Adding air break to stop siphoning

[Madrab]

..... if it is a shallow bath trap then it would have been fine as long as the hopper was there, if the hopper is now gone then it really should go to a 50mm seal.

 

[marblerun]

Hi again. Took the first step of changing the pedestal trap. The basin is draining far better now. However the bath trap is still getting siphoned. Next step will be to remove the bath panel and check if it the trap is shallow.

 

[Madrab]

Is the bath trap definitely getting syphoned, or could it be getting blown? The anti vac valve should certainly be stopping the basin creating a vacuum when draining and then pulling on the trap, or is the basin upstream on the run?

 

[marblerun]

Is the bath trap definitely getting syphoned, or could it be getting blown? The anti vac valve should certainly be stopping the basin creating a vacuum when draining and then pulling on the trap, or is the basin upstream on the run?

Definitely getting siphoned. The two waste pipes combine on the outside wall as seen in 1st pic. Gravity then kicks in. The basin siphon is working as I can hear it in action.

 

[denso13]

As you mentioned, check the bath trap if you can. You can get an anti vac trap for that as well if it fits.

 

[Madrab]

Ah OK , should have checked back on the pic, that's the issue. Given the bath waste is downstream of the basin waste run, the slug of water from the basin emptying is rushing past the bath waste at the tee - given the basin waste is also 40mm at that point, that then creates a venturi effect and a vacuum in the bath waste run, pulling on the trap. In that scenario an antivac on the basin wouldn't really help.

Normally the basin waste would be tee'd in downstream of the bath, and the basin waste is 32mm so it's a lower volume running into a larger 40mm waste pipe, that coupled with the antivac would break the vacuum in that scenario, it won't work the other way around.

The bath run will need vented.

 

[ChrisJP]

We had a hopper before and it constantly filled with leaves even though we had a grill on it. Hepvo I’ll look into it.

Ahh....that hopper would have served as a vent and prevented the siphoning. Did a plumber change the hopper to unvented pipes? If so, did he warn you about the likelihood of siphoning?

 

[marblerun]

Hi again. I’ve eventually removed the bath panel to view the bath trap. Would an anti siphon trap work here and resolve the issue? Pipe run is very tight. See pics. Thanks everyone for the advice thus far.

 

[marblerun]

Ahh....that hopper would have served as a vent and prevented the siphoning. Did a plumber change the hopper to unvented pipes? If so, did he warn you about the likelihood of siphoning?

One plumber did the bath and basin. Another did the outside waste pipe run and pipe combining. No warning and can’t get the outside person back.

 

[Madrab]

OK - there's the problem, it's a shallow bath trap. Whoever changed the outside works and removed the hopper didn't check the bath trap. That would have been fine with a hopper, given it's now a longer waste run with a vertical section then it will pull on that shallow seal.

I don't think anyone does an anti vac shallow trap but there may be enough clearance there to get a standard bath trap in. There is a little pipe left on the downward leg where the elbow above could be cut off and the setup altered if needed to take a new standard trap with a vent.