On 31/01/2011 20:36, Kostas Kavoussanakis wrote:
On 30/01/2011 20:09, Roger Mills wrote:
On 30/01/2011 19:47, Kostas Kavoussanakis wrote:

I too am considering the options in case my 25-yo boiler gives up the
ghost. The combi needs good pressure mains, I believe; have you got it?

It looks like, if you are upgrading or replacing (as opposed to
installing fresh), the combi is the most drastic option.

Indeed. Replacing a stored hot water system with a combi often involves
a lot more plumbing changes than installing a replacement conventional
boiler.

Thanks Roger.

I see from the
wiki and from the Vaillant and Worcester websites that there are two
non-combi options: open-vent (the same as what you and I have) and
sealed system (which only does away with the cold-water tank). I am
still not clear of the benfits of each set-up, so I will stop here.

If you are referring to a non-vented stored hot water system, this has a
mains pressure cold feed to the hot cylinder rather than having a
gravity feed from a cold storage tank. The advantage is that you get
mains pressure hot water, and you don't have to accommodate a cold tank.

Ah, I thought that modern open-vent systems might pressurise the hot water.

The disadvantage is that, if the mains fails, you have no hot water -
and you need a load of pressure limiting valves and (internal or
external) expansion vessels - and the whole thing can only officially be
installed and maintained by a suitably qualified person.

Great analysis, thanks.

How complicated is it to convert from open-vented to sealed? I am
thinking isolating the cold water tank and rerouting mains water, but
there may well be more I haven't thought of. I am very limited by
hardwood flooring and a single hatch near the boiler that, alas, has a
dwarf-wall between it and the hot-water tank.

Also, is it correct that with open-vented I don't have an expansion tank
to inspect for water loss? I am thinking of relocating the boiler
directly above its current location, which would impair access massively
(unfloored loft and awkward, tri-bearing trusses).

Cheers,

Kostas (expecting this is useful to the OP as well)


I fear that you may be confusing your water circuits! A house with a
central heating and stored hot water system has two separate water
circuits. The *primary* circuit connects the boiler to the radiators and
to the indirect coil inside the domestic hot water cylinder. This can be
vented or unvented (pressurised). If vented, there will be a small fill
and expansion tank - usually in the attic. If unvented, there will be a
filling loop with pressure gauge, a pressure relief valve (safety
device) and an expansion vessel. Converting from vented to unvented is
fairly straight-forward and involves removing the F&E tank and adding
the other components mentioned above. Whether vented or unvented, the
primary circuit keeps circulating the *same* water - and usually has
corrosion inhibitor in it.

The *secondary* circuit contains the water which flows to your hot taps,
and is stored in a hot water cylinder. It is heated by water from the
primary circuit which flows through a coil inside the cylinder, but
doesn't mix with the secondary water (you wouldn't want to bath in
inhibitor!). The secondary circuit can be vented or unvented. If vented,
there will be large a cold water header tank which feeds cold water into
the bottom of the hot cylinder to replace the hot water flowing to the
taps. If unvented, the cold water feed to the cylinder comes from the
mains rather than from a header tank - and you then need non-return
valves, pressure limiting valves, an expansion vessel and an
over-pressure relief valve.

Converting from vented to unvented is a major operation because the hot
cylinder has to be replaced with one which is capable of sustaining the
much higher water pressure, in addition to adding the other components.
As I mentioned in my previous post, this officially has to be installed
by someone who is suitably qualified because there are lots of safety
issues.

Which circuit were you asking about?
--
Cheers,
Roger
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