air admittance valves

Hi,
I have heard that AAV should be fitted when a pipe connects to a soil stack, to prevent siphoning of the trap. Is it wind blowing across the top of the soil stack that causes the siphoning or is it changes in the pressure in the sewers?
Are they only needed on shallow traps because I would have thought most basins and baths are connected to a boss on the soil stack these days and most traps I have seen under sinks are not anti-siphon ones.
The reason I ask is that our downstairs cloakroom smelt of drains the other day and the only thing I could think is that the trap which is connected to the soil stack had been sucked dry, though considering the nice weather we have had, it could have been a combination of that and evaporation too. I didn't actually look how much water was in the trap but I ran the tap to top it up and the smell went so I think that was what the problem was.
To fit an AAV like this: http://www.toolstation.com/shop/Plumbing/Air+Admittance+Valves/Air+Admittance+Valve+White/d20/sd3136/p54349
Do I just fit it to any accessible horizontal run of waste pipe or does it have to be at a certain location?
TIA
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wibbled on Friday 25 June 2010 16:51

Hi
Depends. Do you have an outside main vent (open ended pipe up top of house)?
That will deal with any general pressures (which may be +ve or -ve) in the sewer and your branch.
The AAV is really about local suction effects. When you flush a load of bog paper + gallon of water down a 4" pipe, you create a piston which as it goes of course creates suction behind it.
That will be powerful enough to suck traps out unless your loo pipe is trivially short (before it connects to a vented section like the main outside downpipe).

Any trap. Bath traps and shower traps are often shallow out of necessity (eg 40mm seal depth).

It's likely.

http://www.toolstation.com/shop/Plumbing/Air+Admittance+Valves/Air+Admittance+Valve+White/d20/sd3136/p54349
Depends. Needs to be above the flood line of the loo at the very least. If it is a Floplast valve then theirs have an extra rating that allows them to be blow the flood line of any connected appliances (basin) but still must be 200mm above the lowest wet connection to the main branch (in practise just a bit higher than the loo rim).
Ideally you'd fit a 110m AAV next to the bog, but one like yours is better than nothing.
In which case, stick it vertically off a tee to the pipe under the basin so that water cannot run into it in normal operation.
Another option is switch your basin trap for a HepVo.
HTH
Tim
--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes.
Thanks, I hadn't realised flushing caused it. I always thought it was wind across the top of the stack.

When you say connected appliance do you mean connected to the aav or connected to the stack? This basin is downstairs, so if the latter it is beneath all the upstairs appliances. Would flushing the upstairs loo drain the downstairs trap or is that too far away to affect it?
If we are talking about connected to the valve, then the basin is the only thing connected on that branch and the basin pipe work is above the height of the toilet seat.
What is the flood line of the basin though? Surely it is the top of the basin? In which case I can see why you say to put it on a raised spur from a tee in the waste pipe but is that really necessary? I have seen antisyphon traps and obviously traps fit beneath the level of a sink rather than above its flood level; why doesn't that cause problems?
Thanks again.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wibbled on Saturday 26 June 2010 22:04

Excellent - so you really are only dealing with the effects on the pipe betwixt bog and main stack.
You'll probably (almost certainly I would have thought) get away with a 32/40/50mm AAV rather than a full on 110 device. Unless your bog branch is very long (like half the house) - is it?

OK - this is less obvious now...
I thought the basin and bog were together.
Can you describe how your 4" branch pipe runs from the loo and to which bit the vent to the open air at roof height is connected.
Do you have a blockage in the air vent up top? Like a birds nest?

Yes
It is necessary to put any AAV raised above the flow of water in the pipe - so at the very least, put it topside of the run under the basin, not horizontal or bottomside.
Some AAVs require to be sited above the flood line of all immediate appliances (ie top of basin). The Floplast does not - it has some extra BS standard. It does however, want to be above the flood line (rim) of any adjacent loo for obvious reasons.
I think it would be worth you having a look at:
http://www.floplast.co.uk/uploads/Air%20Admittance%20Valves.pdf
That has a most excellent diagram of AAV siting.
I'm a bit lost on what your system looks like, so it's hard to be specific.
Cheers
Tim

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Sorry, I was confusing things. You are right to think that this is a downstairs toilet and basin. I was just wondering out loud whether the downstairs basin could be sucked dry by the upstairs toilet flushing. Now that I have re-read the posts, I think that the sucking only occurs at connections before the vented soil stack. If the upstairs loo and downstairs basin both connect to the stack, as they do here, I think I am right to say that because the stack is vented, downstairs will not be affected by upstairs. Am I right?
Back to the downstairs toilet, I am not quite sure where that connects. It disappears into the concrete floor so I don't know where it goes after that. We have two manhole covers so whether the downstairs loo connects to the soil pipe or whether it connects to the sewers separately, I have no idea.
This makes me think that the smell was perhaps caused by something else. Perhaps evaporation in the dry weather, exacerbated by lack of use of the basin (we didn't use the facilities whilst redecorating it). It may be due to pipe length, as suggested in the other reply.
The pipe is 1.2m horizontally but then it does have a vertical drop to align with the boss on the stack. I guess the vertical section is about a foot-eighteen inches. Since this falls foul of the 22mm per metre, perhaps this drop is causing the siphoning?
I guess the regs did not exist when they built the house in the 70s.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wibbled on Tuesday 29 June 2010 17:12

It *might* depending on whether the upstairs loo connects to an unvented internal stack that also picks up the downstairs loo before it hits the vented section.
If however, you have an outside main stack with the vent on top and the upstairs big goes through the wall into that, it's unlikely - unless the vent is blocked (it happens - birds). If you can see a bird guard on top though, it should be OK.

If you lift the lids and get someone to flush each loo in turn you should gain some insight into what goes where.

You could do an experiment. Get someone to flush all the loos, one by one - lots of bog paper too to get a good "ram". You stand by the basin. If one of them is causing major suckage, you'll hear it as slurping - it will be quite obvious.
But I think it's more likely to be evaporation now you said it was unused for a while - does happen and it stinks when it does. Have you ever had the problem before? Badly designed drains would be expected to cause ongoing problems, not a spurious one off.

Fresh water down a basin - I doubt it. It will tend to run down the sides of the vertical rather than form a solid column of water. That's been my experience anyway.

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes, that's what we have.
[about manhole covers]

They are big concrete ones and look impossibly heavy, so I've put off ever lifting them ;)

We haven't had a problem before. I think it must have just dried out partly because we didn't use it for two or three weeks whilst we were redecorating and partly because it has been hot lately.
Thanks again.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
[Default] On Fri, 25 Jun 2010 16:51:16 +0100, a certain chimpanzee,
wrote:

Approved Document H gives limits on the falls and lengths of branch (waste) pipes of certain diameters. For a wash basin, a 32mm diameter branch should be no more than 1.7m long for a fall no steeper than 22mm per metre.
If you exceed these sizes/falls, the branch pipe should be ventilated. One of the options is an air admittance valve.
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
randomly hit the keyboard and

Probably better using a HepVo in that case. Then the fall does not matter so much as the descending water is not going to draw water out of the HepVo trap as it is dry.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.