Kitchen waste moved to underfloor drain - vent?

29 Apr 2017
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United Kingdom
I’ve recently relocated my kitchen sink (and associated dw and washing machine) so it will now discharge directly into the soil pipe under the house. Previously it went out of the wall into an outside drain.

Does this mean I will need to install an air inlet valve? I believe there’s a vent out the roof on the first floor bathroom but it’s not clear to me if that’s enough venting for the whole system.
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Not much to see yet, but:


The waste runs under the floorboards to the soil pipe that runs from the upstairs bathroom out to the main drain to the street.
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I’ll google, but help for a DIYer would be appreciated.
You could put a small inlet valve on the sink pipework - fit it high up under the sink top above the overflow from the sink bowl.
Just because you can buy diy plumbing products off the shelf dont mean you can dismiss the relevant regulations.Its your call (y)

I’ve read the document and it’s not terribly clear. It’s also good to know what to ask my plumber for. They’ve not suggested it’s required but I generally only know what to ask by asking here or googling. I appreciate the warning. I’ll ask them to explain the relevant document to me.

Before you say, they are very highly regarded so I trust them but it never hurts to ask.
Ventilation for the drainage system will be fine with a single vent on the system as you describe. This is normal practice, and as long as the drain is ventilated somewhere on the property, that is unlikely to cause any problems.

The issue arises with the alteration of your sink waste. Does the waste pipe now exceeds the maximum length permitted in Part H, and is it connected to the soil pipe correctly? 3m is maximum length of run for a 40mm sink waste, (can increase to 4m if pipe is upsized to 50mm), and connection to the soil pipe must be minimum of 450mm above the Invert, (bottom point of the drain), to prevent back pressure when WC above is discharged.
That’s very helpful!

No, it’s only about a 150cm straight run and attached to the top of the soil pipe.

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