Draining unvented system

I want to tap into the cold water main to take a feed for a new electric shower. The house has an unvented hot water system (Megaflo type).
Are there any special things to do/be aware of before I shut off the main stop cock and drain the system/refill afterwards?
Many thanks in advance.
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Just make sure you've turned everything "OFF" before you do the job. Then make sure you turn everything back "ON" and check for leaks when you're done.
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Christian McArdle wrote:

The only reasons I can think of are: 1) S/He wants to add a shower but getting the HW over is really disruptive? (as opposed to the mains CW and the electric and the switch gear... Um...) 2) S/He's afraid the boier will pack in - but Megaflows come delivered with a (double thermostat) immersion, so just run 15A to it!
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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