Why are SVPs so big?

[MortarThePoint]

Looking at Part H I see a conflicting pieces of information:

 

 

 

So which is it, 32mm, 50mm or 75mm?

 

I understand the need for a large diameter if it is to allow air into the stack and there is no AAV as it has to provide a reasonable flow of air as the waste water rushes through the lower pipes. Even so, air moves much more easily through a pipe than water does. I was planning to just have an Durgo/AAV, but think I need an SVP to allow positive pressure from my sewage treatment plant (STP) to vent. Any such venting from the STP would have low flow rates and could therefore go though a narrow pipe (to a roof tile vent). Even 20mm pipe would be plenty big enough 

 

I don't fancy a 75mm pipe going up the bathroom wall and a 32mm one would be much more acceptable. Can I plead the case if I have a Durgo/AAV for the high flow inlet requirement?

[Russell griffiths]

Vent your STP at the tank, run a vent pipe out from the tank and up behind a shed or behind some shrubs, there is no smell from mine whatsoever. 

[JamesP]

I have a vent pipe  about a metre before the treatment plant main inlet and an AAV just above upstairs WC.

Did not want a vent pipe penetrating the building.

BC happy with that.

[SteamyTea]

4 hours ago, MortarThePoint said:

Even so, air moves much more easily through a pipe than water does

Oddly enough, it doesn't. 

Now that may sound controversial but it is to do with fluid dynamics, which are often categorised by a Reynold's Number, which is a unitless descriptor. 

Another way to think about it is not by the volumes being moved, but the masses moved.

Reynold's Numbers tend to be big, 

Air is around 1500 and water around 1000. This does not imply a 50% difference and temperature and pressure make a huge difference, as does turbulent or laminar flow (pipe size and shape).

That is a very brief, and simplified explanation, fluid dynamics is a horribly complex area of science/engineering.

 

Apart from that, I can't help, but it probably explains the odd numbers quoted.

[MortarThePoint]

I may end up plumbing the detached garage just for this reason. Will probably be grateful of having a sink in there too

[Oz07]

Do people ever worry about thermal breaks with soil pipes? It seems we go to a lot of trouble to minimise and then have a network of pipe running thru the house. I suppose they sit at a higher temp than outside air. 

[MortarThePoint]

5 hours ago, JamesP said:

I have a vent pipe  about a metre before the treatment plant main inlet and an AAV just above upstairs WC.

Did not want a vent pipe penetrating the building.

BC happy with that.

 

I have already been including AAVs at the end of all my significant branches (before any vertical sections of 110mm pipe so that should satisfy any negative pressure requirement (i.e. stop traps from being sucked out).

 

I understand that septic tanks need a low level inlet and high level outlet so as to create a flow though of fresh air, but a STP has a pump so any outlet I would expect to be OK surely. Maybe not one of the BioRock type or spinning disc ones though, but I'm not considering those. I am considering Graf One2Clean and Marsh Ensign systems. The Graf looks to have significantly cleaner effluent, easier to have granular backfill rather than concrete and also has a non-continuous pump which saves energy. They do talk about having a 'chimney effect' though which I assume is to prevent smells coming out of the low level pipe.

 

[Nickfromwales]

7 hours ago, Russell griffiths said:

Vent your STP at the tank, run a vent pipe out from the tank and up behind a shed or behind some shrubs, there is no smell from mine whatsoever. 

Done this for 2 other jobs, no issues, no smells, and only AAV (Durgo's) inside the house with zero roof penetrations.

[G and J]

Presumably with a mains sewer connection one still needs a soil stack onsite. 
 

 

[JamesP]

4 hours ago, MortarThePoint said:

I may end up plumbing the detached garage just for this reason. Will probably be grateful of having a sink in there too

I have just done this, WC, shower, sauna and a kitchen sink.

We have a Vortex TP, simple but functional design with an air bubbler. Its about 30 metres from the house.

No penetrations through the building as @Nickfromwales mentioned.

Scrappy image below.

 

[Nickfromwales]

The only major consideration should be keeping the vent away from an elevation that has openable windows, or otherwise take the SVP up to gutter height, attached to the building.

 

19 hours ago, G and J said:

Presumably with a mains sewer connection one still needs a soil stack onsite. 

Not always. If you have a reasonably strong case to not have an SVP to atmosphere, you can ask the BCO for the 1-in-5 or 1-in-10 exclusion; this is where you’re not end of line and have at least 5-10 neighbours who all have visible SVP’s. 
 

The requirement of an SVP is to vent the network sewerage system, not to vent your house btw. 

[saveasteading]

15 hours ago, MortarThePoint said:

considering Graf One2Clean and Marsh Ensign systems. The Graf looks to have significantly cleaner effluent, easier to have granular backfill rather than concrete a

I've only ever used Marsh,4 times. The effluent always looked and smelled clean.... but I washed my hands.

The new Marsh tank has improved numbers. 

Granular backfill easy too .... what concerns you. 

[Kelvin]

19 hours ago, MortarThePoint said:

I may end up plumbing the detached garage just for this reason. Will probably be grateful of having a sink in there too

Exactly what I did. I have an SVP out the garage roof, AAV in the roof void, and another SVP after the tank. No smells. No penetrations through the house and a clean roof with no unsightly pipes coming out of it. Graf system. 

Edited Thursday at 12:38 by Kelvin

[Mike]

On 25/06/2025 at 14:00, MortarThePoint said:

Looking at Part H I see a conflicting pieces of information

So which is it, 32mm, 50mm or 75mm?

They're referring to different circumstances. In particular A.8 is referring to the case when the ventilation pipe is separate from the soil pipe (the old way of doing it, before combined SVPs took over).

 

As for a solution, I agree with the others above - vent at the STP, or via a convenient nearly garage / outbuilding.