Part H: Soil Vent Stack

Clseeper

All,

Trying to understand better Part H with a view to minimizing the impact of a soil vent stack in my main bathroom.

My original plan was to just box in a 110mm extension of the main stack through the vent tile in the roof but looking to minimise this taking space from the bathroom.

Option 1: Reduce the diameter of the pipe above the wet section and route through studwork. However I'm reading two conflicting statements in Part H as to how small I can make this.

1.2.3.7 Size of stack ventilation pipes: The diameter of the part of a discharge stack which serves only for ventilation (the dry part above the highest branch) may be reduced in one and two storey houses, but should be at least 75 mm.

But later in the document is states

1.4.5.2 Ventilation stacks serving buildings with not more than ten storeys and containing only dwellings should be at least 32 mm diameter.

Option 2: Could I get rid of this completely with the use of an air admittance value (AAV) as outlined in 1.2.3.8. This would mean I wouldn't have to route any vent pipe. House has MHRV so extracting the required amount constantly - can't remember exact figure but above minimum.

A different perspective on the building guidelines would be appreciated.
Question 1: Which is the correct min diameter of the vent section 75mm or 32mm?
Question 2: And any cons to the AAV option other than it's a mechanical part that may break?

Thanks.

Calahonda52

1.4 refers to Larger buildings so will not apply to your situation..
I personally don't like the AAv's but they do work

SC024

I'd look into using an air admittance valve with MHRV check out if it can be done. your altering/playing with air pressure just check it out would be my advice.

Clseeper

Thanks for your input guys.

Calahonda52: Can you explain why it's a different diameter for larger buildings? I would have thought that the venting requirement on a larger building would be more as there would be more toilets/shower attached to it? Just curious.

SC024: I'll do some digging. My hope is that the MHRV is balanced so shouldn't create too much of a pressure differential.

Based on this - I'll look at capping the stack with an AAV within the first floor.

BryanF

Clseeper wrote: »

Thanks for your input guys.

Calahonda52: Can you explain why it's a different diameter for larger buildings? I would have thought that the venting requirement on a larger building would be more as there would be more toilets/shower attached to it? Just curious.

SC024: I'll do some digging. My hope is that the MHRV is balanced so shouldn't create too much of a pressure differential.

Based on this - I'll look at capping the stack with an AAV within the first floor.

Can the aav not be outside?

Clseeper

BryanF: Potentially, but this would be very close to windows etc so no way to keep the 900mm distance required. Or does the AAV negate this requirement?

Calahonda52

BryanF wrote: »

Can the aav not be outside?

Apparently not
1.2.3.8 Ventilated discharge stacks may be terminated inside a building when fitted with air admittance valves complying with I.S. EN 12380 Air admittance valves for drainage systems - Requirements, test methods and evaluation of conformity. Where these valves are used they should not adversely affect the amount of ventilation necessary for the below ground system which is normally provided by open stacks of the drainage pipework. Air admittance valves should be located in areas which have adequate ventilation, should be accessible for maintenance and should be removable to give access for clearance of blockages. Air admittance valves should not be used outside buildings or in dust laden atmospheres.

OP, re the pipe size, the building section of Part H refers to a ventilation stack which is a different design concept from the top end of the discharge stack for domestic.
There are also other considerations such as the probability of all toilets in an apt block being flushed together, there is much more "spare capacity".

Notwithstanding that, you are obliged to follow the applicable part of H