On Thu, 20 May 2004 09:29:51 +0100, "David Hearn"
wrote:



On the heating circuit the hall radiator (where the room thermostat is) is
non-TRV. In the bathroom the TRV is set to max (when we arrived, it was the
only radiator with a TRV bizarrely!). So, that satisfys the bypass for the
heating circuit. At present, even in the hot weather we've been having, the
bathroom radiator gets hot, so I assume Max means no-thermostatic control
and fully open. No problems when running with heating only.

On the hot water circuit there is no bypass, but then there isn't a TRV so
that's not a problem. Problem occurs when running with hot water only.

The only issue I can see then is if the boiler fires whilst both valves are
shut/jammed. The wiring is such that when the boiler is on, at least 1
valve is open, so only the jamming situation is an issue. Is it normal to
have a bypass for jammed valves situations? Incidentally, there is a 15mm
pipe (whereas rest is 22mm) with manual valve (same as balancing valve after
HW zone valve) which bypasses both valves. I don't know what this is for
and has always been shut.

That is a boiler bypass to cover the case when both HW and CH circuits
are closed off and the boiler has been firing.

Normally it should be open a bit. The boiler should have an
over-run thermostat to power the pump so that water continues to be
circulated away from the heat exchanger after the other circuits are
closed.

These can be improved by fitting an automatic bypass valve which opens
only when the pressure exceeds a certain value - i.e. both valves
closed, pump running.

However, none of these help if the pump setting is too low.


One other thing that occurred

Did you try running the HW with a completely cold cylinder? Also with
the pump on high? These would tell you if the problem has to do
with getting the heat away from the boiler.

Presumably you did flush the pipes thoroughly when you changed the
boiler and they are not full of sludge?






Thanks

David


..andy

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