It occurred to me to wonder to what extent the recommended cold charge
pressure for a sealed system depends on the position of the pressure vessel
I have seen lots of suggestions saying that the easiest way to convert an
existing vented system is to replace the F&E tank in the attic with a
pressure vessel in the same position.
However, many systems (particularly if using a system boiler) with have the
pressure vessel and gauge quite close to floor level on the ground floor. In
an average house, this represents a difference of about half a bar of static
pressure between these two positions. If the gauge is in the attic, do you
pressurise to half a bar less than if it were at ground level?
The pressure would be the same throughout the system if the whole thing is
sealed. The pressure vessel is not actually replenishing or supplying the
system, as would happen with a normal header tank, but only compensates and
allows the system to expand and contract as needs be. The cold start
setting is only set to allow the pump to run without struggling against the
static weight of water. It helps to give it a gentle push if you like.
No, the pressure depends on the height at the point you measure it. The
static pressure will be higher at the ground, compared to the loft, quite
Basically, yes. There are some parameters about the system that are
important to design in. This will help you determine vessel sizing and
1. The boiler probably wants 1-1.5 bar cold pressure. (Check the manual!)
2. The boiler probably wants you to stay below 2 bar when hot. (Check the
3. The amount the pressure goes up depends on the pressure vessel. You need
a bigger one if running at 1.5 bar, to reduce the pressure rise to contain
it within 2 bar.
4. The hot pressure must be below 3 bar at all points in the system.
5. The hot pressure must be below 2 bar at the position on the pressure
6. The cold pressure should be above around 0.5 bar at all points in the
7. 5m height equates to around 0.5 bar.
These requirements are very easy to achieve in a 2 storey house. If the
boiler is wall mounted on the ground floor and the pressure gauge in the
loft, you'll probably need to fill to around 0.6 bar (but no lower than 0.5
bar). However, there is nothing stopping you have the filling loop next to
the boiler, where it is more convenient and filling to 1 bar.
Thanks for that. It all makes perfect sense - and is more or less what I
expected to be the case - but I have never previously seen it spelled out.
Incidentally, my current boiler is suitable for both vented and sealed
systems, and requires a static head in the range 1 - 30 metres - so there
seems to be plenty of scope.
On Mon, 03 Nov 2003 02:45:09 +0000, BigWallop wrote:
Please see the FAQ. The pressure is not the same throughout the system and
sometimes and allowance for this must be made.
There is no need to add the filling point in the loft space or except in
excpetional circumstances an extra pressure vessel.
Typically it can be added near the boiler or near the airing cupboard.
And maybe 0.25 bar is the difference between the boiler and the filling
Ed Sirett - Property maintainer and registered gas fitter.
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