hi there I'm studying for plumbing cert nov 2016 exam only a week away and have come across a question 5 in the june 2013 exam that has me confused.
Its a Q about fully vented modified stack. It has two WC's on a common discharge pipe which can be sized @ 80 mm (which is one of the questions) however when asking for the total FU's of the stack it is saying that the WC's are 4 FU's each when table 6.1 of asnzs 3500 says that WC's discharging from 80mm are 2 FU's.

Any help much appreciated
 

Linkback: https://www.plumbers.nz/study-base/70/exam-prep/2044/

Thats what i went with when i answered the question but when i marked it off the answers it would be wrong? but it is correct according to the code.

ALSO if there is three WC's connected are the first two at 2 DU's and the third at 4 Du or are they all 4 DU??

only a max of 2 wc is allowed at 80mm - so to get three in there you would have to go to 100mm at which point you are now 4du's per wc

see 7.3.4

This also relates to your initial question where if sizing for 2 wc only then 2du's in 80mm dia - more than 2 wc then 4du's in 100mm dia


Depending on what else is connected to the stack then that may determine the correct answer - are there other wc's connected elsewhere?  If so then that would require the use of 100mm dia pipe and hence 4du's - if not then I say the boards exam answer is wrong and it should be 2du's.

I think you are right - b = 100mm, c = 80mm

a - 100
b - 100
c - 80
d - 600
e - 450
f - 2500
g - 600
h - 40
i - 65
j - 50



LOL - how did i do?

well that is interesting because my last answer was based upon total stack du's = 56 which is based on the wc's discharging to 80mm at 2du's for the first two

56 du's is the max permissible for 50mm dia vent

so if their answer is 65mm dia vent then they have allowed for 4 du's on the wc's which means they are sizing them at 100mm dia which then makes the pipe sizes for the discharge pipe (c) in the question wrong

so I think they have definately got the answers wrong

it is either 56du's for the stack + 50mm vent size + 80mm for the first two wc's at 2du's each

or

64du's for the stack + 65mm vent size + 100mm for the wc's at 4du's each



I think the former

Yeah I dont think it is anything to worry yourself over - good luck and all the best for the exam!