Low Pressure Hot Water Cylinders: 

Traditional water heating in New Zealand was with what is known as a low-pressure hot water cylinder system as in the illustration above. In the illustration the vent pipe is routed into the header tank but often is routed through the roof instead. Other types use a pressure-limiting valve on the cold water inlet rather than a header tank.

Low pressure hot water cylinders are typically made from copper with a galvanised outer casing and are most easily recognised by the copper pipe protruding through the house roof. This is known as an open-vented hot water cylinder.

The copper vent pipe regulates the pressure in the cylinder and acts as an overflow dispenser should the water expand beyond the capacity of the system to cope with. The height of the copper pipe determines the water pressure available for household use and as a guide the pipe length regulates to about 10Kpa per metre up to a maximum of about 76Kpa for a low pressure system or about 7.6 metres in length.

Advantages of a low-pressure hot water cylinder system are simplicity and longevity however with modern living the water flow is slower and baths and showers take longer, (it takes longer for the water from the hot tap to "heat up" and the further away from the cylinder the longer it takes). Older hot water cylinders may only have a 1kw heating element as well which means longer re-heating times compared to a modern 2 or 3kw element.

Many households these days are choosing to upgrade to a mains-pressure system when replacing their aged or worn out low-pressure hot water cylinder.