ᐅ Drinking Water Installation DIY / Pipe Diameter

Hello everyone,
I would like to install the potable water system for a small accessory apartment using multilayer composite pipe, but I am unsure about the pipe diameter.

Here is the setup:
Supply line on the ground floor approximately 10m (33 feet) to the upper floor (no outlets on the ground floor)
Main distribution line on the upper floor, about 2m (7 feet) with a washing machine connected; at the end of this 2m (7 feet) main line, a T-junction splits into two branches:
-> Branch to shower, toilet, and hand wash basin (total length 4m (13 feet))
-> Branch to kitchen sink and dishwasher (total length 7m (23 feet))

Would a diameter of 16mm (5/8 inch) be appropriate for the two branches and 20mm (3/4 inch) for the main supply line?

daytona schrieb:

I would like to install the potable water system for a small granny flat using multilayer composite pipes, but I am unsure about the pipe diameter.

Check out online tools or simplified methods according to DIN standards.

daytona schrieb:

Diameter 16mm (5/8 inch) for the two branches and 20mm (3/4 inch) for the main supply line?

Inner diameter? The outer diameter varies significantly depending on the pipe, so inner diameters differ as well. Definitely do not use 16mm (5/8 inch) for the shower if you want proper water flow. Also, keep in mind that you need to install both cold and hot water lines.

Apart from that, what you are planning is not allowed as a DIY project.

nordanney schrieb:

Apart from that, what you are planning is prohibited for DIY.

ok, I’m not passing Go, I’m going straight to jail

daytona schrieb:

ok, I’m not passing Go but going straight to jail

Reporting yourself and accepting the fine is also an option ;-)

Apart from that, you didn’t answer the question about the pipe. Hand washbasins and similar fixtures don’t require large pipes. Showers are a different matter. What about the hot water? Circulation pipe? As planned or just branch lines without circulation?

The diameter is crucial not only because of the lengths but also due to the fittings (elbows, tees). Each fitting creates flow resistance. Depending on the system, the fitting's inner diameter is smaller than the pipe's (Zeta value). If possible, it is better to bend the pipe rather than use angled fittings.

nordanney schrieb:

Reporting yourself and receiving a fine is also fine ;-)

Apart from that, you didn’t answer the question about the pipe.

Hello,
-> Yes, see the first post "...multilayer composite pipe..." (from the market leader)
-> branch lines, hot water in the same nominal diameter as cold water
-> my plumber suggests DN16, supply line DN20