Toilets and air admittance valves

[jonah]

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! 🙂

 

[Mike]

2 hours ago, jonah said:

Fitter #1 says we must keep a full-size AAV and it must go 200mm above the height of the toilet/sink

If you have a WC, you need this (the height just needs to be higher than the highest bathroom fitting, so potentially a bit less than 200mm - check the AAV instructions).

 

A small AAV on the sink is only protecting the trap in the sink waste.

 

Edited February 22, 2023 by Mike

[Conor]

Yea, the AAV needs to be on the main 110mm stack and higher than the highest trap.

[Nickfromwales]

You can reduce to 2x 50mm pipes and rise to the attic, or 3x 40mm ditto, or 4 x 32mm ditto. They can be hidden in a stud wall and will have the capacity to deal with the same volume that the 110mm AAV provided. I would, at that stage, also install the mini AAV on the basin ( anti-siphon trap ) to further help out.

Fitter #2 is wrong, and is using "wishful thinking", as there is no way that the 32mm anti-siphon trap is man enough to do the same job. The AAV is tiny on one of those.

I've been redirecting SVP's to AAV's like this for a couple of decades, works fine and BCO have never said a word against it. Use compression AAV's not solvent weld as these do fail now and then. They must be accessible to, so factor that in when relocating them.

[jonah]

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
 

[markc]

AAV’s are needed to prevent siphon pulling water from bath or sink traps when toilets are flushed. A toilet flush can/does create a considerable vacuum behind it. A AAV is an easy path for airflow balance this

[jonah]

29 minutes ago, markc said:

AAV’s are needed to prevent siphon pulling water from bath or sink traps when toilets are flushed. A toilet flush can/does create a considerable vacuum behind it. A AAV is an easy path for airflow balance this

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)?

[Nickfromwales]

51 minutes ago, jonah said:

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)?

B.Regs are indiscriminate, but they're that way simply to prevent people straying from them, executing their "unique" own interpretations , and then falling onto their own ill-informed swords.

 

You can redirect with 2x 50mm with ease. 

 

Crack on, or you won't get a sign-off. Simples.

[Mike]

5 hours ago, jonah said:

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.

 

Well my knowledge of BS EN 12380 : 2002 has improved. Valves rated Class A can indeed be installed below the flood level. However it looks like they're only tested under a 1m head of water - presumably a static head.

 

BBA certificates are the gold standard - with one of those you should be able to convince Building Control, provided you install it according to the Certificate. But the BBA search seems to be down and I can't spot one elsewhere for the Ventapipe valves.

 

However I do have one for Durgo (BBA certificate 06/4325), which states that the '82 mm and 110 mm valves are for use on discharge stacks serving up to 10 storeys' subject to a maximum flow rate of 37.1 litres/second. Their 82mm valve is A1 rated, so could be used below the flood level; the 110mm one that is normally be used is Class B, so couldn't be. You'd need a 110 to 82mm adaptor, and it would need to be on the soil stack.

 

That might give you some extra flexibility. However I've seen AAVs that have leaked foul air into a room, so although I might be tempted to use one below basin spill-over level, I wouldn't below the spillover level of a WC!

 

Edited February 24, 2023 by Mike

[jonah]

On 24/02/2023 at 10:37, Nickfromwales said:

You can redirect with 2x 50mm with ease.

 

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?(!)

[Conor]

The critical issue here is not the type of AAV, it's the location. It needs to be on the same stack as the toilet as that's where the vacuum will be generated, i.e. behind the toilet, so the AAV needs to be on the same pipe and "upstream" and above. A sketch would help here to see where the sink is in relation to the toilet.

[Nickfromwales]

2 hours ago, Conor said:

The critical issue here is not the type of AAV, it's the location. It needs to be on the same stack as the toilet as that's where the vacuum will be generated, i.e. behind the toilet, so the AAV needs to be on the same pipe and "upstream" and above. A sketch would help here to see where the sink is in relation to the toilet.

It only needs to be teed in immediately before the stack goes from horizontal to vertical ( drop ). This doesn’t need to be upstream, as any length of horizontal pipework will introduce a natural air break ( where the air flows up and over the body of water ) and the issue only becomes an issue after the pellet of water has started to fall rapidly south, creating behind it a vacuum. It is that vacuum which causes the air / water to be sucked out of anything with less resistance ( so a basin or shallow shower trap vs the toilet bowl ) in the absence of an AAV. 
 

2 hours ago, jonah said:

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?

If it was sufficient, nobody would be paying twice the price for twice the AAV   

 

I come up with plenty of good ideas / cheats / hacks ( as the kids call them nowadays ) and none of the get off the ground unless the BCO is happy. 
 

Ultimately, fit what you wish, and demonstrate it functioning to your BCO. If they’re happy it matters not a jot what I say, or anyone else on da ’net. 👊. 
 

Most BCO’s say to me “if the internal stuff ain’t right, they’ll be calling you back, not me” then they shrug their shoulders and walk off to look at drains.