Drainage flow rate calculation

[mfgs]

I have been asked what the flow rate of the drainage is on a project we are doing of 23 flats. I’ve never had to do this before, can anyone advise on how this is done?

 

[oz-plumber]

It should be in the regs.
Fixtures, fixture units and fixture unit ratings should all be covered.
eg: a basin might be 1 fixture unit and a pan might be 5.
Add up the fixture units and size the drain accordingly.
Just give the total capacity of the drain and the total fixtures and fixture units connected to the drain.
If they want more than that - tell them to get an Hydraulics engineer in.

You are only expected, as a plumber, to work within the regulations.
You can't be expected to have the knowledge or the access to the design criteria of drain sizes that the regulation tell you to use.

 

[mfgs]

Yeah I managed to find it in the building regs. I had all the unit values but didn’t know how to work it out as obviously not everything is in use at the same time in multiple properties. It’s in part H of the building regs. Got a table going up in values of 5.

 

[oz-plumber]

It's not your job the work out the flow rates.
It's your job to install plumbing to the regulations, with fixture units loadings and appropriate drain size.
Who knows what design criteria the Regs are built on.
That's why they gave use fixtures unit ratings and drain size