Expansion Vessel Pressure?

[diydavzo]

Hi I recently had my hot water tank expansion vessel topped up by British Gas. Now the vessel is specified at 3.5 Bar and was pressurised to that. However the last guy that did it, pressurised it to 2.5 Bar so that it would
match the pressure from the cold water rising main. This rising main enters house via stopcock and has a regulator to limit the pressure to 2.5 Bar.

So what pressure should it be at 2.5 Bar or 3.5 Bar or does it not really matter?

Thanks.

 

[secret squirrel]

It does matter, have you got the users manual? that should tell you what it is,

If you have not got the manual, if you speak to the manufacturer they will tell you what it should be?

Lastly, you'll probably find out what the pressure is on the manfacturers website.

 

[Bernie2]

Hi! Squirrel!

You caught my mail just in time. Thanks! I was talking about the boiler water content and got mixed up. The enquiry was about pressure vessel charge not system pressure charge. Sorry about that I apologise you are right they should check the manual.

Getting old!

Bernie

Hi! diydavzo

Its not easy to give a straight answer as there is not much info to go on. I think Secret Squirrel has the right approach.

The expansion vessel charge pressures are reliant on knowing quite a bit about your system and working it out from there. Its easier just to look it up in the manufacturers instructions usually.

But some don't tell you, then you have to work it out.

You have got to know the volume of water in the system.

Initial pressure of system worked from its static head.

The boiler flow temperature.

and a bit more.

So as you can imagine its easier to look it up in the manufacturers manual.

 

[diydavzo]

Thanks for the replies 🙂

I looked up the vessel on website and while could not get the manual the vessel is rated as 3.5 Bar which I presume is the maximum pressure it should be charged to?

Does that mean it should be charged to 3.5 Bar or to 2.5 Bar to match rising cold main?

In general should the hot and cold pressures in a system be the same?

 

[Bernie2]

Hi! diydavzo

No its not that simple the info can be found in British Standards 7074 Part 1 1989.

But you can get simpler info from some of the boiler makers sites such as Ravenheat.

As a guess yours does seem slightly over pressured but may not be as it depends on a few things which I don't know about your system.

Good Fortune

 

[migoplumber]

i agree with bernie,
if you dont want to use the calculation and work it all out, 0.5 bar below working pressure is a good guestimate.

shaun

 

[Puddle]

The second engineer was correct the expansion vessel should be pressurised to the pressure of the water coming into the cylinder,in this instastance,2.5 bar maybe .1 bar on top,to cover any intolarance however the first engineer 90% of the time would not course any problem pressurising to 3.5 bar in practice
The reason for this is the operation of the expansion vessel,if you pressurise at water pressure coming in, you maximize potencial expansion take up of vessel
Inside expansion vessel you have a rubber diaghram one side air one side water in cylinder,if pressures the same rubber diaghram is sitting central giving maximum system pressure take up
If you pressurize air side to 1.5 bar,and water side (when cold) 2.5 bar the rubber diaghram will be aready partialy compressed before heat applied to water,therefore expansion vessel capacity has been reduced
like wise if pressurized to 3.5 bar the diaghram in the vessel will be pushed into the water side and the expansion vessel will not take up any pressure increase/expansion in the cylinder until pressure has increased to 3.5 bar,this would also cause uneven pressures in hot and cold also,
of course you open the tap and after several seconds pressure would be equalised,however on short pipe runs this could cause a problem,with hot over powering cold causing possible scolding (in therory) ect .ect

 

[jonathanb]

Once again Puddle tells em like it is - keep up the good work mate.

 

[diydavzo]

Thanks Puddle - that makes sense to me 🙂

I have a few more queries on how my CH system works and will probably start some other threads!

Thanks for all the information guys.

 

[bobpape]

The expansion should be charged to slightly above the working pressure, all it does is accomodate the amount water expands due to the heating of said water,
The expansion vessle looks after the primary water circuit and does not have any input
on the domestic hot water supply to the taps thanks to the diverter valve.