Have a vaillant ecoTEC plus 624 which we've had for about 3 years or so. Very reliable, apart from if we're away for a few days and I switch both heating and hot water off, the system loses pressure and damp patches appear under boiler. Engineer who fitted system and services it cannot find any problem. However, today on returning from a few days away caught the thing actually dripping water! Took the cover off and water is dripping from the left hand black rubber hose which is about 4 inches long. There is another on the right, but it doesn't leak. The hoses are connected to the copper pipes with factory fitted type of butterfly clips. Any thoughts on what the problem may be and why it only seems to happen when system is idle? Many thanks.