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Dishwasher Waste Pipe Advice as it's Causing Siphon and Damaging Motor

Postby MarkDOT » Sun Apr 16, 2017 1:48 pm

Hi there, I'm new to the forum but would thoroughly appreciate anyone's advice on the problem we're having!

Our dishwasher was working okay - but not great - for quite some time but eventually failed. The manufacturer repairer told us that the waste pipe was creating a siphon that was effectively hurting the motor inside the dishwasher.

I've been reading up about air admittance valves and dishwasher high loops and decided to have a look at our existing configuration.

Attached shows the dishwasher running into a waste pipe that existed long before we bought the house. The waste pipe then runs along (at ground level) behind a couple of units before draining into the waste pipe using a self-tapping valve. It is not draining into a u-bend or anything (the sink is above).

Is this setup - at present - correct and safe? My understanding was that it needed to drain into the 'clean' side of the sink u-bend? Is the fact that it's not causing the siphon? To me it looks like a bad job / solution but perhaps this is what is acceptable?...

Can this be fixed or does it need a new pipe running directly into the sinks' drainage as with other installations?

Any advice appreciated! Thank you very much!
Plumbing Setup.jpg
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Simply Build It

Postby Southsidebobby » Fri Apr 21, 2017 1:04 am

Have you ever siphoned petrol of of a motor car fuel tank, using a long piece of plastic tube? All you do is push the tube below the fuel level in the tank, suck on the end of the pipe, and the petrol starts to flow on its own as long as the end of the tube is lower than the level of fuel in the tank? Well, that is what is happening to your dishwasher! The drain pipe is one continuous piece of pipe, with the drain lower than the water level in the dishwasher, so when the machine fills, water will start to drain out, often as fast as it is flowing in, all by itself without the drain pump running! This will give terrible wash results as the detergent is flowing away down the drain, and the machine will constantly be refilling with cold water. The answer is that you must have an air break in the pipe, higher than the water level, to break the vacuum that is causing the siphon. That's why they use a large diameter waste pipe with the dishwasher drain pipe pushed in, with an air break. It's no use lifting the pipe higher, as the siphon will not be broken. Look carefully at the manufacturers installation instructions on how to fit a waste pipe with an air break. I found this situation thousands of times in my working life!
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