Hoorah, I did just that and it’s all fine.
One snag was that the lockshield on the other side was seized and when I tried to use a bit of force on that, I cracked the bloody thing. I had a new one of those handy too so I changed that. Topped up the pressure and haven’t needed to bleed the system yet. Running it up to temp at the moment and I’ll go round checking for air in rads when it’s cooled down although I do have an auto air bleed valve in the system.
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Over the years, I’ve replaced a couple of the TRV heads and I’ve always bought a new complete kit with TRV and lockshield valves as it was only a few quid more than buying just the head - I was advised to do this by the bloke in my local plumbers merchants. Turned out he was spot on.
I drained probably half a bucket of water out - do you think that would have significantly diluted the inhibitor?
Edit: Now that the fresh water from the top up has had a chance to circulate the system for 20 minutes, I’ve drained off half a cupful from my mag filter and dropped a steel nail in it, along with half a cupful of tap water in another container as a test and I’m going to see if the system water makes the nail rust. If it doesn’t, does that mean it’s okay?