are Screwfix 15mm isolation valves non leaking at the screw?

[Best]

Those isolating valves should NOT be fitted on heating pipes at all!
They can't take the heat and will leak and are too restrictive on the flow which will affect the flow if on any circuit with a few rads on it.
They are pathetic valves for any job anyhow.
Only morons fit them to heating pipes IMO.
A full flow valve might be okay, but I would use connectors, normally soldered copper fitting if possible.

 

[jaydebruyne]

Thanks for all your replies, I agree about it being a bad idea to put them under the floor, there are four altogether which blanked off pipes ends before rads were installed. I've decided to replace them all with straight connectors if the 20mm length difference can be made up by slack in the pipes. If not and they are not weeping, I'll leave them be! Shame they are so rubbish, I guess they are ok on cold water pipes maybe ?

Oh I didn't realise these were on CH pipes. Oh dear.. If I were you, I'd replace that section of pipe with soldered connections

 

[AJ16]

Good advice, I'll get them replaced asap! :-/