there should be a drian off valve at the bottom somewhere, connect a
hose to that (with the other end outside or in a toilet), Switch water
off, either by the tank or to the whole house, Switch boiler off and
open drain off valve.



If no drain off valve, then you can turn boiler off, turn water off and
open hot and cold taps and this will drain it all down. I would open all
taps just to make sure there is no water left in pipes.



If you have a drain off valve, this is the best option as it drains it all and prevents any risk of water coming out.



The pipe should go across and down, the best bet is to take the tapping off of the bit that goes down.
Replace faulty gate valve 1st-u can isolate then but no need to drain full system,drain valve will probably be goosed anyway
Personally I would never advice any body who has little or no plumbing
experience to work on a system which is half full. If it all goes wrong,
he will be stuck. And also the fact that it is easy to forget that
there is still water in some of the system and cut through the wrong
pipe. Easier to completely drain down. Unless you know what you are
doing.
Gate valves often let by a little when they get older,one of these is better if you're going to change your existing one.

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