Changing both rad valves

[CoinBa1982]

I’ve been renovating toilet suite and part of it’s replacing the radiator and both valves. Both valves are turned off atm with old rad removed and system has been running for a few weeks as normal while I was waiting on new rad and valves to arrive. I’m confident in fitting the new rad and would ideally like to not drain the full boiler system down. What’s the easiest way to swap the valves over so that the water doesn’t shoot everywhere all over the new painted walls

 

[Mottie]

Do you have a sealed or open vent system?

 

[CoinBa1982]

Sealed, Combi boiler system with no water tanks in loft.

 

[kidgreen61]

I close the flow/return valves on the boiler which preserves the pressurised water in the exp vessel, and prevents air being drawn into the auto air vent.
You just need to depressurise the system via a drain and crack on.
If you have a enough play in the pipes, it's easiest to undo one side and fit a cap to rad tail, that way you can do it and only lose a few drops of water.

 

[CoinBa1982]

Could I depressurise by undoing both nuts on old valves and let water trickle into a bucket. Would I be losing much water in the system by doing this?