In article ,
Roger Mills writes:
On 15/05/2018 22:29, Andrew Gabriel wrote:
In ,
Roger writes:

The airing cupboard would be by far the easiest place to install the
kit. There is ready access to mains water for the filling loop. Because
the secondary HW system is unvented, there is a tundish into which
anything coming out of the hot cylinder's temperature and pressure
relief valve would spill - so that could also accept the output from the
primary circuit's PRV.

No, the PRV outlet must go outside without any air break, and it
should point back against the outside wall. If the system boils,
it will be ejecting super heated water and steam.


But surely the same thing could happen with near boiling water coming
out of the hot cylinder's temperature and pressure relief valve? That
goes into a tundish with an air gap. Why the distinction?

There's a massive difference between nearly boiling, and super-heated
water and steam. Also, an immersion heater is typically 3kW when it gets
stuck on (think, a kettle boiling). A central heating boiler is going to
be 25-50kW when stuck on - a completely different ball game and very
dangerous for anyone caught in the steam, particularly in the confines
of a house. Yes, mostly it just handles a failed pressure vessel, but
it is also there to handle much more serious failures.

There is an uninterrupted connection to the
boiler - albeit in 15mm pipe, and probably 3 metres vertical and 3
metres horizontal away.

The boiler's installation manual says to put the the PRV on the flow
pipe, close to the boiler and before the pump inlet. But that seems to
assume a single pump on the flow side, prior to any zone valves, etc.
But this system ain't like that! There are two pumps - one for CH and
one for HW - both on the return side, feeding *into* the boiler.

It still wants to go on the flow pipe near the boiler and before
any other things that might restrict flow.

OK, if I move a lot of stuff in the garage, it could go in the flow pipe
very close to the boiler, with no intervening flow restrictions.
Technically it would be *before* the pump inlet - since the water then
goes right round the house to get to the pump, which is quite close to
the boiler on the return side. Does that sound ok?

It sounds OK, but you really need to read the installation manual,
which I haven't. You become responsible for the installation, even
any related parts done wrongly initially.

Are there any technical - or even legal - reasons for not installing all
the kit in the airing cupboard - some way from the boiler, but directly
connected to it? If so, could I put the PRV near to the boiler, and the
expansion vessel and filling loop somewhere else (preferably in the
airing cupboard)?

Filling and pressure gauge can go anywhere (but should be next to
each other so you can see the pressure as you fill the system).

Expansion vessel should ideally be as near to the pump inlet as
possible (heating pump), as that will give longest pump life and
lowest pump noise. You can put it anywhere, but then you will lose
those benefits.

In that case, it could go in the garage near the heating pump inlet
which, as noted previously, is in the return pipe.


PRV must be connected exactly as it says in the boiler installation
manual or the system won't conform the manufacturer's safety
requirements and would fail commissioning.


OK, it makes sense to put it close to the boiler. what "commissioning"
are you referring to? It would be me doing it!

The commisioning instructions will be in the installation manual.
Some of them will need to be repeated since you are changing the
system design. Commissioning means testing to ensure the system
is operating according to the manufacturer's design. It might
including things like testing that you have sufficient pressure
vessel capacity for the system volume at max running temperature.

--
Andrew Gabriel
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