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I have an indirect central heating system that has developed a leak. The boiler is a Harcal 430, heating the radiators and the hot water tank. When the system is cold I can hear the central heating header tank filling, and if I turn off the stopcock or tie up the ballcock the tank gradually empties.

The header tank doesn't fill when the heating is on, or for some time after it is turned off, only when the system is cold.

The property is a 30 year old bungalow, with the most of the pipes buried in the concrete floor, and I can see no sign of the leak inside.

A heating engineer has told me that if I can't locate the leak the only solution will be to re-plumb the system.

Can anyone advise me whether the age of the system, and the fact the it only leaks when the water is cold, might indicate that the leak might be in the boiler. In that case I may get away with a new boiler rather than an entire new system.

Alternatively, can anyone suggest a method of finding the leak.

When pipes are heated they expand, so this will probably stop any leaks. When the pipe cools and contracts, this is when the leaks will start.

Do you get any discolouration of water, when using the hot taps ? could the heating coil in the tank be the problem ? corroded - heating water mixing with hot water.

A leak of this size from a boiler - I think would be noticeable, can you feel the heat in the pipes under the floor, a good sign of a leak is if you can feel where the pipes run when the heating is on - if there's a leak there'll be a 'spread' of heat where the leak is as the pipes will heat the water soaked concrete (hope you understand what I mean) .

Hope this helps. Will look out for future posts.