Can Air Admittance Valves Stick Intermittently?

[endaxi]

Hi,

I started a thread some time ago about an intermittent odour in a bedroom that has an en-suite shower room. The toilet, shower and sink in there are all used daily. A few weeks ago I realised there was another shower connected to the same soil stack that is hardly ever used so I now run some water into the plug hole periodically to keep the trap full. I also ensure that every other plug hole in the house gets some water down it at least once a week.

Everything has been fine for a good three weeks but my wife said she could smell a faint odour again that soon went away when the window was opened. Just one faint smell reported in three weeks and nothing again since.

The soil stack has an AAV which is nearly twenty years old and solvent welded. When I flush the loo and look into the sink and shower plug holes there is no more than a ripple in the trapped water.

So, is it possible that the AAV is sticking just now and again and water is being syphoned from a trap thereby letting an odour out? Or do they just get stuck and never work again?

I had already bought an AAV and contacted a plumber asking if he could fit it but he never responded. So, thinking I had resolved the problem anyway I returned it and got a refund.

Any advice on this would be much appreciated.

 

[Hugh Jaleak]

Possible they can stick, but I'd also be looking at the possibility you are getting positive pressure building up in the system, which is then looking for the easiest way out, which could be the cause of your issue. AAV's do what they say on the tin, but if the is for example a partial blockage in the system, any discharge above the blockage needs to displace the air trapped in the pipe, so this air needs to escape somehow, if an AAV is blocking the head of the run, then an alternative route is found.

 

[endaxi]

Possible they can stick, but I'd also be looking at the possibility you are getting positive pressure building up in the system, which is then looking for the easiest way out, which could be the cause of your issue. AAV's do what they say on the tin, but if the is for example a partial blockage in the system, any discharge above the blockage needs to displace the air trapped in the pipe, so this air needs to escape somehow, if an AAV is blocking the head of the run, then an alternative route is found.

Thanks for this. Can I just confirm I've got my head around it?

I think that when for instance the toilet is flushed...

1. The rush of water will build up pressure ahead of it and a vacuum behind it. The vacuum behind will open the AAV and let air in to maintain atmospheric pressure, and if there's no blockage ahead then the water will flow freely through the pipe to the sewer.

2. If the AAV sticks closed then the vacuum behind the water flow will suck water from shower or sink traps thereby allowing foul air to come into the house through the plugholes that are now not blocked by water in the traps. When the traps are then refilled with water it will stop the foul air coming through.

3. If there is a partial blockage ahead of the flow then as well as the AAV allowing air in behind the flow, pressure will build up ahead of it and push foul air back through the water so that the air behind the water is also now under pressure... and foul. This pressurised foul air will then bubble up through the traps into the house every time the toilet is flushed.

Am I understanding that correctly?

If I've got it right then I think a partial blockage would lead to odours most of the time (which I don't have) whereas an intermittent sticking AAV would only lead to odours on the occasions it sticks... say every couple of weeks or whatever (which I do have).

If I'm thinking this through right, would you therefore recommend that I get a new AAV fitted at the top of the stack in the loft as a first course of action?

Thanks for your help with this.

 

[denso13]

would you therefore recommend that I get a new AAV fitted at the top of the stack in the loft as a first course of action?

As a first step yes, but you are overthinking the theory. Things like this can be hard to track down, but try a new AAV first.

 

[endaxi]

As a first step yes, but you are overthinking the theory. Things like this can be hard to track down, but try a new AAV first.

Thanks. Ever since I discovered there was such a thing as an AAV and that I had one I've thought that was the route of the problem. I just like to understand how things work and what problems can occur. If the problem was there all the time it would be easier to diagnose but it seems I just need to find a plumber who will be willing to do the job and then see what happens from there.

 

[Hugh Jaleak]

Thanks for this. Can I just confirm I've got my head around it?

I think that when for instance the toilet is flushed...

3. If there is a partial blockage ahead of the flow then as well as the AAV allowing air in behind the flow, pressure will build up ahead of it and push foul air back through the water so that the air behind the water is also now under pressure... and foul. This pressurised foul air will then bubble up through the traps into the house every time the toilet is flushed.

Am I understanding that correctly?

Yes, but in that instance it is unlikely there will be any negative pressure in the pipework so the AAV wont open. If there is a blockage downstream effectively sealing the pipework off, and the AAV is sealing the end of the upstream pipework, when you flush the WC for example, the 6 litres of water etc from that flush is going into what is now a sealed void. The water needs to displace the equivalent quantity of air, it cannot go downstream as the way out is blocked, and it cant get out past the AAV, so looks for the next easiest path out.

 

[endaxi]

Yes, but in that instance it is unlikely there will be any negative pressure in the pipework so the AAV wont open. If there is a blockage downstream effectively sealing the pipework off, and the AAV is sealing the end of the upstream pipework, when you flush the WC for example, the 6 litres of water etc from that flush is going into what is now a sealed void. The water needs to displace the equivalent quantity of air, it cannot go downstream as the way out is blocked, and it cant get out past the AAV, so looks for the next easiest path out.

Thanks for the explanation. I've got a good idea of what can happen now.

My soil stack at the front of the house goes right up to a roof tile that is vented and is barely noticeable. Why they had to put an AAV on the stack at the back of the house is a mystery to me. Such is life.

 

[Hugh Jaleak]

My soil stack at the front of the house goes right up to a roof tile that is vented and is barely noticeable. Why they had to put an AAV on the stack at the back of the house is a mystery to me.

Simply because it's often cheaper to fit an AAV than to break through the roof for an Open Vent.

An issue we find in the Drainage Game is people blocking off open vented stacks with AAV's, we come along to jet a nearby sewer, (which has been done on a preventative maintenance basis for years without issue), then to have the Occupiers screaming as we've just blown a load of filth out their toilet.... Simply because the bow wave of pressure that was escaping out the stack, now cant and looks for the next easiest way out.....

 

[endaxi]

Simply because it's often cheaper to fit an AAV than to break through the roof for an Open Vent.

An issue we find in the Drainage Game is people blocking off open vented stacks with AAV's, we come along to jet a nearby sewer, (which has been done on a preventative maintenance basis for years without issue), then to have the Occupiers screaming as we've just blown a load of filth out their toilet.... Simply because the bow wave of pressure that was escaping out the stack, now cant and looks for the next easiest way out.....

Oh well... such is life I suppose.