On 17/08/2018 08:12, Chris B wrote:
On 17/08/2018 00:29, John Rumm wrote:
On 16/08/2018 21:47, wrote:
On Thursday, 16 August 2018 20:36:57 UTC+1, Chris BÂ* wrote:
OK I thought I had got my head round unvented systems but clearly not.

and having looked at the wiki and several web pages havn't cleared
things up.

If I have an unvented system and I need to bleed a radiator the
pressure
in the system will drop as I bleed it.Â* Having completed the bleed I
now
need to top up the system using the filling loop until a set
pressure on
the gauge. My assumption here is that incoming water is compressing the
air in the expansion vessel as I top it up.Â* Having reached the
required
cold pressure I isolate the system at the filling loop.Â* Now as the
radiators heat up the water in the system expands out into the
expansion
vessel and the pressure rises to the working pressure.Â* All the water I
have been talking about so far is the heat transfer fluid (ie that with
inhibitor).

Now the real crux of the problem - I simply want to change a hot tap
washer. The tap does not have one of those little quarter turn isolator
valves like all the youtube videos seem to conveniently have.

The hot tap is pressurised.Â* Do I simply turn off the rising main at
the
cold water inlet point and let the pressure out of the hot tap?Â* If so
how to I "repressurize" the system when the job is done?

Any help appreciated.

You don't need to do anyting, the incoming mains water will
repressurise it. The 2 pressured circuits are different in nature.

+1, yup what he said - I left that off my earlier explanation.


Thanks to all for your help, just one last thing: when "repressurising"
the HW cylinder there is no gauge I need to look at? I just turn the
rising main back on and the cylinder design just takes care of the
pressure?


Yes, that will do.

You might want to download the manual for your particular DHW cylinder
and follow the procedure for recharging the inbuilt pressure vessel.
That's supposed to be done annually but I know there are systems that
have been installed for years without it ever being done - mine for
instance! You are also supposed to test the pressure relief valve, etc.

Yours, living dangerously,
GB