Not worth getting a freezer kit for a one-off AND still leaving yourself without a drain-off. Just in case, have you checked OUTSIDE the house as well as in all the obvious places inside? Some thoughtful installers provide external drain-offs. I wish more would.
You don't say whether the valve is seized open or shut. Anyway, it's probably not on the lowest rad in the system. You'll need: a spare radiator valve, a hose, a 15mm compression T and a new drain-off cock, plus other odds and ends. Read ALL the following before you start, so that you understand all the steps and have the necessary bits to hand.
- locate lowest radiator in system with easy access to tails below valves. In particular, you'll need to be able to 'spring' the valve away from the rad either to
- IF (and only if) valves BOTH work and look in good condition, turn both off and open air vent at top of rad.
- IF air and / or a bit of water comes out, then stops, the valves are both holding the pressure from the rest of the system and you can proceed.
- CLOSE valve on F&E tank supply or string-up valve to make sure no more water can enter heating system
- join valve to a short piece of 15mm copper tube, then push end of hose onto the end of the tube and fix with a jubilee clip.
- loosen the nut on the joint between radiator and valve (NOT between supply pipe and valve!) and drain water out of rad into a suitable container. WATCH OUT FOR BLACK, SLUDGY WATER!! Alternately loosen, drain, and retighten nut whilst emptying container until rad empty.
- when you're SURE it's empty, loosen the nut completely and GENTLY 'spring' the valve away from the fitting in the rad. If it won't come completely clear, you'll have to undo BOTH ends, lift the rad off the wall and move it out of the way.
- take the 'tail' off the new valve and fit the new valve onto the existing tail. Valve bodies and tail fittings usually fit together 'well enough' for this purpose - try to match the new and old valves as closely as possible: you may also need a bit of PTFE tape to seal the temporary joint.
- replace airvent screw in radiator. Make sure hose end is in a drain or somewhere harmless outside - the water will probably be black and / or soupy! Open other valve. Water should flow through the rad and down the hose.
- when the F&E is empty, loosen the airvents on all the other rads, starting from the top, and drain the whole system.
- fit the new drain-off to the T and cut the T into one of the vertical pipes feeding the radiator. There will be a small flow of water when you cut the pipe below the level of the rad.
- reattach the old valve to the rad and replace the other valve you wanted to change in the first place.
- don't forget to make sure ALL airvents AND the new DoC are SHUT!
- refill the system, carefully checking for leaks at all the places where joints have been undone.
- add corrosion inhibitor.
- vent all the radiators.
Allow several hours to do the work and then get the airlocks out of the system.
And before you ask: - Yes, this is why plumbers charge serious money for this sort of work. There's quite a lot to do and it can take a while!
