Kitchen sink vent not working

[Sadruddin]

I'm in a new build. We have a sink and dishwasher venting into a 110 pipe. I have a reducer on the pipe to 40mm and am having trouble with a vacuum causing the sink to drain very slowly. So I've put in a vent pipe with a macalpine vp2w venta pipe on top of it. It's near as dammit vertical, but I still get a vacuum, and slow drainage, unless I prop the air release tab open, in which case I get good flow but bad smells.
This is driving me crazy. Any ideas, please?

 

[just pumps]

The problem is within the pipe from the sink to the 110 pipe if all other traps are draining well. Remove trap from sink and clean then use a snake to clear the pipe. If you can see the pipe, check it has no dips over its length.

 

[Sadruddin]

It's the only pipe draining into that waste.
Sink outlet (and dishwasher outlet) go to u bend, the pipe (which is actually vented) runs down to t join, bottom going to 110mm bung type reducer, top to vent pipe. Pipe goes behind dishwasher, but no blockages anywhere in the system.

 

[martygturner]

I know this will sound a bit dumb but why is your outlet the highest point ? Normally its the lowest point after the trap.

 

[conny]

This is why you have a slow drain problem.
You need to replace that U bend with a shorter one and lower that rear pipework to just below the level of the flexible pipe on the right.
Currently you are trying to flush waste water uphill by the force of gravity and it simply ain't going to work effectively.

 

[tell80]

OP,
As above, & much of what I see in that under the sink plumbing is wrong.
You need to dismantle the whole plumbing set-up below the sink including the sink waste, & take advice from on here about how to plumb it back together with little chance of smells or leaks.

Is the soil pipe internal or external?
Is the W/M on the right hand side like the D/W? Where/how does the W/M discharge to?
Are you prepared to pull the D/W out to expose (for a pic) the run of the sink waste pipe & its connection to the soil pipe hub?

 

[Nige F]

It's the only pipe draining into that waste.
Sink outlet (and dishwasher outlet) go to u bend, the pipe (which is actually vented) runs down to t join, bottom going to 110mm bung type reducer,

The bung reducer - is it on the top of a 110 pipe going through the floor - like a diaphragm ?

 

[Madrab]

No wonder it isn't draining correctly. The U bend part of the P trap should be turned the other way around so the downward leg is the longer length of the u bend and the short leg is then upward to the elbow into the outflow pipe heading right and that outflow should have a suitable fall.

The outflow connection to the stack will obviously have to drop by the same amount.

 

[Sadruddin]

OK. So. Done that. But.
No success.
Photos enclosed.

Still slow, unless I prop open the tab under the air inlet. Still unsure what's going on.

 

[Madrab]

Can only be positive pressure then, if the AAV doesn't sort it. There has to be a restriction in the waste/drain somewhere and when the pipe is full when draining it can't get past the resistive pressure. Take the pipe out of the push fit where it drops into the soil pipe, pop that into a bucket, half fill the sink and pull the plug, if it drains properly then the soil pipe has a restriction further downstream.

You didn't need to put the vent in BTW, if an antivac trap has been fitted. Also be careful about using pushfit, it has a nasty habit of blowing off when a plunger is used

 

[Sadruddin]

I'll give that a try.
But if the aav is completely removed (and left open) it drains really well. That said, I'll give it a go.

 

[Laurieo]

Does the sink have an overflow ?,is it blocked.

 

[Sadruddin]

Does the sink have an overflow ?,is it blocked.

It does have an overflow and it isn't blocked

Can only be positive pressure then, if the AAV doesn't sort it. There has to be a restriction in the waste/drain somewhere and when the pipe is full when draining it can't get past the resistive pressure. Take the pipe out of the push fit where it drops into the soil pipe, pop that into a bucket, half fill the sink and pull the plug, if it drains properly then the soil pipe has a restriction further downstream.

You didn't need to put the vent in BTW, if an antivac trap has been fitted. Also be careful about using pushfit, it has a nasty habit of blowing off when a plunger is used

OK. So. It looks like it's in the drain. I didn't remove the pie out of the push fit, but I checked where the drain goes into a manhole in the garden. It is presumably blocked, as it doesn't seem to be running into the main drain. If I have the tap running into the sink, it doesn't show in the manhole about five metres away. I'm guessing we have a serious problem now, as any water from the sink and dishwasher have got to be going somewhere....... Help?

 

[Laurieo]

Do the other basins flow into the main drain.
Your kitchen waste might be plumbed into a rain water drain or soak way .
Try rodding back through main drain or take off the fittings in the kitchen and rod from there .

 

[Ltnman]

If it is a new build get on to the house builder to fix it.