kitchen sink bowl depth / waste pipe height

12 Apr 2004
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United Kingdom
calling all those in the know:

just checked the waste plumbing on my new 1.5 bowl ceramic sink and am finding that the water in the main bowl is (too) slow-draining.

the main bowl has a depth of 20cm. Fitted into the work top, this leaves it a minimum of 17cm from sloping horizontal waste on the wall at the back of the unit.

Total length of the horizontal waste is 2.6m, other inputs to it are a p-trap for washing machine (precedes main sink bowl on outflow route) and a separate p-trap for smaller bowl (comes after main bowl on outflow route).

No problems on drain rate of smaller bowl, and although I've discovered that trying a new trap configuration on the large bowl helped to a degree, it's still too slow.

I suspect it's just too deep for the waste pipe height (sadly, changing this is not an option). Any ideas? Is there some kind of formula/ratio relating to downward flow and distance that needs to be taken into account when buying a new sink? (Looks like that might be the next step!)

Thanks in advance for your time
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Waste water pipes should go downhill (!). The gradient quoted is 1 in 40 (one inch per metre), but on kitchen sinks etc, steeper is better. Why is it not an option to make the waste pipe lower?
Yep, gradient subscribes to that standard pretty much exactly.

Lowering waste all way to vertical stack would be too disruptive, alas. Shallower sink, if necessary, is lesser of two evils.

Would you say then it's just a case of height and slope, and I don't have enough of a combination of these?
what about the diameter of the pipe. Presume this is 40mm.

Could try anti-syphon trap or air admittance valve on waste pipe.

Why not run the waste into a bucket until you identify the bottle neck.
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On trying a 3rd alternative trap configuration (initial drop lowered a fair bit with a telescopic fitting), and filling washing machine trap at start of waste (it had been open before), the problem seems to have resolved itself.

Panicked a bit there: never could get my head around physics at school so was at a loss as to the 'why' of the problem. When things are wrong I hanker after science, but when they work I'm quite happy with a little bit of magic (ie I get to keep my fab sink!)

Thanks for suggestions guys

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