Dishwasher standpipe overflow

22 Sep 2008
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United Kingdom
Problems with Overflowing standpipes - I thought I would add my own experience with a 4 year old Bosch dishwasher and an overflowing standpipe. We had the problem for months. Having dismantled and checked the U trap several times at the bottom of the standpipe (virtually clean as a whistle)and still got overflows, I was convinced it had to be either the dimensions of the standpipe (which seemed correct) or the machine itself somehow. We had put several machine cleaners through on the hottest cycle and that made very little difference. We use liquid detergents and, having hard water, salt as per instructed.
In the end, before radical surgery, I thought I should make sure again the waste pipe was not actually blocked and this time dismantled the plastic waste pipe just beyond the U trap, before it joined the waste pipe going outside. This was fairly inaccessible under the sink and I had concluded it was probably not possible for this area to be blocked. Hallelujah. It was almost completely blocked for several inches by a hardish white deposit and thick black sludge - bar a very small hole say 5mm in diameter. I attach a photo of this just to prove how much blockage you can get. The sludge has all evaporated leaving a white crumbly deposit which I gather is detergent - it is not limescale.
So as some basic advice to readers, as my father in law always said - look for the simplest thing first, even if it does not seem likely.
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