I've recently had a mains pressure unvented hot water system fitted and I am
absolutely delighted with it (fantastic shower pressure without any noisy
My question relates to the pressure vessel for the central heating system.
The vessel has a pressure gauge and a water feed attached and my plumber
recommended that I have it set at 1 bar pressure. When the system is idle
the pressure is currently 1 bar and when it's in demand it's nearly 2 bar.
Is this correct or should I have it running at 1 bar ? I don't want to run
it at too high a pressure and risk the rather aged pipework springing a
Heat causes things to expand and water is no exception. The rise in
pressure is natural.
Just make sure that all the radiators are full of water and not air, but if
it is a neaw system, then this will already have been done, so leave it
alone until it doesn't do its job correctly.
In other words, If it ain't broke, then why try to fix it. :-))
That sounds correct. 1 bar when cold will ensure highest points
in the system stay pressurised as long as they aren't more than
30' above the height of the pressure guage. Pressure always goes
up when system heats up. For most systems, it mustn't get to 3
bar, or the pressure relief valve will operate (would indicate
the expansion vessel is too small, or lost it's pressure).
That sounds like it is behaving precisely as designed. When filling or
checking the system, ensure it is cold. If it rises above 2 bar, then you
may have a problem, though. This would probably be an undersized or
malfunctioning expansion vessel.
BTW, this is totally unrelated to the fact you have an unvented hot water
cylinder. It is the pressurisation of the primary circuit which requires the
pressure gauges and filling loops. You can have this system on a gravity fed
hot water cylinder too. It sounds like it was converted at the same time as
the cylinder was installed, though.
Take a look below. The rise is rather on the large size and if there
were a lot of rads and/or the old fashioned 'school' type then another
expansion veseerl might be needed - Or maybe the existing unit is
faulty or wrongly set up.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
I realise that the unvented water system is not directly connected but I
just wanted to show off ;-)
I had the system installed a few months ago and although the plumber fired
up the heating to test it it wasn't run for any length of time and hasn't
been used in earnest until now. After the first few days i had to bleed a
lot of air out of the system. When I bled the air out I had to let some
water into the pressure vessel to compensate. Now it seems to have settled
down nicely and I am glad that it is running at the correct pressure (1 bar
when cold and 1.5 (ish) when up and running)
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