Air Admittance Valve - 40mm on a Toilet

[Force 8]

I’m looking for guidance on whether I can get by with a 32/40/50mm AAV for a toilet connected to a septic tank – the tank receives only toilet waste, there are no other connections, no basins, no showers; nothing.

This is a remote location with 20 toilets in 20 huts spread over 50 metres, all connected to the tank. All toilets are at approximately the same elevation. 19 of the toilets have 110mm AAVs on the stacks.

Because of space limits, the final toilet (nearest the tank but still about 25m from it) can only take a 110mm AAV if the stack runs up the wall in the bathroom. I’d like to avoid this. Can we safely use a 32/40/50mm Durgo, tee’d off the soil pipe as it leaves the toilet?

If this is problematic, could we fit 2 x 50mm Durgos on the tee that comes off the same soil pipe – I presume the air restriction is the opening of the valve, not the size of pipe it is attached to?

All help appreciated.

 

[Nige F]

So you want to put aav's on all 20 Wc's - No mention of an open vent @ the furthest point from the septic tank.

 

[Force 8]

The contractor who put in the septic system (and installed the 19 toilets, leaving a stub for the last one closest to the septic tank) put 110mm AAVs on all 19 toilet stacks.

If I lift the AAV off the furthest stack, can the last one safely use a 32/40/50 Durgo?

 

[Hugh Jaleak]

No, the highest point (or last toilet) on the system should be vented to atmosphere. You need to remove the AAV and extend the soil pipe to terminate in an open vent to allow the drainage system to breathe, or you might have an unwanted surprise at the end nearest the tank. Seen it happen and it isn't very pleasant....

 

[Force 8]

OK, thank you. If I do as you say at the furthest end can I use a 32/40/50mm Durgo at the nearest toilet?

 

[Madrab]

Surprised at the contractor, they should know that a waste run/sewer can have positive as well as negative pressure, AAV's only works one way and wont balance a positively pressure waste run, hence the need for at lease one open vent.

Will a 50mm AAV work, possibly ... but if a number of toilets let go at the same time then that's a lot of sheeite running past that last (first) branch and could suck at the pan seal if the vent can't equalise quickly enough.

 

[Force 8]

Great, thank you

 

[Hugh Jaleak]

You also have the risk with a septic tank, if the drainage outfield cannot get rid of the discharge quickly enough, and/or the water table is high, then effluent can back up the inlet pipework to the tank, again if the system cant breathe, any positive pressure need to be able to escape. Always wise to keep an eye on things.

 

[John D v2.0]

Theoretically no as the 50mm one is rated 6.7l per second but in practice people get away with far worse. If the toilet outlet is not too far above the underground drain pipe you might not need an aav at all. Large aavs are designed to cope with tall stacks of multiple storeys so your hut should be fine.
But as mentioned, if your drain surcharges you're going to get a lot of bubbling in your loos! Need an open vent on the system at the far end.

 

[Nige F]

Or No smoking in the cubicles

 

[Hugh Jaleak]

Or No smoking in the cubicles

Nige, that would be the least of my worries. Had a large septic tank at a Transfer station I worked in a few years back, the drainage basically started at the 'In' gate weighbridge hut at the top of the site, ran down to the main building, picked up the WC's and kitchen, then out into the yard, and across to the 'Out' weighbridge picking up the WC there, and then a few yards on to the tank.

After some commotion in the 'Out' weighbridge hut one morning, we went to investigate, and found the Weighbridge Clerk honking his guts up outside, and inside, the walls of the toilet had been redecorated a lovely shade of brown.... Some kind of blowback had happened, I spent the rest of the morning fitting a stack and vent to the outside of the hut, to prevent a recurrence.