Where's Leaky? Wetness, swelling and white streaks!

[PhilCSwift]

Combination tank on a raised shelf in a 2 door cupboard. Top cupboard door is jammed shut as it has 'swelled up'. Some evidence of 'wetness' but hard to say where it is coming from but...the ceiling above the cupboard, outside of the cupboard, is showing wet patches. Is there a plumbing equivalent of fluorescent dye used in automotive?
I think it could be

1. leak from roof into cupboard (obviously nothing to do with plumbing
2. condensation from header tank
3. header tank overflow pipe/joint leaking
4. header tank ball and/or valve worn
5. something I have no idea about

I'm going up sometime today to have a sniff about!

 

[ShaunCorbs]

Leak from the roof

 

[Chuck]

Your list of possibilities looks about right to me.

Is there a lid on the header tank? (There should be!) Use a pair of binoculars to check the area of roof above the cupboard for missing/broken/split tiles which are often visible from ground level.

You can get tracer dyes, such as fluorescein, from a merchant or a lot of people use food dyes. I doubt they will help in this case. If people are going to wash in water from the tank after you've added dye to it, make sure you check nobody is allergic. Once you've added dye to the system it'll a heck of a job to get it clean again....

 

[PhilCSwift]

Your list of possibilities looks about right to me.

Is there a lid on the header tank? (There should be!) Use a pair of binoculars to check the area of roof above the cupboard for missing/broken/split tiles which are often visible from ground level.

You can get tracer dyes, such as fluorescein, from a merchant or a lot of people use food dyes. I doubt they will help in this case. If people are going to wash in water from the tank after you've added dye to it, make sure you check nobody is allergic. Once you've added dye to the system it'll a heck of a job to get it clean again....

Thanks Chuck, I very much appreciate your reply. I did quite a bit of work on the issue yesterday. Went up onto roofs and photographed all potential outdoor leak areas then realised that the tank is not directly below any flashing or wall joints. So it must be condensation or a leak from a flat above it.
I did not know about lids and that nowadays they are meant to be sealed to stop insects. That touches on byelaw 80 I guess. There was an old aluminium lid in the cupboard, but not on the feed/top cold water tank. It was just big enough to not drop in but does not seal. I put it on top and put kitchen towel down on flat surfaces.
Most of the black mould was above the level of where the hot tank ends and where the cold feed tank is so I think this is condensation issue, or possibly overflow or water level height issue.
The cupboard opens out into a hallway that can get very cold. I have put a wall vent in above the cold tank height in the plasterboard plus I will try and check the age of the combination tank; it's an old Telford.
There were some water puddles on floor that were a red-herring but this could be wet push-bikes left in the hall by tenants.