Minimum pipe size to AAV for 110mm stack (Durgo, Air Admittance Valve)

[MortarThePoint]

@Nickfromwales and others have kindly pointed out on thread(s) the minimum size requirements for AAV on 110mm stacks, which to summarise I understand as 1no. 3" vent or 2no. 2" vents. However, what about the pipe size leading to the AAV? Can you have a 50mm pipe which steps up to 82mm where the AAV is?

 

The area of a 50mm pipe is about 2,000mm2 and an 82mm valve would have to have a stroke of 7.6mm to open up an equal size area [1]. How big is the stroke on these AAVs? If less, should be OK to use 50mm pipe. In theory 😃.

 

Am I on to something?

 

[1] That area would in the form of a slot be more restrictive than a round opening by a good amount, but let's ignore that as the pipe has length.

Edited June 23 by MortarThePoint

[MortarThePoint]

Wow these sorts of thoughts can lead you to strange places. Looking at BS EN 12380 : 2002, the flow rate quoted by manufacturers is at a pressure drop of 250Pa. Also the valve must be open by 150Pa and have a measurable flow.

 

250Pa = 25mmH20 = 1inH20.

 

Most 82mm AAVs have a flow rate of ~40L/s according to BS EN 12380 : 2002. 

 

Using this duct calculator, 40L/s creates a pressure drop of 137Pa/m of 50mm ID pipe (only 18Pa/m for 82mm OD = 75mm ID pipe). I hoped the pressure drop would be much much less than that across the AAV itself, but it isn't. Hmm.

 

It is worth noting though that the resistance of two 50mm pipes across 2m is the same as one 50mm pipe across 1m. I have seen some say that running two 50mm pipes to AAV is OK. I could get away with a section of 50mm ID pipe only about 0.4m long before swapping to 82mm pipe.

Edited June 23 by MortarThePoint

[Nickfromwales]

On 23/06/2024 at 13:26, MortarThePoint said:

Wow these sorts of thoughts can lead you to strange places. Looking at BS EN 12380 : 2002, the flow rate quoted by manufacturers is at a pressure drop of 250Pa. Also the valve must be open by 150Pa and have a measurable flow.

 

AKA, "overthinking it" lol.

2x 50mm pipes to 2x 50mm AAV's is more than adequate. Usually these runs aren't much more than 10-15m max, and I've shown these working (to the BCO's) and they've been happy that they function so 'tick that box'.

 

Step away from the scientific calculator