'tank in a tank' hot water tank issue - is it broken?

[boltwhite]

Hi all,
Moved into a house built in 2008 with a ground source heat pump that does both the hot water (with immersion heater also) and wet underfloor heating. The hot water tank is a 'tank in a tank design' where there is an inner tank for the domestic hot water and an outer tank for the underfloor. I recently had the immersion heater changed. Since then I've noticed the expansion tank making a lot of noise - i had thought this was the expansion volume but checked the pressure and it was okay. I've also since noticed that the underfloor heating pressure was high. I released some of the pressure using the relief valve, with water flowing out - but the pressure slowly refilled even though the manual filling pipe was closed. I'm worried that there's a hole between the two tanks...
I appreciate this is an unusual design (it seems a bad design to me!) but any advice or experience would be hugely welcomed!
thanks
Hugo

 

[Madrab]

Make and Model of Cylinder?

 

[boltwhite]

The manual calls it a 'UniHub Thermal Store' unvented copper dual mantle (tank-in-tank). I attach a photo and page from the manual... Nobody ever seems to have heard of this design...

 

[Madrab]

Unihub is by Unico SYS (UCC) I believe but it's not one of the better known manufacturers TBF and it's more of a complete solution provider rather than a component manufacturer. They are better known in the US as they provide a full HVAC system with AC and ducted warm air solutions, with the hub cylinder at it's core. The cylinder can use multiple input sources.

Not much in the way of documentation around the unihub itself but if the CH system is sealed and pressurised and the core cylinder is unvented and the system pressure is rising - have you actually disconnected the filling loop if there is one? - then yes, mains pressure could be bleeding from the core vessel into the system, creating the creep in spressure. Without seeing a diagram of the internals it's difficult to say if there is any other direct path between them apart from the internal cylinder being pinholed.

Can you give a close up of the data plate on the cylinder showing all the info and any specifications/data/model info that might be in the manual.

 

[polesapart]

Shut off cold supply to cylinder and open a hot tap and leave overnight - what happens to system pressure?


* Edit: check in your documentation for the commissioning procedure "first filling" that its ok to have inner cylinder at atmospheric pressure and outer tank pressurised.

 

[boltwhite]

So I did that briefly at the weekend - shut off main supply; opened a hot tap full (which slowed down to a dribble but didnt stop); and watched the underfloor heating gauge which started dropping; only did it for a couple minutes; when i opened the mains again the underfloor heating pressure refilled. Filling loop def closed!

Boiler plate photo below. thanks for you help, its a complicated setup that gets more complicated the more i look at it (i haven't even mentioned the Unico air heating upstairs and seperate buffer tank)!