trickle from hot water taps

Boilermate 2000
all the taps in the house trickle when the hot taps are open, i take it the heat exchanger is scaled up

whats the quickest way to replace the heat exchanger?

do i have to drain the tank and heating sysytem?

Yes drain the unit.
Turn off water and run hot taps.
Undo PHE use a container to catch any water.
Put towel over pump etc.
Fill unit and don't forget to put inhibitor in as per manual.
Mike
MGC/H2O
Norfolk