This is the proposed new layout for a dishwasher waste with the waste hose entering at A. All waste pipework is 40mm.
Sink || P trap A || || || || ||==========P trap || || External drain
Is there anything which governs the minimum length of the vertical pipe between A and its associated P trap?
When the pipes are new and clean, the stand pipe can be just high enough for the end of the drain hose to be clear of the water in the trap.
The problems come if things start to get clagged up and the water starts to meet some resistance as it flows through towards the drain. That'll create back pressure which'll cause the water level to rise in the stand pipe when the dishwasher is pumping out. Obviously, it'll be bad if the water level makes it to the top of the pipe, so it's just one of those cases of the longer the better.
"Standard" stand pipes seem to be about half a metre long. I've never known one to overflow.
If you're worried that your stand pipe might be too short for comfort, have you considered changing the sink trap for one with an appliance inlet and doing away with the stand pipe altogether? Then any overflow will then just come up into the sink. You can extend the dishwasher drain hose if needed.
Cheers,
Colin.
Many thanks for the response.
As you have gathered the height is a bit of a problem without getting fitted kitchen units out and making major changes to the current waste pipework. At the moment I have got 210mm but may be able to squeeze another 30/40mm. There is a water alarm fitted in the area (early problems with original water filter) so may have to rely on this to signal an overflow.
The reason for the shallow P trap immediately under the sink is that the cupboard space is taken up by an inline water filter and pull-out rubbish and recyclables bins so the over-riding need was for maximum headroom.
Another possibility is to use a closed inverted loop of pipe, with an air admittance valve on the top, or better still, a vertical standpipe with air admittance valve on top, and a directed T feeding the waste in near the top of it (directing it downwards, of course).
I have found an air admittance valve at Toolstation and will fit it at point A with an appliance tee between it and the associated P trap.
Many thanks for the advice.
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