Choosing a Hot Water Unit | Electric - Gas - Heat Pump

Due to the many different types, brands and sizes, along with new government regulations regarding hot water units, it can be a difficult process choosing the correct unit for you and your family.

To help you make the right choice, you need a plumber who understands the regulations and can outline the different options, including the type, brand, model and size that suits your needs.

To help you make an informed decision, the following is an outline of some of the aspects you need to consider. Please feel free to contact me to discuss your particular situation further.

Restricted Electrical Licence

Kyoritsu insulation tester

Does your plumber have one?

It is a legal requirement that the plumber holds a Restricted Electrical License if they will be disconnecting or reconnecting the electrical wires to a hot water unit. This includes the work needed to replace the thermostat and element.

When you're choosing a plumber to do the job, please ask them whether they hold a Restricted Electrical Licence and if they will be carrying out the required electrical safety tests.

I hold a licence and ALWAYS carry out the required testing. This ensures you and your family are safe from a possible dangerous situation and also ensures my insurance will cover any liabilities on my behalf.

You will also receive a Certificate of Electrical Safety at the completion of the job.

Government Regulations

Note:The following regulations have recently been repealed by the current Queensland Government, coming into effect sometime in early 2013. This means that homeowners will now be able to install any type of hot water system they choose. Until the time the repealed laws come into effect, the following regulations still apply.

Since January 1 2010, any existing house or townhouse within a reticulated natural gas area must install a greenhouse-efficient hot water unit (either gas, heat pump or solar) when the existing electric system is replaced.

If the existing electric hot water unit is still functioning, there is no requirement for it to be replaced.

If you are in a reticulated gas area but do not currently have a gas supply on your premises, these regulations will only apply to you if a gas distributor will install a gas supply to the boundary of your property at NO cost to you.

If you don't currently have a natural gas supply within your premises and are unsure whether you are in one of these areas, please feel free to contact me and I will enquire on your behalf.

Also, the gas distributor will only bring the gas supply and gas meter to your property boundary, the property owner will then be responsible to have a licensed gas fitter install the gas supply pipework on the premises. If you would like a quote for this work, please contact me and I will be happy to inspect the job and supply you with a detailed quote.

Please viewthis PDF file for more information regarding the government regulations.

Small-scale Energy Certificate Scheme

Small-scale Technology Certificates (STCs) are part of a Commonwealth Government scheme promoting the use of energy efficient hot water units.

STCs are available to the owners of new Solar and Heat Pump hot water units, who can either sell the certificates themselves or transfer them to an agent for an agreed amount.

The owner of the STCs can either place the certificates on the open STC market where they will sell for whatever the price is at that particular time. Or alternatively, the certificates can be placed in the STC Clearing House, where they will, at some point, sell for a Government guaranteed $40 per STC.

The $40 per STC that is offered by the Clearing House is well above what the certificates are selling for on the open market, but there is no guarantee how long it will take for those certificates to be sold.

If you choose me to supply and install your new Heat Pump hot water unit, I will help you through the paperwork process to ensure you can receive the correct amount of money back as soon as possible.

More detailed information is available at the Federal Government's  Clean Energy Regulator  website.

The current spot price of STCs on the open market can be viewed at the  Clean Energy Council  website. The price is displayed at the top of the page as 'STC(SPOT)'.

Note: The rebates offered by the Queensland Government for installing a Solar or Heat Pump hot water unit ended on 28th September 2012. The STC scheme described above is still operational.

Tempering Valves

Tempering valve

To prevent the possibility of scalding, it is now law in Queensland that any hot water being delivered to bathroom fixtures, (showers, baths and basins) be no greater than 50 degrees Celsius.

Tempering Valves are therefore required to be fitted to any new or replacement hot water unit.

A Tempering Valve works by mixing the heated water (at least 60 degrees) coming out of the hot water unit with cold water to regulate a temperature of no greater than 50 degrees.

Hot water to the kitchen sink and laundry tub is still permitted to be delivered at 60 degrees, so, when possible, it is preferable to leave the hot water line to these fixtures untempered. Although, this is often not feasible at certain houses, due to the existing pipework being hard to access. In these cases it is far cheaper to temper the hot water to the entire house.

Power Tariffs

There are 3 different power tariffs available to residential customers - Tariff 11, Tariff 31 and tariff 33.

Which tariff you should be on will depend on a few different factors, including:

- Type of unit

- Size of the unit

- Number of people in the home

Tariff 11

Tariff 11 is a continuous tariff which will supply power 24 hours a day. This is the most expensive tariff and is unnecessary for domestic hot water units.

Tariff 33

Tariff 33 supplies power for at least 18 hours per day, usually outside the peak period of 4pm-8pm and costs approximately 32% less than tariff 11.

This tariff should always be used for storage units of 125 Litre capacity and below. This is because at these smaller sizes, the hot water can be diminished very quickly and if power is not available to the unit, you may have to wait until the following day before the water will be reheated to the desired temperature. This tariff may also be required for larger units depending on the number of people in the household and their hot water consumption.

Most Heat Pumps of any size are also recommended to be on tariff 33, but certain high quality units such as the Siddons Heat Pump may function well on the cheaper tariff 31.

Tariff 31

Tariff 31 is the cheapest power tariff and costs approximately 52% less than Tariff 11.

This tariff is recommended for storage hot water units of 250 litres capacity and above as long as the unit is sized correctly to the number of people in the household.

Tariff 31 only supplies power for a minimum of 8 hours per day, usually between the times of 10pm-6am.

Types of Hot Water Unit

Rheem storage hot water unit Rheem storage hot water unit


There are three main types of electric hot water unit, Mains Pressure Storage, Heat Exchange and Heat Pump


Mains Pressure Storage units consist of a holding tank, heating element/s and a thermostat. Water is stored in the tank, which is heated by the element to the temperature defined by the thermostat. (Must be at least 60 degrees Celsius to prevent the possible growth of the legionella bacteria)

As heated water is drawn from the top of the tank, it is replaced by cold water, which causes the temperature in the tank to drop. When the temperature in the tank drops below that set by the thermostat, the heating element will turn on and the heating process will begin.

The time it takes to recover the water in the tank to the desired temperature will depend on the size of the tank, the power of the heating element/s and the power tariff that your hot water circuit is on.

Heat Exchange

Heat Exchange units work by heating the water in the tank to a very high temperature. When a hot water tap is opened, cold water flows through a copper coil inside the tank and is heated as it passes through the very hot water stored in the tank.

The mass manufacture of Heat Exchange units ceased in 2011, so they are no longer an option for a replacement, but there are still thousands of units operating reliably throughout Brisbane and Logan. Parts are also still available for servicing.

Heat Pump

Siddons Heat Pump Rheem heat pump

Heat Pumps are a Mains Pressure Storage type of hot water unit, but are far more economical to run than a standard electric storage unit. Good quality Heat Pumps can use as little as 33% of the power of a standard electric unit.

They work by taking the hot air surrounding the unit and transferring that heat into the water in the tank. Electricity is still required to raise the temperature, but the amount required is far less than a conventional storage unit.

Heat Pumps come in two styles: a single unit, where the heat pump is connected either on top or on the side of the tank, or a split system, where the tank and heat pump are two separate units.

They can be installed either indoors or outdoors, but a split system unit is required for indoor use. In this case, the storage tank is installed indoors, and the Heat Pump is installed outside nearby. Split systems can also be installed completely outdoors.


Gas storage hot water unit Continous flow hot water unit

The two types of gas hot water units are; Mains Pressure Storage and Continuous Flow (also called Instantaneous.)


Gas Storage works exactly the same way as the electric version, except that instead of an electric heating element, it contains a gas burner, which activates when the temperature drops below that set be the thermostat.

Also, unlike their electric counterpart, gas storage units don't operate on a tariff and can supply heat 24 hours a day, when required. They will also recover to the required temperature far faster than an electric unit.

Continuous Flow

Continuous Flow units don't utilise a holding tank to store the hot water, instead, the water is heated 'instantaneously'.

When a hot water tap is opened , cold water flows through the unit activating the gas burner. As the water continues through the unit it passes through a heat exchanger which has been rapidly heated by the gas burner and the hot water is then delivered to the tap.

The main advantage of Continuous Flow units is that gas is only ever consumed when hot water is required, making them very economical to run.

While Continuous Flow units have been around for decades, the technology has made huge advancements in recent years with the introduction of electronically controlled units.

Previously, these units were ignited by either a continuously burning pilot light, a battery pack igniter or a water flow activated igniter. They also required the installer to adjust the unit's gas and water flow to allow it to operate satisfactorily during both summer and winter when different water temperatures and hot water flow rates were required.

While these units all had their time and place and are still being manufactured, they are far less efficient, reliable and 'smart' than electronically controlled units.

Electronically controlled units will sense the slightest change in water flow and constantly make adjustments to always supply consistent water temperatures to multiple outlets running at the same time and can also supply a far larger volume of hot water than the older style units.

One small downside to installing a new continuous flow unit is, because they are controlled electronically, a power outlet will need to be installed within 1.5 meters of the unit.

Sizing Hot Water Units

Which size of hot water unit you require depends on a number of factors including;

- Type of unit

- Number of people in household

- Power tariff

The tables below are a guide to help you understand how hot water units are chosen for a particular situation.

Storage sizing table Rheem heat pump sizing table Siddons heat pump sizing table Rinnai heat pump sizing table Rinnai Infinity sizing table

* Continuous Flow hot water units, are labeled with a number that represents the maximum amount of water delivered at a temperature rise of 25 degrees Celsius above ambient water temperature.

For example, if the ambient temperature of the cold tap water at your home is 15 degrees Celsius, the Rinnai Infinity 26 will be able to deliver a maximum of 26 litres per minute of water at a temperature of 40 degrees Celsius.

Of course, this doesn't mean that the unit can only supply heated water at 25 degrees above ambient temperature. The water will be supplied at higher temperatures if less water is being used.

For example, if you have a unit that can supply water at 60 degrees (with bathroom fixtures tempered separately to 50 degrees) and you are only using a kitchen tap, the unit will easily supply water to that tap at close to 60 degrees. This temperature will decline if more taps start being used (more water flow) and may eventually get to the maximum supply rate of 26 L/min at 25 degrees above ambient temperature.

The variance in litres per minute for some units in the table above, e.g. 16-20 for the Rinnai Infinity 16, is dependent on the climate of the area in which it is installed. Brisbane and Logan residents should be able to expect something at the higher end of the range.