I have just fitted a new bathroom suite which looks great, however I also want to replace the rad and valves to complete the new look. Turning off the water and putting in new pipework for the suite was easy, however I have a sealed, pumped C.H. system and want to try and avoid draining the system. Next to the boiler is a tail with isolation valves either end, to adjust the pressure in the system to a working pressure of 1.5bar also a valve under one end of the tail for releasing the pressure. If I isolate each rad (I am in a bungalow so all rads on one level) and release the pressure to nil, besides the water in the rad that I want to remove, will I get away with little water loss when I remove the existing rad valves or will I flood my floor? I have an outdoor drain tap, but if I did drain the system is it easy to refill it by simply opening up the tail valve next to the boiler? Help. I am not a plumber just a good DIY'er.

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To refill it have someone open an air bleed valve on a rad while you let the water in (using the valves that you normally use to repressure the system). When they start getting water coming out of the air bleed have them close it and move to another rad and repeat. You could do it on your own but don't just leave the filling valve open. You need someone to keep an eye on the pressure to ensure it doesn't go too high.

Before you start don't forget to put some corrosion inhibitor in the system (eg Fernox F1). They do a concentrate you can inject into one rad via an air bleed valve or it can be put in via the filling loop (the tail with isolation valves you mentioned). They provide instructions.  

http://www.fernox.com/files/Fernox/Content/PDF/English/HTSSuperconcentrate.pdf