Replacing radiator valves without draining your central heating system is something that can be achieved with a few simple additional tools (bungs), a heating system that's in reasonable condition, and a small helping of luck!
This is a perfectly safe & viable way to do small changes or radiator removal / replacement / new valves etc without emptying the system and avoiding needing to flush & refill.... Or so various plumbers seem to reckon at any rate!
Without further ado, the process is started by first either closing off the heating's feed & expansion tank outlet/feed into the heating system, or by fitting a "bung" into the tank's outlet pipe. Once done, repeat for the heating's open vent pipe.
Didn't have a suitable purpose-made bung available so simply used a speedfit 15mm stop end to seal the vent.
Next comes the first test of faith... With a suitable container strategically placed to catch water, crack open one of the old radiator valves, catch the water, and wait for the flow to stop.....
Assuming you've stopped the F&E tank supply from reaching the system & that the vent's sealed, once the water stops you have successfully produced a vacuum in the system & can start work!
If however the flow doesn't stop and rebunging / tightening valves etc doesn't help, I'd suggest you stop here & drain the system down below the level of whatever you need to work on!
As we're replacing the old rad with a new one that's about the same size, no other work is needed except replacing both valves & fitting the new rad.
Step 1 - cleanup the pipework.
Given the state of the old valves & pipes, it's a good idea to remove as much of the old paint as possible around where the new valve will seal as paint doesn't tend to help watertightness.
It's also much easier to do this before you've removed the old valves & fitted the rad to the wall....
Step 2 - brackets & test fit.
Determine where the radiator's going to be positioned & drill the necessary holes for your radiator brackets. Measure carefully - easier to get it right first time...
Once the brackets are mounted, fit the radiator & determine where the valves need to be positioned.
As a suggestion, attach radiator tails, bleed valves & blanking plate before mounting the radiator on its brackets.
Also always helps to ensure the radiator's level... Ensure a good 6-9 turns of PTFE tape are wrapped around the new radiator tails before screwing them into the radiator.
Step 3 - remove old valves & replace
Assuming you've got a good vacuum in your system, go ahead & undo the valve... I opted to simply cut them off as the new radiator was a little taller than the original (and therefore I needed lower valves).
Clean up the pipe end, ensuring you've got bare copper where the valve's olive will seal as otherwise you'll be faced with remaking the joint as you start trying to fill the system.. A scenario best avoided really!
Make up the valve, ensuring retaining nut & olive are on the pipe. Position the new valve body suitably angled/aligned for your radiator's tails and hand-tighten the connections.
Tighten everything up using a suitable pair of grips or wrenches, and ensure the valve is actually closed...
Then, repeat for the other valve.
Assuming that you're fitting a TRV to one end, ensure it's on the flow pipe unless the one you've got works in either direction & use the TRV head to close the valve..
Step 4 - feeling confident?
Unbung/reopen the F&E tank supply & unseal the vent pipe.
Assuming nothing's leaking yet open both the rad valves.
Check for leaks as water starts to fill the new radiator, tightening any connections as needed.
As it's filling, open the radiator bleed valve to ensure all the air's removed.
Once the radiator's full, close the bleed valve and assuming everything's still dry, switch on the heating & check the new radiator heats up.
Enjoy & file the idea away so that next time you need to replace a radiator or change a valve, you avoid emptying the system!