At my mum's house the kitchen has a full-size steel sink with a half-bowl
drainer next to it. When the plug is pulled out on the larger sink water
flows away, but slowly, and some comes up into the half-bowl drainer.

Underneath there's lots of 2" (or so) diameter plastic pipe in various
lengths fastened together with screw-rings on some pieces threading onto
threads on other bits, with rubber washers, and in one or two cases fitments
that seem to use a compression washer and fit against an unthreaded plain
pipe. These pieces connect the half-bowl to the main sink's outflow, above
the trap, then the flow from the trap goes into plastic pipe that looks to
me as if it's solvent welded to adjacent bits.

This latter welded pipe starts at the extreme back righthand side of the
undersink cupboard. I cannot get any tool or my arm or anything else
further to the right). The pipe has a downward section (3" perhaps) then a
right-angle bend and flows from right to left by about 12", then another
right-angle bend towards the rear of the under-sink area and then it goes
out through the house wall. I think it then meets a cast-iron downpipe,
about 2 feet above a small access hatch in that 4" downpipe.

I've taken apart and cleaned all the screwed-together bits of plastic pipe;
they were a bit gungy but nowhere near blocked. On the other hand the fixed
plastic pipe that all the screwed-stuff is connected to feels to to me as if
it may very well be gunged-up, though I took as much gunge out as I could
with my finger.

I bought some "one-shot" drain cleaner at B&Q; it's clearly concentrated
sulphuric acid. I've read some reviews of it on Amazon; some people say
it's great but a significant minority say their use of it has burned through
their plastic pipe, or maybe through washers/seals leaving them with a
really horrible acidic mess to clear up, and I'm not keen to add that to the
problem. The instructions for the stuff say one pours 125ml of acid down
the sink, waits 5 minutes, then adds some water... but it seems to me that
all that that would do is replace the water in the trap by acid, maybe
diluting it a bit in the process. There's no blockage in the trap or before
it. Then more water would flush some or all of the possibly diluted acid
into the lower reaches of the drain, where some of it might linger long
enough to make a difference.

Maybe I'd be better to try to tip it directly into the solvent-welded
section of plastic pipe, rather than the clear section above the trap?

Maybe I'd be better not to use it all? Can one get bendy brushes (like
bottle brushes?) which can be pushed down a 2" pipe to extract gunge?

I guess I can open the small hatch in the downpipe, but ecen if there's
signs of any blockage there I can't see that I've much chance of doing
anything useful because the kitchen drains enter that pipe 2' higher up.

Any advice?

--
Jeremy C B Nicoll - my opinions are my own.

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