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I bought this house and the bedroom radiator pin was seized. I freed it but it became leaky; no amount of PTFE could repack the gland nut correctly. I have decided on replacing the TRV.

I bought a 15mm TRV but when I've opened up the old valve, the union nut on the new TRV appears much smaller than what's currently on my radiator. The 15mm pipe size looks correct.

Photos show the old valve/union connection, the new 15mm TRV, and the radiator tailpiece.

Is my current setup using a 15mm union nut while the new TRV has a 22mm union (measuring 30mm at the nut, 26mm on the other nut)? Do I need to buy a TRV specifically with a bigger union connection, or do I need to replace the radiator tailpiece?

30mm nut and too large pipe

26mm nut unopened

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Both of those connections are compression fittings.
You need to replace the union with a 15mm radiator tail, like this:

enter image description here

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  • Thanks for the input. Which tools would I need for that? A torch? Commented Nov 27 at 20:37
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    A couple of spanners are all you need. If you've never used a compression fitting before, though, you should read up on how to do it. Essentially, make sure you use the olive, finger-tighten and then tighten a further 1/4 turn with the spanners. Only tighten further if it leaks - another 1/4-turn at a time. The worst thing you can do to a compression fitting is over-tighten it. Commented Nov 28 at 9:48

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