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[orainsear]

I personally wouldn't run a central heating system without inhibitor.

The semi-permanent pipe is called the filling loop and should really be taken off after use - you don't want to get any inhibitor, or anything else for that matter, in your drinking water after all.

Also you can just remove say a single TRV valve from a single rad, empty that rad of water and using the rad bleed plug and pipe/funnel to get it in that way.

I did it last year but completely drained the system and gave it a good flush too, removed a rad and dropped it down the pipe then refilled the system.

[oldfogy]

1. The semi-permanent pipe is called the filling loop and should really be taken off after use - you don't want to get any inhibitor, or anything else for that matter, in your drinking water after all.

2. Also you can just remove say a single TRV valve from a single rad, empty that rad of water and using the rad bleed plug and pipe/funnel to get it in that way.

3. removed a rad and dropped it down the pipe then refilled the system.

(TRV valve) = Bleed screw.

1. Cross contamination is not possible because by law these valves have a anti-syphon valve fitted (one way only)
The only real reason to disconnect them is to stop people, mainly children from turning them on and over pressurizing the system and causing the excess water to exit via the over-flow pipe.

2. If only removing enough water to be able to top-up with a "inhibitor" the best/correct way is to:
a) Undo the main drain plug just enough to relieve the pressure from the system, then tighten it back up.
b) Remove the bleed valve from the highest radiator on the system.
c) Undo the main drain plug again and measure how much water is taken out through the drain plug, which is usually at the lowest point in the system,
(on some systems you may need to use a garden hose to be able to eject the water out of the property)

VERY IMPORTANT
Whichever method is used to top-up the system, DO NOT close the radiator bleed valve before topping-up the system (because by closing the bleed screw helps to push any air in the system around the system), with one person standing at the radiator "ready to turn it off" get someone else to "slowly turn on the pressurising tap/valve, then once the air appears to have stopped, close the bleed screw.

Now run the system for about 15 - 30 minutes, then leave turned off and leave to stand for a further 15 - 30 minutes to allow any air still in system to settle, then, and only then, check all of the radiators for air.


NB.
Radiator bleeding should only be done with the heating turned off, or you risk the chance of actually sucking in air.

[sevenlayermuddle]

Just one thing I'dd add to all that excellent advice is, if you mess around with a sealed system, try very hard when adding water not to over-pressurise at any time. 

It's not that it's dangerous to do so, as the pressure relief valve will open.  The trouble is, each time it opens,  crud  from  the system gets washed past it, and sometimes gets trapped in the seat.  They are then notorious for not sealing properly again.

Or so I've been advised by several plumbers, some of them bad, but also some not yet known to be bad.

[oldfogy]

Although more than likely not relevant to the current system (Kitz's)

Be warned, the newer heat exchange type systems actually produce minute amounts of "ACID" which is ejected out of the over-flow pipe, that being the reason that the over-flow pipe does not just stick out of the wall like older systems, but goes down to ground level (or beyond).