Boiler leak issue driving me up the wall

[Cainan]

Ok working at a church, job was to change 32 thermostatic radiator valves.....easy
change a few taps ok.....easy
fix pressure drop on boiler.....not so easy

the boiler house is situated about 6 metres away from the church so the pipework goes under ground and then into the church.
pressure drop is pretty much top up to 1.5 in the morning by night it's sitting at zero.
Done the obvious things, checked pressure on expansion vessel which is all good
been under all the floor boards in the church all bone dry
checked prv over night with container under again that is fine
i then isolated the flow and return under the floor boards when it was entering the building so I was only testing from boiler to church under ground pipework and again pressure down at zero so I can pretty much rule the church out completely
not sure why I am writing this as I think I know what is required but just in case some one has any other suggestion thought I would come on here.
am I faced with the sad fact that we are ripping up the concrete base outside and digging to the pipes? No leak on boiler, no leak on prv, expansion vessel fine but there's a pretty major leak somewhere and that's the only place it can be......right?

 

[IDCHAPPY]

can you not just run new pipework between boiler house and the church?

Churches are notoriously tight, even tighter than us Scots(supposedly )

 

[Corgiguy187]

is it a condencing boiler. cos if it is isolate the boiler itself and see if the pressure drops. possibly pin holed main hex going down condence line.

Had this 3 or 4 times and has caught me out with green star Worcesters. Cap flow/return see if pressure holds.

 

[Cainan]

Ok this was sometime ago but thought I would just give quick update as I was on here. I ended up after trying every single other bloody thing cutting the tar out and digging a track between the hall and the church. Found the old pipework, cut into it and fed new hep through and into church and boiler house, connected up, come back the next day and pressure was solid. Got company to re tar and the job was success, so somewhere under ground there was a leak. Thanks for all help at the time of this going on

 

[galaxy_plumbing]

nice to hear some feedback after this mad job

 

[kris]

Yeh well done give yourself a pat on the back. Its good to here actual fixed solutions to problems started on here.

 

[Cainan]

Cheers guys, meant to update ages ago. I was relieved as anyone one when it worked as like mentioned on the posts everything else possible was tried.