Fitting New CH Pump and 3 Port Valve?

Hi,

Further to my on going saga...

I had elected to change the 3 port valve - as it had an intermitant fault ('end-switches'?) and I also had to hand a 'new old stock' pump which I had elected to fit at the same time.

Yesterday the old pump - already fitted to the CH system - failed!

Time then to drain the system and fit the new pump and 3 port valve along with some new gated valves etc. and top up with additive then..

My question is; do I only need to drain down the CH system and header tank for such? We have a 'partially pumped/partial gravity system with an expansion header tank, I presume that the pump is used for the CH system (rads) and for the 'coil' within the cylinder (so as to heat up the surrounding hot water) for our hot water etc. and therefore do I only need to shut off the water supply - ensure that any ball-valves are either tied up or isolated and drain down the CH header tank etc. so as to fit both the pump and the 3 port valve?

Am I also right in thinking that a small quantitiy of - residual - water will be present in the pump and the 3 port valve and I should be prepared for such?

Also the 3 port valve will need rewiring - to the junction box - along with the pump, is this usually wired in with the 'ring main' or the circuit for the room-stat and timer etc....There doesnt seem to be an individual RCD/fuse on the consumer unit for such - although there is for the immersion heater. Shall I isolate the ring main or the whole lot? I will be checking and double checking the electrical continuity anyway.

Many thanks in anticipation.

Regards,