A question regarding how expansion vessels work ..

[KeithP]

Hi friends ..
Still playing nice with my overactive Ariston c0mbi ...and still trying to determine the nature of my fault if it actually exists..
During my investigations I found that even after several hours of use the expansion vessel and the pipe connecting it to the boiler remain stone cold to the touch...
Have I grasped the principle of its operation correctly ?
I assumed the vessel contains a diaphragm ...on one side pressurised and the other side a space into which heat expanded water would be pushed as the boiler heats up ...thus providing somewhere for the excess pressure within the sealed system to go..
If this is true then what does the vessel in my system remain cold ?
..and is it possible there is some sort of airlock preventing expansion ?
Could this be responsible for my boilers excessive activity (firing up every couple of minutes) ...
And finally ...if any of this is true ...
..Is it potentially dangerous ...and is there anything I can do to about it ..
Please don't larf if I have misunderstood whats going on..
rgds
Keith

 

[kevplumb]

could be a blocked pipe
is the pressure rise exessive when hot

your understanding of the principle is spot on BTW

 

[KeithP]

Thanks for the reply Kev
My system is pressurised at 1 bar when cold and rises to approx 1.5 when the ch is in use ..
is this within acceptable limits ?
rgds
Keith

 

[ChrisR]

Getting to the top end of what would be normal for say a 3 bed semi's worth of rads.
Suggest you check the precharge. Do it cold and with the pressure let off the system at a drain cock. Then open a drain tap on the boiler to let water out as you're pumping it up. Do a search on "pressure vessel"!

 

[kevplumb]

yeah thats normal

(another door slams shut)

hope you find the prob

 

[KeithP]

Thanks for the reply Chris ..
Sorry to appear thick but don't understand the term precharge ...can you perhaps explain in laymans terms ..
very much appreciated
rgds
Keith

 

[KeithP]

Thanks for that Kev ...
I can only assume then that the CH system ...pipes rads etc ...are absorbing the pressure without untilising the expansion vessel to any great degree ..
Pipework is mostly 22mm with short 15mm spurs to rads ...
Rads total 7 in all ..
thanks for letting me bend your ear m8

rgds
Keith

 

[croydoncorgi]

don't understand the term precharge

Precharge is the pressure on the 'air side' of the expansion vessel diaphragm when there is ZERO pressure on the water side.

Without enough precharge, the space available for expansion of water into the vessel is too small when the water in the system is raised to its optimum pressure.

If there's too much precharge, the water pressure will get too high as the system heats up.

As a rule of thumb, the precharge should be half the intended system pressure. (Most vessels come factory-precharged to 0.5 Bar for this reason.)

 

[KeithP]

Many thanks for the explanation croydoncorgi ...
That makes sense ...though I have no way to check the precharge pressure on the air side of the diaphragm within the vessel ..
The vessel has never been tampered with ...so unless faulty in some way .. I guess it is still set as it was at manufacture.
rgds
Keith
If nothing else .. I'm learning a lot

 

[weargas]

keith, it is perfectly normal for the piep to ypur epxansion vsesell to be clod. teh fcat taht tits not gong hihger tahn 1 bra meens its okaaay.