15 or 22 mm copper? 16 or 20 mm multilayer? For housing the choices are surprisingly standard. Below are the practical tables for water and heating, plus what you need to know about gas.
Drinking water
Pipe sizes are always outside diameters. Copper has a thin wall, multilayer pipe a thick one: that is why 20 mm multilayer has a bore comparable to 22 mm copper, and 16 mm multilayer to 15 mm copper.
Application
Copper
Multilayer pipe
Main pipe after the water meter
22 mm
20 to 25 mm
Branch to a bathroom (several outlets)
22 mm
20 mm
Single outlet: basin, toilet, washing machine, outside tap
15 mm
16 mm
Kitchen tap
15 mm
16 mm
Shower
15 mm
16 mm
Bath (fills faster)
22 mm
20 mm
Torn between two sizes on a long run or with lots of simultaneous use? Go for the bigger one: one size too big costs a few euros, one size too small gives permanent pressure loss.
Central heating
Application
Copper
Multilayer pipe
Boiler connection and main pipes
22 to 28 mm
20 to 26 mm
Connection per radiator
15 mm
16 mm
To the underfloor heating manifold
22 to 28 mm
20 to 26 mm
Heat pump: connection and main pipes
28 mm (35 mm above approx. 10 kW)
32 mm (40 mm above approx. 10 kW)
The same principle applies to heating: main pipes generous, end connections small. On long runs with many radiators, a proper calculation based on output pays off.
Heat pump: one size up. A heat pump works with a small temperature difference between flow and return, around 5 degrees versus 20 for a gas boiler. To deliver the same heat, roughly four times as much water has to circulate, and that calls for a thicker pipe. Too thin and the pump gets noisy, does not reach its output or cuts out. The exact size is in the installation manual of your heat pump. More on connecting a heat pump with press fittings.
Gas: read this first
Gas is not DIY work. The diameter of a gas line has to be calculated from the output of the appliances and the length of the line, and work on gas lines belongs with a qualified gas installer. Use this page for gas only to understand the conversation with your installer.
Pressing on gas is allowed only with press fittings approved for gas: recognisable by the yellow marking, for example Viega Profipress G, Geberit Mapress Gas or Sanha NiroSan Gas.
The O-ring in a gas fitting is yellow or made of HNBR. A standard water fitting with a black EPDM O-ring is forbidden on gas: it swells up from hydrocarbons and starts to leak.
As a guide: a cooker is usually on 15 mm copper, a boiler on 15 or 22 mm depending on output and distance. The calculation stays with the installer.
Taking it to the job?
These tables are also on an A4 sheet you can print or save on your phone, with a QR code back to the page. For now the PDF is only available in Dutch; the sizes are the same.
The outside diameter. 15 mm copper is about 13 mm inside; 16 mm multilayer pipe with a 2 mm wall is 12 mm inside.
Yes, with a transition fitting. Make sure you press each side with the profile that fitting specifies.
Usually 28 mm copper or 32 mm multilayer pipe for the connection and main pipes, and one size up above about 10 kW: 35 mm copper or 40 mm multilayer. A heat pump circulates much more water than a boiler, because the temperature difference between flow and return is much smaller. Check the installation manual for the exact size.
With 19 inserts, our complete set covers the common sizes from 12 to 32 mm in the TH, V, U and M profiles. If you press Uponor S-Press Plus or Uni Pipe Plus, take the UP version: it adds UP16, UP20, UP25 and UP32. See which press jaw do I need.