Shower Pump Feed from Top of Hot Cylinder - Drain to Disconnect?

[RealPolitik]

Attempting to replace a dual shower pump, which has been plumbed to serve one shower (cold feed from a CWST) and three mixer taps (cold feed from mains).

The hot feed to the pump inlet is taken from the top of the top of the hot water cylinder via a copper tee, the other side of which is the overflow feeding back into the CWST, as normal. There is no stop tap or isolation valve on this hot feed to the pump (the tank side outlet/discharge with stop tap is capped) - do I need to fully drain the hot water tank before taking off the connection to the pump, or not as it‘s a vented tank? Pictures attached for reference.

 

[RealPolitik]

I assume that I can just isolate the cold flow into the cylinder to stop flow out of the cylinder (assuming nobody turns a tap on)?

 

[Johntheo5]

No, shut the cold supply from the CWST to the cylinder and open d/stairs hot tap. There may (should) be two cold water outlets from the CWST, one to the HW cylinder and one to any gravity fed taps and the pump cold water supply, shut both.