C/Heating sealed system pressure loss - fault finding

[davidg12]

Newbie looking for advice please....
Had new condensing boiler and rads fitted to existing pipework in May 07.
All okay until Jan 1st. Pressure fell overnight and I now have to manually fill 3 times a day otherwise pressure warning light on boiler. Still same after 3 weeks - boiler manufacturer changed secondary heat exchanger. Installer added fernox leak sealer after no sign of leaks at boiler or rad connections (or any staining on ceilings etc.) when system hot or cold. No change. Installer also needed to replace faulty auto valve on hot water feed pipe as hot water was being heated continuously when c/heating on. I thought that suspicious and more than co-incidence but he thinks not - only way to tell is to replace hot water tank he says. Is there any way to test the h/water tank as source of leak? Also, how do you find the leak if its elsewhere - is their anyway to pressure test the system without exposing all pipes for physical examination. Any advice appreciated

 

[ChrisR]

You can check the levels in the tanks, and you can put valves in the primaries to the cylinder to pressure-test the rest of the heating system.

Do you have any pipes buried in concrete floors?

 

[davidg12]

You can check the levels in the tanks, and you can put valves in the primaries to the cylinder to pressure-test the rest of the heating system.

Do you have any pipes buried in concrete floors?

I do have a ground floor extension with a long run of pipes beneath it which is why isolating the problem is paramount.

The hot water cylinder already has the auto valve on the feed and a valve on the return from it (i think - its a gate valve with a red circular tap). If I switched hot water off (so auto valve is shut) and shut the other valve is this effectively shutting off the cylinder and preseeure testing rest of the system. Is there any danger doing this or should I leave to an expert?