor based on maximum system temperature, you get:

Total system volume = V x ( 1-(p1/p2) ) / e

For a system max temperature of 85 deg C, e is 0.0324

If you fill your system to 1 bar that gives you an absolute pressure of

2 bar (i.e. add 1 bar atmospheric to it)

If you decide the maximum pressure you want to run at is 2.75 bar, then that's an absolute pressure of 3.75.

So your 10 litre expansion vessel allows:

10 x ( 1-(2/3.75) ) / 0.0324 = 144 litres

[PeterC]

The PDFs on

the volumes for Stelrad radiators. Assuming that yours are similar, the figures should be near enough.

[thirty-six]

IIRC it is usually somewhere between 2% and 5% (but I honestly have only little idea on this and would recommend you find an official source). It really depends on what is the maximum circulation temperature your system has been designed for. Although it might be a reasonable idea to fit a bigger expansion vessel to cope with a one- off burn-up of 90 deg C, in practical terms, you should not ever require an average system temp over 70degC and it would be fair to assume that this, 90degC, would be a rare requirement, the current expansion vessel will either cope with it or you will lose some heating water through the pressure relief valve. If you ever require these high temperatures and you suspect there has been a water loss (due to a high pressure indication), then it would be a good idea to leave the heating on constant at a reduced burner setting to reduce the contraction of the water, during the exceptional cold spell. This would mean if a top-up is required because a top radiator did not heat, it would only be the once when the weather was warmer.