On Fri, 22 Oct 2004 12:36:48 +0100, "Set Square"
wrote:


Unfortunately, your carefully logged details are missing one vital piece of
information - namely what the heating system was doing at the time in terms
of being switched on and off - either by a programmer or room stat.

Nevertheless, from what you say, you have a problem either with with your
pressure vessel or with your pressure refief valve.

Thanks for your input on this. I think the pressure relief valve must
be OK because it's brand new. I replaced it a few days ago, because
the old one was my chief suspect, but the new one behaves exactly the
same as the old one.

When everything is working normally, the pressure should be about 1 bar when
cold and 2 bar when hot. [A cold charge pressure of 1.75 bar sounds a bit
high to me - have you always used this pressure?]

Well, to be very honest, since I instaklled the system, I've been
topping up the sytem to 3-bar! I thought mistakenly that the 3-bar on
the guage was 2-bar. It was only in the past could of weeks that a
Brittony boiler expert made me aware that I shoul only top it up to
1.5 or 1.75 bar. Nevertheless, I would imagine that the pressure
relief valve has been sparing the system from any damage resulting
from my over-topping-up of the presure....

When the system heats up, and the water expands, the pressure vessel is
supposed to be able to absorb the expansion with only a modest rise in
pressure. If the pressure rises above 3 bar, the pressure relief valve is
supposed to open - to let some water out and reduce the pressure.

Yes, that certainly happens. In fact, water starts trickling out at
about 2-bar (on both the old and the new pressure relief valves). Both
of them tend to blow when the pressure guage is not much past 2-bar.
But I reckon it's unlikely that they are both faulty in exactly the
same way. They don;t even look the same. The replacement I just put in
is a new design. (Designed to be better than the older design, I
guess).

First check your pressure vessel. Let some water out of the system to reduce
the pressure to well under 1 bar. Measure the air pressure in the pressure
vessel with a car-type pressure guage applied to the Schrader valve.

Schrader valve??? Never delved into this area of my boiler before, but
I am willing to venture into the unknown.. Looks like my time has come
to be a hero.... ;-)

If water comes out of the valve when the pin is pressed in, the diaphragm is
split, and the pressure vessel is shot. If no water comes out but the
pressure is low (say, less than about 1.3 bar) pump some air in to increase
the pressure. This might cure the problem.

If you still get water coming out of the pressure relief valve, you need to
determine at what pressure this is opening. If possible, stand by the system
at a time when water is coming out and observe the reading on the pressure
gauge. If this is substantially less than 3 bar, the relief valve is shot.

See my above comments on the valve...

If the pressure vessel and relief valve are both ok, try running at a lower
charge pressure. If you still have problems, it would suggest that the
pressure vessel isn't big enough, and that you need to install a bigger one.
This sounds unlikely if the system has worked ok for 4 years - unless it has
recently been extended by adding additional radiators.

No, it hasn't been extended. There is, of course the possibility that
the pressure guage is incorrect - but I suspect not... because I've
noticed the boiler starts making a moaning noise (or whining noise)
when the pressure drops to around 1-bar. That symptom is the same as
it always as.

So I guess the chief suspect now is the expansion vessel.....

Many thanks for your help.

Dave L