"Grunff" wrote
| 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.

The 'drain' should be copper pipe, turned back against the wall. It is the
outlet of the pressure relief valve and squirts out boiling water.

| 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 valve is opening and releasing water. If the system is within
pressure (check gauge on boiler) the valve is faulty. If the system is over
pressure then the pressure vessel (which may be within the boiler or
elsewhere) is failing.

Owain