In Hot Water?

27 Nov 2018

In response to some specifier requests we have added to the hot and cold water sections details on hot water temperatures and instantaneous electric water heaters (similar to gas continuous flow units).

Hot Water Temperatures

Due to some BCA's wanting hot water temperatures listed on consent documents, Masterspec is adding a clause to help set this out. Temperature requirements come from NZBC G12/AS1, 6.14 Safe Water Temperatures, but it is more complex than it first appears. One thing to keep in mind is that G12/AS1 is largely written around storage hot water heaters, although maximum temperatures still apply. Boiling units are excluded from all these requirements.

Water Heater Temperatures

Storage water heaters are to store water at not less than 60°C to NZBC G12/AS1, 6.14.3 Legionella Bacteria. Legionella can grow between 20 to 50°C and survive in temperatures normally up to 55°C, but at 60°C Legionella bacteria dies in about 30 minutes. Non-storage heaters (flow throughs, instantaneous etc) do not require a minimum temperature as they are not storing water, particularly not at the Legionella critical development temperatures. NZBC G12/AS1, 6.14.4 also requires 60°C (or alternative sterilising system) to be maintained in flow and return circulating pipe systems as the pipes are effectively storing potentially large amounts of warm/hot water.

Temperatures at the outlet

Water temperature at the outlet (usually a tap) is defined by NZBC G12/AS1, 6.14.1 Maximum Temperature, and although not directly stated this relates to scald times (see below).  To get the full picture, you have to read what this clause says and what it does not say, hence the complexity.

Basically G12/AS1, 6.14.1 covers a hot water piping system, with temperature controls where necessary (tempering valve etc), that is to provide water at the outlet at the following temperatures:

  • For personal hygiene fixtures (showers, baths, wash hand basins etc) temperatures to be close to but not to exceed:
    • 45°C - for early child hood centres, schools, elderly facilities, hospitals, psychiatric or disabled institutions.
    • 55°C - for personal hygiene fixtures in all other buildings.

The key words to focus on are "personal hygiene fixtures"  which means fixtures such as showers, baths, wash hand basins, bidets etc. in all situations (refer G12/AS1, 6.14.1 comment 2). This is where it gets complex, personal hygiene is covered in G1/AS1, which includes showers, baths, wash hand basins, bidets etc. but does not include kitchen sinks (and appliances), laundry tubs (and appliances), cleaners sinks, industrial fixtures etc. So those fixtures that are not included can be classified as non-personal hygiene fixtures. As G12/AS1, 6.14.1 is silent on non-personal hygiene fixtures it can be deduced that there is no restriction on maximum temperature (related to scalding).

So for non-personal hygiene fixtures something like the following statement could be used to describe the hot water temperature at the outlet:

  • For non-personal hygiene fixtures (kitchen sinks and equipment, laundry tubs, cleaners sinks, industrial fixtures etc) temperatures are:
    • Unrestricted - direct from water heater, approx. 60°C, must be less than 65°C (for kitchen sinks and equipment, laundry tubs, cleaners sinks etc) - in all buildings.
    • Unrestricted - direct from water heater not tempered (for industrial fixtures and specific items etc) - in all buildings.

Simple right?  No!  It is all subject to the hot water system design, water heating types and required temperatures and scald times.

Scald Times

Virtually any hot water is capable of severe scalding, that is a 2nd or 3rd degree burns, it just depends on time and skin condition. Lower temperatures reduce the risk of scalding by increasing the length of time it would take to get a burn by effectively providing enough time to react to the heat and withdraw from it. For fixtures with the primary purpose of skin cleaning, the Code reduces temperatures, which in turn increases reaction times. This is particularly so in areas where there is expected to be concentrations of people with reduced/impaired reaction times or at risk skin condition (young children, elderly, disabled etc.).

 Period to receive severe scald burn (approximate)*
 45ºC  More than 1 hour
 55ºC  17 - 20 seconds
 60ºC  3 - 5 seconds
 65ºC  2 seconds
 70ºC  Instantaneous
 * - Represents 2nd degree burns to adult, sources vary about times


For non-personal hygiene fixtures (kitchen sinks and equipment, laundry tubs, cleaners sinks, industrial fixtures etc) where skin cleaning is not the primary purpose temperatures above 55°C may be desirable or for industrial processes required.

In domestic situations approximately 60°C (preferably just below) may be OK for rinsing greasy dishes, but it would be extremely dangerous for it to exceed say 65°C.

High temperature industrial processes may be different, as safety measures and equipment should prevent skin contact.

Hot Water System Design

System design starts with the water heater, either storage heater or instantaneous heaters (flow through etc). The storage heater outlet will usually, depending on energy source, initially provide water at or just over 60°C, and then normally via a tempering valve the temperature would be reduced to a required fixture temperature, say 55°C. Instantaneous heaters can use tempering valves but many of them are equipped to directly deliver water at a maximum prescribed fixture temperature without a tempering valve.

Issues arise if mixed temperatures are required or desired. Obviously a single storage heater providing water at 55°C to all fixtures in the building is probably going to be the cheapest and easiest system to provide. A mixed temperature system will get more complex needing multiple hot water lines and possibly multiple tempering valves (or multiple heaters), or even a flow and return system.

It is highly likely the mixed temperature system will need designing and documenting, showing how the different temperatures will be achieved and where the extra pipes (or extra heater) will go and which fixtures they serve and at what temperature.

Instantaneous Electric Water Heaters

Instantaneous electric water heaters are non-storage units, usually very compact, single or 3 phase and high amperage. They only heat water that flows through the unit which is activated by a tap being turned on. These types of heaters are particularly effective for remote low-use fittings (say whb) with limited space for a conventional water heater and where higher power costs are offset by heat losses from long pipe runs. The lower amperage heaters tend to work best with low-flow taps/fittings.

Most units do not require separate controls for, temperature (tempering valves), expansion or pressure, under normal conditions.  All that is usually required, in close proximity, is a 2 or 3 phase power supply of higher than average amperage plus a cold water feed with an isolating valve.

Masterspec Sections

Masterspec has added to the hot and cold water sections a clause detailing hot water temperature requirements.  Also to these sections, clauses on instantaneous electric water heaters have been added to the range of water heater option already able to be specified. There have been some very minor tweaks to guidance notes in the electrical sections to also allow for instantaneous electric water heaters.