I need to drain our Megaflo to replace the immersion heater(s). The system
is a mains-fed one, ie no tank in the loft. I just want to check the
procedure for draining and re-filling the cylinder, please.

1. Turn off cold water feed.
2. Open hot tap and let as much water drain out as the air pressure thingy
inside the Megaflo expels.
3. Connect hose to drain and drain it.

Question: The Megaflo will need to fill with air when the water drains out.
Where does the air come from? Is there an automatic valve somewhere, or do I
need to open something? Or does it get sucked in through the open hot water
tap?

Filling:
1. Close drain valve, and open cold water feed.
2. Anything else???

Do these things come with an instruction book? If so, I have no idea where
ours is, assuming that the builders ever left it.

Thanks for your help.