Originally Posted by Aidan

doozer wrote:
I think I have found the tundish.

That's it. The pictures aren't clear, so I can't guarantee that what
follows is correct. The two pipes going into it are from the expansion
relief valve, and the T&P relief valve (directly above the tundish).
The thing you've labelled as the T&P relief valve seems to be the
Pressure relief valve for the heating. The red expansion vessel is for
the heating (primary) and has nothing to do with the unvented hot water
storage system.

The tundish has scale on it from when one of the two valves was
discharging or dripping. It was probably the Expansion relief, the T&P
relief looks like the older lever type.

Is it normal to have what we have which is basically just a cut out
bit
of pipe for a tundish? Surely that would cause water to flood out all

over the floor rather than down the emergency vent pipe if there was
a
problem.

Yes. As mentioned previously, the D2 pipe to outside should be sized so
that it will carry the water away. It will only overflow the tundish if
there's a blockage in the D2 pipe. If it couldn't then overflow, the
cylinder would then be subjected to excessive pressure, possibly
leading to catastrophic failure.

The 'multi-function control valve' looks crusty and should be replaced.
It may be a pressure reducing valve, in which case there should be a
strainer upstream of it. A replacement would probably have the strainer
built-in.

What would be the effect of having no bubble?

The pressure would rise as the water heated up. The expansion relief
valve would/should open and discharge the excess water. In the UK,
unvented systems are required to have a bubble or expansion vessel to
prevent the waste of this water. Elsewhere, there's no provision to
accomodate expansion and the water goes down the drain. IMHO, it's best
to avoid operating the two safety valves unless necessary. Sometimes
they won't re-seal and have to be replaced.

What I have marked as the immersion heater

I can't make that out.

and we get a burst of hot water the first time we turn the tap on.

That could be normal, the pressure will rise and the air bubble will
get compressed as the water heats up and expands. The additional
pressure will be discharged when you open a hot tap.