Question about designing branch waste pipe run for basin

[Pzzszk]

I am looking for advice/guidance on the re-design of the branch waste pipe run for a 1st floor basin. The starting point is the 35mm chrome pipe from the trap on the back wall of the house on the 1st floor. The end point is a pre-existing boss in the 4in cast iron soil stack located around the side of the house down near ground level. Normally I would use a McAlpine MS4-ISO 90 compression bend outside and then use 32mm welded pipe to get to the boss. There is only the one basin (no WC) connected to this branch waste pipe.

However, in this case, the waste pipe from the trap passes out through the wall and then needs to run 1.5m along an outside wall, then around a 90 corner and then down 2.4m to get to the level of the boss and then 1.2m across to the boss. So, a total of 5.1m (2.7 horizontal + 2.4 vertical) and 4 bends. If I read the regulations strictly, this isn't allowed without an additional venting (very difficult to achieve in this case) or a AAAV (even harder to achieve).

My questions are:
Do I need to count the vertical drop in the calculation? If not, then it becomes 2.7 and I can use 40mm pipe.
As this is one basin only (with a 35mm outlet), would converting to 40mm pipe after exiting the wall (about 6 inches from the trap) be sufficient to prevent the trap from autosiphoning?
Would it be better if I inserted a 60cm drop (in any pipe size) immediately upon exiting the wall, so that no water plug can be formed (basin only - no WC).
Would I solve (avoid?) all problems by just using 50mm pipe upon exiting from the wall?

Any help or advice will be appreciated.

Steve

[mredd]

Mcalpine do in line 1 1/4 aav's. Which you can Tee in outside.
http://www.mcalpineplumbing.com/productlist.asp

[AWheating]

can you change the basin trap for one with an aav valve?

[SimonJohns]

By the regs u should also go up a size. Thats a very long run for 1 1/4


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[Pzzszk]

Thanks everyone.

SimonJohns - It's easy to go up a size (or 2 if absolutely necessary). Will this be compliant, if I do?

AWheating - I can't find a chrome trap with aav. Does it exist? Maybe I could use a deep trap (ie. 75mm water seal) that is described as re-sealing instead?

Mredd - These aav's are for inside only (according to Mcalpine's documents). Do you know differently? Are there inline aav's for outside?

[SimonJohns]

Mcalpine do in line 1 1/4 aav's. Which you can Tee in outside.
http://www.mcalpineplumbing.com/productlist.asp

Shouldn't be done outside because the diaphragm can freeze shut in the winter. They have been designed for indoor use only


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[SimonJohns]

If you really wanted to have good ventalation on your pipe work. Do it the old way. Put a vent in. I would say that 3/4 will do it. Just run it high.
Preferably 500mm above an openable window.


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[AWheating]

soil_waste_price_list.jpg

like simon has said you could vent it with a normal cowl, you get them in 50mm but im not sure if you can get em smaller. (see top of pic)

[Pzzszk]

One approach would be to create a 50mm vented & cowled stub stack that is vented 500mm above the height of the bathroom window (this is on the side wall where there are no windows within 3m). And then use 32mm along the back wall, turn the corner and then connect into the stub stack (a total of 1.7m). No need for aav.

Any improvements on this?

Anything obviously wrong?

Steve