Changing TRV valve without draining the system

[curium]

I’ve been decorating my daughter’s room so removed the rads for painting.

The tails have got blanking caps and the TRV and lockshield valve are closed.

Before the room is carpeted I wanted to change the TRV to reduce any issues in the future with stuck pins and to have a more modern looking head that isn’t covered in paint.

The heating is provided by a combi-boiler.

I’ve watched a few YouTube videos and have seen that this can be done without draining the entire system provided the new valve is ready to install immediately and there are plenty of towels.

My question is in regards to jointing compound which I intend to use as part of a belt-and-braces approach. Screwfix offer 2 products from Fernox - *Water Hawk* and *Hawk White*.

Does it matter which of these I use on the valve or is either suitable?

Cheers!

 

[justigator]

Personally I never change a valve without draining as much of the system as necessary first. Don't like floods! With respect to your question, if the tails are in good condition or can be made so, I just use a little silicone grease around the olive; makes any future dismantling easier. If I really have to use a jointing compound, I find LS-X is excellent.

 

[dilalio]

If you're confident that the nut can be reused on the new valve (with a tiny amount of paste applied to the socket of the new valve) then just drain enough water from air vent or rad valve so that system pressure drops to zero.

You can then undo tail nut, keeping downward pressure on the old trv and then snatch it off and swap for the new one, then do the nut up and repressurise to test.

 

[ChrisJP]

You could also use freezing kits to form a plug in the pipe feeding the valve you want to change. These typically give around 30 minutes of work time before the plug begins to melt. I've used them successfully on several occasions

 

[CountryFan]

I’ve only ever encountered one issue when draining a heating system, but it was years ago - should have had towels then, however, I’ve changed plenty of TRVs on combi systems and never had to drain the system, only the leg.

Half the work is already done, so either connect something to the valve or angle a receptacle and open the valve, water should cease to flow in approx 60 seconds or less. Same applies to the other valve. This forms a plug in the system and is good for working on. Sometimes I’ve even had to cut the old olive off with a junior hacksaw, as per @dilalio the nut was a different size. Once reconnected and filled test for leaks. Ls-x is ok, as is the 2 mentioned by you and ptfe is ok too.