Venting of bathroom waste system

[trossachs]

Hi all, I have recently moved into a victorian flat, it is a 3 storey building with the bottom floor being a basement. My bathroom setup is that I have a sink which discharges into a run of 42mm pipe, I have a bath which tee's into this run. The pipe then is unioned with a small section of lead piping which feeds into the top of a 3" cast iron stack, via a 90 degree bend. The stack runs right down to the bottom of the property with the floor below tied into it. There is a separate 4" stack that sits beside it where the toilet discharges.

The 4" stack continues up through the roof to an open vent, but the 3" stack is not vented at all.

My problem is that when I run the sink I get gurgling noises from the bath trap. I have places a thin piece of paper over it and it looks like it is a positive pressure problem. I am assuming this is because there is no vent, and the sink when run is filling the pipe causing a pressure increase in the pipe. Given the bath has a shallow trap it wont take much increase in pressure to blow the excessive pressure through the water seal.

If I am right, I was going to run a vent line which would tee into the line after the bath and sink tie in's but before it runs into the vertical stack. This would run up through the wall into the loft and through the roof. Is there a minimum of vent size, i.e. would a 1/2" hose suffice or does it need to be 32mm/40mm. If I could get away with a hose it would save me having to rip apart wall etc.

Appreciate any advice guys.

Thanks,

 

[Hufty]

If there's room and it won't be visible, how about changing the shallow bath trap for a Hepworth HepVO trap (and 90deg knuckle adapter). Dry trap = no gurgling.

 

[Morris40]

why not just replace the traps you have with anti syphon traps, easy to do and small cost

 

[Hufty]

If the bits of paper are being blown off the bath plughole then somehow there's positive pressure at the bath trap. I wouldn't have thought an anti-vac trap would help in this case?

By the way have you tried cleaning out any matter from the section of pipe between basin and bath. Trick that sometimes works for pushfit pipe is to rotate pipe 90deg then run loads of water through.

 

[Morris40]

as already stated fit anti-syphon traps, sorted

 

[MJN]

as already stated fit anti-syphon traps, sorted

For positive pressure? How?

I'd go with a HepVO as Hufty suggested as they're pretty good at coping with low levels of positive pressure as the membrane closes tighter with the increased pressure and of course being waterless there's no bubbling/gurgling.

Mathew

 

[Morris40]

Still disagree, air admittance traps will fit as a straight swop so no pipework changes needed and are equally as good as dealing with positive pressure and the hepVO could be very difficult to fit under basin without much pipework alterations under the bath would be easier though.

 

[MJN]

Still disagree, air admittance traps will fit as a straight swop so no pipework changes needed

Again, how is an air admittance trap going to help with positive pressure? Please explain otherwise your duff info could lead the OP to wasted efforts.

We need to either allow air to be expelled (which we don't want to do internally for obvious reasons) or be able to accomodate it without breaking a seal (and bubbling). An air admittance trap does what the name suggests - admits air - this is of absolutely no use in this scenario of positive pressure.

under the bath would be easier though.

Yes, I should've clarified - that's where I meant.

Mathew