Another Durgo valve question

[Penners]

With thanks to jsouthwood for reminding me, I've been meaning to ask another question about the new mini-bathroom installation that I'm about to start.

Drainage of the toilet, washbasin and shower (in that order) will all be into a horizontal run of 100mm pipe, that in turn will feed into an existing vertical stack.

My building regs consent requires me to include a mini-stack on this "branch-line" drain, with a Durgo valve at the top.

My question relates to the location of this mini-stack on the horizontal pipe run. Should it be placed before the toilet, so that the run consists of mini-stack/Durgo valve, then toilet, then basin, then shower?

Something in the back of my mind tells me I read somewhere that the air admittance valve should not be the last thing on the run, but that depends what one means by "last". Does it mean upstream or downstream (or should I just ignore that whole dilemma)?

Any advice please?

 

[Lime]

Durgo air admittance valve
The Durgo valve is designed to reduce the number of ventilating pipes and subsequent roof penetrations in domestic, commercial and public buildings.
Suitable for use in sanitary pipework systems up to ten storeys high, the valve must be fitted in a vertical position above the flood level of the highest appliance connecting to the stack. Valves should be installed within the building in a ventilated duct or roof space where there is no risk of freezing and must be accessible for inspection and testing.
The 50, 82 and 110mm size valves have been assessed by the British Board of Agrément and awarded Certificate No 97/3427 which permits their use in accordance with the Building Regulations. A copy of the full certificate is available and provides comprehensive information on their use and installation.
When installed the valve will remain closed unless the system is subject to negative pressure whereby the diaphragm will lift and allow air to be drawn in to eliminate symphonic action.
Positive pressure ensures the valve closes and prevents foul air escaping from the system.

Each valve is supplied boxed with a polystyrene insulation cover that should remain in position after installation, as this will protect the valve against freezing, particularly when installed in a roof space.

To ventilate the underground drainage system and to minimise the effects of back pressure should a blockage occur, the branch or main drain serving a stack or stacks fitted with Durgo valves may require conventional venting at a point upstream of the stack connection.

For up to and including four dwellings, 1, 2, or 3 storeys in height, additional drain venting is not required. Where a drain serves more than four such dwellings equipped with the valve, the drain should be vented according to the following rules:
5 to 10 such dwelling – conventional ventilation to be provided at the head of the system.
11 to 20 such dwellings – conventional ventilation to be provided at the mid-point and at the head of the system.
For multi-storey domestic dwellings (other than those referred to previously) and non-domestic buildings, conventional drain venting should be provided if more than one such building, each equipped with the valves, is connected to a common drain which itself is not vented by means of a ventilating stack or a discharge stack not fitted with a valve.

Stacks should not be fitted with valves when the connecting drain is subject to periodic surcharging or is fitted with an intercepting trap.
An open vent must be provided and this also applies to stacks that discharge to a cesspool or septic tank.

I don't think it makes much difference where the valve branch goes as long as it is near the top of the pipe (upstream)

 

[Penners]

Many thanks for that, Lime. As you say, they seem unconcerned about where the valve is fitted.

Logically, it seems to me that the valve should be the first item on the "branch line" (ie: farthest from the main vertical stack).

I wonder whether anyone has any other opinion?