Central heating pressure drop

[cam23]

Our central heating system loses pressure in a dramatic way (gauge is down to zero within 2 weeks of topping up the system). This has been going on for several months. We've looked at all the radiators and at the wall/floor around the boiler, and re-checked an underfloor pipe that was mended a few years ago, and there are no obvious water leaks that we can see. Also, our downstairs neighbours know about this but haven't reported any damp on their ceilings (yet!).

The boiler is a Vokera Mynute, 9 years old. Nothing plumbing-related has been changed in our house to explain the problem starting. Even at very low pressure, the central heating system works correctly. It doesn't seem possible that we could have a water leak going on for months at a stretch without our neighbours finding water patches. Can anyone suggest anything?

 

[cam23]

Just read the Wiki, going by the definitions there I *think* this is an open vented system, but I could be wrong. It features a valve and tap by which you can add water to the system, a small vent pipe that goes through the wall at one side, and the boiler appears to have a kind of wide chimney pipe going through to the roof. Apologies for the non-technical description - the boiler was installed some time before we lived here.

 

[twgas]

I *think* this is an open vented system

I think not

a valve and tap by which you can add water to the system

a filling loop

wide chimney pipe going through to the roof

flue.

does the pressure go up when the boiler is firing? is there a drip from the pressure releif pipe, the one through the wall?

 

[cam23]

Yes, the pressure does go up when the system fires. Average rise is about 0.5 bar. There are no visible water drips from any part of the system.