On 16/08/2018 20:36, Chris B 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).

So far, that is all spot on.

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.

ok, first thing to realise is that the term "unvented" can be used for
two separate and unrelated systems... it can be used to describe a
sealed primary CH system - which it what you alluded to above. However
that is more commonly simply called a "sealed system".

It can also be used to described a mains pressure hot water *storage*
system. Which has the cold main feed directly into a hot water cylinder.
The cylinder needs to be rated to take that pressure and have a number
of other safety systems included with it. Including its own expansion
vessel. (the cylinder could be heated by a vented or unvented CH system,
or an immersion heater).

However many people will have mains pressure hot water provided via a
combination boiler - that heats water on demand - there is not stored
hot water.

In the case of a combi, shutting off the cold main supply to the boiler
(or indeed the whole house), will also stop the supply of hot water.

With an unvented cylinder, shutting off its cold main supply will also
shut off the hot water - but it will still continue to supply a few
litres of hot which is expelled by the pressure from the expansion
vessel that belongs to the cylinder (not the one in the boiler).



--
Cheers,

John.

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