Difficulty rating : Changing a TRV?

[mjd]

I need to replace the TRV on the radiator in our kitchen. Our collie chew the thing to bits last year, and I'd like to replace it before the winter.
The rad is next to the back door and there is a drain point in the pipework immediately feeding the rad.
All existing TRVs are Siemens, so I plan to replace with the same or at least similar. Boiler is a Worcester 28si II if that's relevant.
Also during one of BG's weekly visits a year or so ago, when the boiler was on the blink, I believe the engineer drained the system, and subsequently I suspect there is no inhibitor in the system anymore.

So just how difficult is the procedure of fitting a replacement TRV?
and given I am draining the system, is it straight forward to add an inhibitor? (I'm assuming this goes in first)

 

[mjd]

Just went to have nose and sure enough, it came off with a quick twist.
Wish I'd asked this question sooner.

Can I still add an inhibitor without draining down completely?

 

[mjd]

That's great - thanks for the info.
The boiler has a filling loop, so should be straight forward.

 

[mjd]

Thanks for the tip.