Changing PHE

Hello,
I'm planning to change PHE myself. Can someone confirm that I'll be doing right thing:
1. Close main water feed and open hot and cold taps downstairs.
2. Close valve (1) on picture.
3. Open valve (2) and wait until water stops. It will empty tank up to the level (4)
4. Change PHE
5. (optional) Install valve on position (3) so next time don't need to drain system, like here http://www.ultimatehandyman.co.uk/diy_plumbing_isolation_valve_fitting.htm but I can't find valve for 20mm pipe.


I understand that I will need to add some Sentinel X100 but I have no idea how much. My system says it is 150L and obviously it will be drained much less but I can't see how high tank is. Also I noticed that when I tried to drain small amount of water, it is rusty and there is a lot of rust in expansion tank. Do I need to do anything with it(adding another chemicals, installing filter just before expansion tank) or it is fine?
Thanks

A litre for every 100 litres.
The unit is 150 thus 1.5 litres.
Then the system which will be at least another 1 litre.
Total 2.5 litres I would use 3 litres.
You could clean the unit and system with sludge remover.
Time is all it take.
To change PHE.
Water off and run taps.
That will be the front top and bottom of the PHE.
It would be better to drain the unit for the other 2 pipes.
As that will help pull sludge from the unit.
Mike
MGC/H2O
Norfolk

Mike,
Thank you for the advice. All done and now my hot water is hot again!!!

Well done