Weird trap seal problem

[Johnysmall]

I work on a council contract fitting bathrooms, kitchens and heating systems in West London.
The discharge hose for the washing machines must be put onto a spigot trap under the sink (no standpipes allowed).
I was called back to a recent property we have done to investigate a problem with the W/M waste coming up into the sink, this is quite common in the area and is due to the stacks or underground pipes being blocked due to years of abuse. This property was different though.
The sink water drained very fast, sucked the seal out of the trap and proceeded to gurgle on and on and on.....I shone a torch down the waste and could see about 5mm of water in bottom of trap.
The outlet from the trap immediately went vertical to the floor where it converted to 3" dia under the kitchen floor thus explaining the trap seal being broken. The tenant then ran the washing machine and as soon as the waste was discharged it filled the sink up, eventually draining away and sucking the seal out again.
I returned to the property and fitted an In-line Anti Syphonic Trap.....No success. Returned again and fitted an Air Admittance valve.....this stopped the seal being sucked out when sink waste released but the W/M waste still filled the sink up.
I'm baffled, at first I assumed it was air in the trap stopping the W/M waste going down...now I don't know.
Any ideas?

Sorry for the long boring post but wanted to give as much info as poss, any help will be much appreciated.

 

[Nige F]

Well, for what it`s worth ........I had a nightmare scenario with a w/m connection trap on a sink ...........and it turned out to be a carp design ..........the little spigot with the hose on it had a cup on the end ....ONLY visible when spigot removed from trap...........I`d got the sob upside down and it was catching a bit of water from sink discharge and taking it to the w/machine each time sink emptied...cut the thing off straight across , put it back and no probs wasted hours b`guring abaht on that one .................try using a hep V O waste valve, or 2 ....they won`t allow backflow at all

 

[ChrisR]

Sounds horrible. We've had something like this on the forum before,, but a long time ago. Only way I'll find it is if I can remember a keyword to search on...

All I can think of is that the 3" pipe might go uphill for a bit before going down again. Then you have a trap in the pipes, to fill, before it sucks itself out. See what I mean? If not I'll do a sketch.

So often these things end up being blockage-related, I'd be tempted to pour some Dambuster or similar down there.

Other thing I'd try would be to restrict the outlet from the WM , if it's slower it might go the right way.

 

[Softus]

I've had a very similar problem; as it happens it was also in West London.

It turned out that the 40mm waste from the sink ran uphill until it turned down to join a 3" waste, which itself turned on an elbow and ran into a 4" CI stack. The bathroom basin and bath waste also joined the 3" section.

The symptoms were that the kitchen sink drained into the bath, and everything was fairly slow to run away.

I was tearing my hair out until I discovered the uphill run behind the new kitchen units - the fitters must have lowered the sink end of the run without altering the stack end.

What continues to confuse me is that the sink preferred to drain into the bath instead of going down 3" waste. I was baffled then, and am baffled now, but the answer was just to correct the pipework so that there was the correct fall away from the sink. It instantly all worked.

I think I posted the experience on the forum, but haven't found the topic yet...

 

[Nige F]

Sounds horrible. often these things end up being blockage-related, I'd be tempted to pour some Dambuster or similar down there.

right way.

I`d run round the kitchen with me shirt over me head singing .......the dambusters song ..@ least they`d never call ME back

 

[Johnysmall]

Sorry, I never posted the outcome of this problem.

I managed to convince the clerk of works to allow me to fit a standpipe under the sink. The problem continued with the sink water flowing away freely but the washing machine waste overflowed standpipe almost immediately.

I then went with half a litre of D-Block, when I ran the sink taps 15 mins later, the acid spilled over the top of the standpipe.

By this time I have got the hump. I decided that if the next step didn't work I was gonna do something I don't like doing.............Give up.

I increased the waste pipe to 2", removed standpipe and replaced with a second air admittance valve. Put w/m discharge back in spigot trap and put a trap extension on the sink.

Worked a treat

So 4 visits covering a good 5 hours work to fit a kitchen sink, not one of my most successful installations.