Leak at UFH blending valve

7 May 2019
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United Kingdom
I have a Reliance Thermoguard UFH blending valve. Flow and return connections on the valve are 3/4" BSP F and boiler pipework is 22mm copper.

I am using a bog standard brass 22mm compression to 3/4" BSP M coupler to join the pipework to the blending valve.

Unsurprising to me, the BSP threads weep, despite application of PTFE and thread sealant.

What's the right way of doing this connection? BSP taper, something with O-ring? I'd rather not just apply more welly doing it up. Its pretty tight already.

I knew it was going to leak when I put it together but couldn't find a bits to do a better job.

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1. 3/4" fibre washer(s) to let flat face of iron mate to flat face of valve via fibre washer seal?
2. Loctite 55 per instructions on container after roughing up male threads gently with a file.
3. Clean all thoroughly and Flomasta Pipe Seal (Screwfix 4373J)
Gas PTFE tape, because it's thicker, works every time for me.

Don't overtighten the fitting, it's a 'feel' thing. When you feel the coupler bite with enough resistance .... stop ..... don't keep tightening it up.

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