jonah

Thanks for the reply, but I still don't get why "2 x 50mm with ease" is OK without working out the numbers, other than that's just what has worked before - one wouldn't put in the biggest RSJ just because it would work, you'd calculate the required size and put in the smallest that works. And you wouldn't put in 2 smaller RSJs just because it would work. Surely with AAVs the same logic applies - we have discharge unit rates, air flow rates and the calculation shows a single 50mm is more than sufficient? I'm just wondering whether this is an area where everyone has just done the same thing since forever (110mm AAVs up through roof) because, well, that's just what everyone does and that's the size part that matches the soil pipe? PS. How would you put in 2 x 50mm anyway, do you run 2 separate vertical runs off a reduced soil pipe, each with 50mm AAVs? Or is there a 2-way splitter?(!)

Yes I understand that, but as per my post above the quoted figures from AAV manufacturers and BS regs suggests a 50mm AAV is more than sufficient for the airflow required by a bath, sink, shower and toilet all discharging simultaneously (which will never happen anyway)?

Thanks for the replies and the tip about compression AAVs. I understand that using a full size AAV on the main stack is what's ideal/recommended and we'd do that as first choice, but we can't easily do that in this bathroom. I still don't really understand why we need to do this given a toilet's discharge rate of flow is the same as a bath? There is a document here: https://bpfpipesgroup.com/media/29598/Air-admittance-valves-for-domestic-properties.pdf ...that talks about type A/A1 AAV's which are pressure-tested and can be below the flood level. It gives an example on p10 of toilet, bath and sink in simultaneous usage requiring a maximum combined 7.6l/s of air flow according to BS regs, so for example here is a 50mm compression AAV that is type A/A1 and has a flow rate that is more than double what is required: https://mcalpineplumbing.com/air-admittance-valves/ventapipe/vp50-100-ventapipe-50-2-3-and-4-pipe Why doesn't this provide enough air flow? (Even the 32mm one provides 6.5l/s which works unless you drain a sink, bath and toilet simultaneously.) Sorry I don't want to sound unappreciative of your extensive combined knowledge, I just like to understand things if that makes sense! I would appreciate it if anyone could explain why the above is not a suitable solution as according to the numbers it seems to be? PS. There was also a previous discussion here about using 50mm AAV's: https://forum.buildhub.org.uk/topic/4459-air-admittance-valve-soil-vent-pipe/page/3/#comment-195188

Hi All We have 2 completely conflicting "recommendations" from bathroom fitters and don't know who is right: We have 3 upstairs bathrooms/toilets in our house, on 3 different sides of the property - bathroom 1 has an external soil vent whilst bathrooms 2 and 3 have internal 110mm air admittance valves coming off the toilet (both boxed in within the room). We are on a septic tank so presumably the 3 soil wastes join up somewhere underground before there. We want to modernise bathroom 2 with a freestanding bath and hence want to remove the existing boxed in soil pipe that travels 2m across the floor to a full-size AAV that is in the corner of the bathroom. As well as looking much neater, we also really need that extra width for the freestanding bath to fit comfortably: Fitter #1 says we must keep a full-size AAV and it must go 200mm above the height of the toilet/sink (can't remember which but basically about 1000mm above floor). This means we either have to keep the ugly boxed in pipes or build a false wall / conduit up into the roof space, which there isn't really space or a place for due to shower and window. Fitter #2 says it's sufficient to install a small AAV on the sink waste hidden within the sink unit and remove the soil pipe AAV completely - this means all pipework is then below floor and out of sight, and we can have a nice neat bathroom with a freestanding bath. From reading here and elsewhere online there seems to be contradictory advice on which is right. Some say you absolutely must have full size 110mm soil waste AAV at 200mm above overflow; others seem to suggest that if you can't accomodate that then a smaller 50mm is OK, and it can even be below overflow level if it's pressure-tested type A? Other advice seems to indicate that as there is an external vent elsewhere on the waste system the smaller AAV would be OK, but is that correct even on a completely different arm of the waste system? I'm really confused - any advice would be very gratefully received thanks! 🙂