are there any service valves on the hot tap ensure your hot water demand is off well in advance to cool the system make sure it cant be turned back on untill done . turn off main stop cock turn off service valve and disconnect if no service valve open tap and another higher in system to drain out off you go .when done turn both taps off turn on main stop cock open lower tap first to fill pipework then higher one till air expelled.
The service valves need moving hence the need to turn everything off.
Thanks
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