"Alex (YMG)" wrote in message
...
"Grunff" wrote in message
...
No, I haven't gone and bought a combi :-)

A friend has a Worcester 240 combi boiler, about 9 years old. The
pressure
started dropping a week ago, and it now needs topping up several times a
day. Further investigation reveals that it is losing water from the
drain
that goes to the outside of the house. This isn't a condensing boiler.

I have zero experience with combis. Any pointer to what this might be,
where to start, etc.? What is this drain? Why is it there? What does it
connect to? What could be causing the water loss?

The pressure relief valve will blow off at 2.5 - 3 bar. The pressure of a
normal system is somewhere in the 1.5 bar region. To prevent a large
pressure increase in the system when the combi fires up the CH, there is a
bladder-like air-sac in a big metal doughnut thingy. This is an expansion
reservoir that takes up the expansion of water in the system as it heats up,
thereby stopping the system pressure climbing to dangerous levels. Your
friend's air reservoir ( I forget the proper name for the item ) either has
lost all its air via the top-up bicycle valve which is attached to the
doughnut, or has a punctured diaphragm inside and has lost all its air into
the CH system. Thus, when the CH fires up, the CH water expands, the system
pressure rises too far and the last-ditch safety feature, the blow-off
valve, does its stuff.

You would probably do best to locate the expansion reservoir doughnut (
possibly painted red, at the back of the combi, about a foot across??? ),
and see if you can pump it up via the bicycle repressurising valve. Then
keep an eye on the system pressure, maybe for several weeks. If the system
loses the ability to regulate pressure again, then it'll probably need a new
expansion reservoir, 'cause the daphragm will be leaking.

Andy.