Charge pressure for sealed CH system

[Roger Mills]

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 and gauge.

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?

Roger

[BigWallop]

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.

[Christian McArdle]

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 considerably.

Basically, yes. There are some parameters about the system that are important to design in. This will help you determine vessel sizing and pressurisation.

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 manual!)
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 relief valve.
6. The cold pressure should be above around 0.5 bar at all points in the system.
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.

Christian.

[Roger Mills]

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.

Roger

[Ed Sirett]

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 point.

HTH