Low Pressure Systems

The most common New Zealand hot water cylinder uses an electric element to heat water, which is then stored until needed inside an insulated cylinder (the hot water cylinder). Whenever you turn on a hot tap, water is drawn off from the top of the cylinder and cold water enters at the bottom of the cylinder to replace it and be heated up ready for use. The majority of New Zealand homes have hot water installations that use low pressure cylinders. This is in contrast to Australia where there is a predominance of high pressure hot water installations. Low-pressure cylinders are made of copper, which is durable but can withstand only limited pressure. Pressure is reduced by running the mains water into a header tank, just like a toilet cistern, which in turn feeds water into the hot water cylinder. As the water is heated it expands and the pressure is relieved by the water rising in a vent pipe up through the roof. Low pressure systems are defined as delivering water at below 150kPa, but in reality many New Zealand homes have a supply below 50kPa and some as low as 20kPa. Installations using low pressure cylinders invariably have high pressure cold water supplies (ranging from 500kPa up to as high as 1000kPa). That means that when water from these supplies mix, there can be a ratio of 10:1 or more.

In Australia, and increasingly in New Zealand, many homes are employing full mains pressure – such that the hot and cold water are delivered at the same or approximately the same pressure (both being above 120kPa). This requires a steel cylinder and a combination of valves and filters to give consistent hot and cold flow that will work best with efficient modern fittings.

The star ratings listed for tap equipment and showerheads have been calculated using the protocol outlined in AS/NZS 3718 and AS/NZS 3662:2005 respectively. As this is currently worded it does not cater for low pressure supply situations. The result of this is that New Zealand products specifically designed for use in low pressure systems will be awarded a very low (or possibly zero) star rating when tested under a mains pressure testing regime. Work has been done by Standards New Zealand to amend AS/NZS 3718 and AS/NZS 3662:2005 to allow for an alternative testing protocols for fixtures intended for use with low pressure supply systems. The Ministry of Consumer Affairs is working with Standards New Zealand to ensure there is either a joint Australia/ New Zealand or New Zealand only low pressure supply system standard that will be cited in the proposed regulations.

Low Pressure Systems – Labels

There is some anecdotal evidence that New Zealand consumers sometimes buy the wrong type of fixture as they cannot easily determine which fixtures are intended for low pressure or mains pressure systems. Some fixtures can be adapted for both situations with the addition or removal of a flow restrictor.

It is proposed that water efficiency labels on shower heads and taps state whether the item is intended for low or mains pressure or both. This would also tie into the testing protocol used to determine the star rating. A fixture intended for low pressure only would be tested using the (yet to be determined) low pressure testing protocol - a star rating would then be given. The label would then state that this fixture was intended for use in low pressure supply situations. The opposite would be the case for fixtures intended for use in mains pressure supply situations.

Although a fixture may have been primarily designed for use in a low pressure situation, many low pressure fixtures can be used in mains pressure systems. In this instance, the manufacturer would still specify the intended supply situation (low pressure) but the label would also include a disclaimer along the lines of "This showerhead is intended for use in low pressure water supply conditions. The water consumption and efficiency figures stated are those produced when tested under low pressure conditions. If the showerhead is used in mains pressure supply the water consumption of this product will be higher than that stated."

Fixtures intended for use in mains pressure supply situations can generally not be used effectively in low pressure situations. However, some fixtures are able to be legitimately (and without negating the consumer's warranty) modified for use in both situations. As above, the manufacturer would specify the intended supply situation (mains pressure) and include a disclaimer "This showerhead is intended for use in mains pressure water supply conditions. The water consumption and efficiency figures stated are those produced when tested under mains pressure conditions. If the showerhead is used in low pressure supply conditions (in accordance with the manufacturer's instructions) the water efficiency of this product will differ from that stated."