URGENT Peculiar problem

[Nischal]

I have a system boiler with 2 unvented hot water cylinders. There is no hot water in the loft bathroom tap. But hot water is coming in the loft bathroom thermostatic shower. Moreover hot water is coming in the other 8 bathrooms. When you switch on the loft bathroom shower, hot water starts coming in the loft bathroom tap. But on its own the loft bathroom tap is giving cold water. Any suggestions would be greatly appreciated. Two plumber *** gas care engineers have already given up. I have installed a new tap and a new thermostatic shower in the loft bathroom to eliminate the problem and come to a diagnosis. But no luck so far.

 

[cjg]

have you plumbed it up yhe right way round. ,

 

[Scott_D]

Do you have isolation valves on the hot and cold supply to the tap?
Try shutting the cold supply valve to heck its not overpowering the hot.

 

[Timmy D]

If you run the shower and tap until hot comes out of the tap. Then close the shower, what happens?

If you run hot at the next closest bathroom and then run the tap, what happens?

Currently thinking a cross over (cold getting into hot), but it needs to be proved.

I’d go around and isolate each outlet one at a time and test the tap.

Is the properties cold supply taken as balanced after the unvented pressure reducing valve?