Grohe shower problem.

[Burnerman]

The next door neighbour has a Grotherm 3000 (I think) built in gravity shower which has worked excellently for years. All of a sudden all water flow stopped.
This shower has the 2 controls: temperature below, which feeds the on/off cartridge above. This cartridge (ceramic) has been taken out for now.
The inlet filters on both sides are clean. When they are removed and the isolators turned on independantly or together, loads of water pours out.
He removed the old thermostatic cartridge and bought a new one. Its a Grohe replacement with exactly the same part number.
The fault is exactly the same - i.e no water flows, hot or cold.
If I remove the old cartridge wax capsule, only hot water flows. Put it back in, all stops.
Please - any ideas whats happening? There's something I know nothing about here!
John

 

[ChrisR]

Not a scooby. But I thought that might make you at least feel better

I suppose there could be a partial blockage on the feed from the cistern. If it's common to H and C you'd get a vent pipe full from the hot.
"loads of water" could be misleading. Maybe. Blast the mains up the cold pipe?

 

[picasso]

hope I am not stating the bleeding obvious but have you tried it with the shower hose removed, they sometimes collape inside.

 

[Burnerman]

Gentlemen, I thank you for your replies!
Picasso - the off/on ceramic cartrige tap has been removed. The neighbour took it out just to ensure it wasn't faulty. So, we have a gaping hole where the cartridge was, which should belch water if the shower is working. Sadly, it remains dry!
ChrisR - you have me thinking - its an ancient farmhouse (3 stories) with gawd knows what living in the loft...where the unlidded tank is
Maybe if I took the filters out again and let the cold water run for a few minutes, this would prove your point.... This I will do!
I guess my next question is, will a thermostatic shower pass any water (!) if one of the supplies is at reduced pressure or volume?
PS The filters were pretty clean, and the shower valve housing is a one piece bit of machined brass, with no obvious obstruction in the drillings.
John