Gurgling Basin Trap - Anti Syphon Trap Fluttering ???

[WhatNext]

Hi

I have a problem with a wash hand basin. It is on the 3rd floor of the house.

When you fill the basin with water and then let it go it pulls the trap.

The trap was a 40mm p trap - I changed this for a 40mm McAlpine anti syphon p trap expecting this to solve the problem.

Now the water in the trap is no longer pulled, but as the water drains the little valve in the McAlpine trap lifts and flutters rapidly up and down and making a noise that you can't ignore

I have enough space to fit a McAlpine air admittance valve ( VP3 ) ( picture attached ) between the original p trap and the wall.

As this AAV will be horizontal with no stand up will this cause an issue ?

When the WC is flushed there is no effect on the wash hand basin trap. The wash hand basin is above the level of the bath.

Many thanks in advance.

 

[Madrab]

If it's a basin then it won't be a 40mm trap, it'll be a 32mm. Installing another inlet valve will probably just do the same.

That aside has it always done it or is this something new?

 

[WhatNext]

Hi there no it is definately 40mm - there is a cross over from the bottom of the sink waste - no sure why this was done.

I think it has always done this - not long moved into the house.

 

[Madrab]

Not sure what you mean, a cross over? Do you mean a reducer attached to the waste then the trap fits to that? If so then there's even less reason that it should be pulling on the trap given the larger pipe diameter, unless it's an unusually long pipe run.

If that's the case though then there's not a lot you can do about it unless there's a restriction somewhere in the waste pipe or check out the stack for restrictions/faulty AAV.

 

[WhatNext]

Hi many thanks for your reply.

Yes there is a reducer fitted to the bottom of the waste which allows the 40mm p trap to be fitted and then use 40mm pipe. As you say when I saw this I thought that it might have been fitted due to long branch lengths, however I have no idea where the branches run under the floor. I am only guessing that they are long looking at the lay out of the bathroom and where the soil stack is.

There is a single soil stack which is vented to the outside via the attic space up to an open vent on the roof.

I did try the VP3 air admittance valve and as you correctly said it "fluttered" as well. I am assuming that it opens, breaks the syphon, closes and then repeats until all the water has passed through the pipe.

If only secondary ventilation back to the soil stack had been run even in 32mm then this would not be happening

 

[Madrab]

There should be no need for secondary ventilation back to the stack unless the run is over the max limit or there isn't sufficient fall on the run/too many bends. If it's all fitted properly then it shouldn't be overwhelming the trap as there shouldn't be enough water in the basin to cause that kind of problem.

So either the pipe run isn't installed correctly or there may be a restriction somewhere.

 

[Nige F]

You could go real retro and fit a puff pipe out through an outside wall - using 22mm overflow pipe

 

[WhatNext]

Morning - thanks for your replies - much appreciated.

What is the maximum length of a run for 40mm pipe from the basin to the soil stack ? I take it that there is a formula of sorts to account for the number of 90 deg knuckle bends that are incorporated ?

 

[Hugh Jaleak]

Regs say 3m max run for 40mm pipe from a basin, although in practice this is often exceeded. Bends, no max number permitted, but most installers have the savvy to keep them to a minimum. In any event, swept bends are preferable to the 'knuckle' type.

 

[WhatNext]

Hi thanks for your reply - I can never figure why swept bends are not the norm - I hate seeing knuckle bends used when there was clearly space to fit a swept bend - from what I can see the bends are the same price.