easy way of replacing broken lockshield valve

[bm212]

I have an upstairs radiator with a broken lockshield valve. The top of the valve (i.e. the bit you turn with a wrench to open/close it) has sheared off, leaving the valve in the closed position and making it impossible to open. The valve isn't leaking or anything but the rad is stuck in the off position. I was hoping I could replace it without having to drain down the system.

I was hoping I can simply shut off all of the upstairs rads (ie close their lockshield valves), turn off the boiler (and presumably the pump) and then unscrew the faulty valve.

Here's my reasoning (which may well be totally flawed as I'm making this up as I go along)...
- it's upstairs so no other rads are higher than it
- we have a combi boiler, no water tank, and the boiler is downstairs
- if the boiler is off, presumably the pump will be off too (vaillant ecotec 937)

Is this a really bad idea? when I searched the forums, people seem to advise draining the system completely? Can I use a freeze kit to freeze the pipework just below the valve? Is there a risk of causing damage by doing this?

any advice much appreciated

 

[namsag]

Turn the isolating valves off under the boiler realise the pressure within the system through letting water out of a bleed valve or cracking the union on the valve you are going to change .
you will get a few pints of water out then it will create a vacum and stop flowing then just change the valve

 

[GPlumb]

It would be better to drain it down tbh. Even if you just remove enough water to empty that radiator. 1.5 bar of pressure in a combi system which will make a mess of your house if you opened it without letting the pressure off a bit.

Its possible to change a valve "live" if its on a open vented system if your quick!

 

[bm212]

of course, the system is under pressure - you just saved me from installing an upstairs pond I think...

Re isolating valves under the boiler; if the boiler is lower than the valve I am opening, presumably I'm not going to empty the actual boiler, so do I need to close the isolating valves? I'm asking because I'm not entirely sure which are the isolating valves...

 

[namsag]

The isolating valves are on the two 22mm pipes either side of boiler jig, one in middle is the gas.
Reason for turnin boiler off is to isolate the auto air vent in the boiler ,if you can see the auto airvent then just screw the cap down on it.
The reason for this is for the vacum to form no air should get into system anywhere