Bathroom basin waste unit leak

7 Nov 2021
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United Kingdom
I have just replaced a bathroom basin waste unit. Its leaking slowly - its wet on the outside of the thread, suggesting its leaking between the unit and the basin. The nut to tighten the seal on the base of the unit is hidden in and behind a porcelain pedestal making it impossible to tighten the nut on the waste unit without taking the sink off the wall. I have found no tool that will do the job with the basin in place.

So far I have taken the basin of the wall twice, tightened the nut and checked no leaks, but every time I put the basin back in place its leaks again. I even tried attaching the waste pipe before I reinstalled the basin in case I was inadvertently torquing the waste unit it when I reconnected the waste.

Any suggestions? Many thanks.
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Basin mates are great especially these days with wonky basin castings.

Either that or fill the gap between the shank of the waste and the throat of the basin waste hole with some silicone (not too much if it's slotted) and run a little around the threads that will sit below the level of the basin waste so when tightening the nut and washer up the nut will gather some of the silicone up and seal it all up.
The biggest concern with cold water becoming overly warm from other pies or sitting too long is legionella. This bacteria is commonly found in most fresh water and all it needs is the right conditions to multiply to large enough numbers to cause the immune system to be overwhelmed.

It lies dormant at temps below 20Deg and is destroyed in temps reaching and over 60deg, hence the recommendations of water storing and pipe run design contained in the regs.

Recommendation is always to run cold water to flush out any possible concentrations, especially if it's become warm or sat for extended periods of time.
THANK YOU all so much. Problem solved.

I ended up hack-sawing off the old nut on the waste unit because it was too tight, saved me taking the sink off the wall for a 3rd time!. Got the basin mate on without too much difficulty. Really appreciated - thank you again.

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