Getting into Hot Water



Related Page: Solar HW page
On This Page: What I've done
Hot Water Links


Hot water can be heated a number of ways:

Hot Water systems can generally be placed in one of two categories: Storage and instantaneous.





The water is heated at a convenient time and stored in a tank for use later on.

  • Electric Hot Water Systems: Electricity is cheaper during non peak demand periods (like overnight), so it is more economical to heat water then than during the day
  • Solar Hot Water systems: Solar Hot water systems depend on the sun for heating the water, so water can only be heated by this means during the day.
  • Water heaters loose the stored heat through the insulation surrounding the tank. The poorer the insulation, the more the heat loss. Energy input is required to maintain the water temperature.


Water is heated as needed.

  • No loss through poorly insulated storage tanks.
  • Usually smaller than storage tank, so can fit in more confined spaces.
  • Incorrectly sized system may struugle to heat the required amount of water.
  • No hot water if fuel or ignition source is removed temporarily

Gas storage Hot Water systems typically have smaller storage tanks than electric units because the water can be heated relatively quickly and there is no discount for using gas at night rather than during the day. A lot of gas systems also burn a pilot flame all the time which consumes a small amount of gas.

See Hot Water Links below for info on some efficient systems.


Hot Water Temperature

Most Hot Water systems are set to heat water to around 65 to 70C. Since this is too hot for most purposes, we bring the water temperature at the outlet down to the desired temperature by mixing some cold water with it. In our house (and I suspect lots of others) the major user of hot water is the shower. The hottest we can comfortably tolerate in a shower is apparently about 43C. If we turned down the temperature at which hot water is stored at, we would:

Note : There is some thought that storing water at 50C may encourage the growth of legionella bacteria, which leads to Legionaires disease. Although mainly associated with air-conditioning towers, some people say that water stored below certain temperatures provide good breeding grounds for the bacteria. Legionaires disease, which was first recognised in 1976, is passed on to humans through fine water droplets. There are certain groups of people which are more at risk of contracting the disease. There seems to be a a lot of differing opinions as to what constitutes good breeding grounds for the bactreia. The Queensland Government Health Department had a web page on Legionaires Disease which included the statement "Maintenance of a domestic hot water system at a temperature of at least 50 C may also reduce the risk of disease transmission.". They have since removed this page. This would appear to be laxer than some other guidelines. On the other hand, some state Health Department sites don't even mention hot water storage on their Legionaires Disease pages. See also the Victorian Govt Health Department page on the subject, or, or the What is it link from that page, which contains more definitions and information from various sources than you can poke a stick at.


Hot Water Calculations

The amount of electricity or time required to heat water (assuming 100% effeciency) can be calculated. Formula and examples are on the Calculating Power Consumption page.


Tank Insulation

Most hot water storage tanks have poor insulation, resulting in up to 50% of power used to heat the water lost through the insulation around the tank. (When I was living by myself in a house with an electric hot water service, it was consuming around 5 kWh a day on average, but the amount of power required to heat enough water for my daily shower was under 2 kWh. I am pretty sure I wasn't using 3 kWh of hot water for washing my hands and doing the dishes!)

There have been voluntary guidelines in place for a number of years specifying how much heat is allowed to be lost through the insulation around hot water storage tanks. The government in October 1999 made mandatory regulations (called MEPS - Miniumum Energy Performance Standards) that all new units have to conform to. The following table, which is drawn from info in Australian Standard AS1056, is shown here to demonstrate the amount of heat which is allowed to be lost through the insulation.

Electricity required to maintain the water temperature as a result of heat losses through insulation surroundeing the storage tank.

Tank Capacity (litres) Voluntary 1977 Voluntary 1985 Mandatory 1999
160 3.4 kWh 2.8 kWh 1.96 kWh
250 3.9 kWh 3.4 kWh 2.38 kWh
315 4.2 kWh 3.8 kWh 2.66 kWh
400 4.5 kWh 4.1 kWh 2.87 kWh

Heat loss can be minimised a little by insulating at least the first metre or so of hot water pipe as it leaves the hot water storage tank.


What I've done

In the past....

  1. I've turned the thermostat temperature of the hot water tank down to around 50C. This often involves removing a flap on the hot water service casing, and tweaking the thermostat adjustment screw. NOTE : Turn the Hot Water off at the main switch or Circuit Breaker before attempting to do this, as often the thermostat has exposed mains very close to it!! If you're not sure what you're doing, call an electrician!!
  2. Wrapped some 'lagging' (special insulated pipe tubing) around the pipe carrying hot water out of the storage tank.
  3. Replaced existing shower head with a AAA effecient head to reduce the amount of hot water used.
  4. Used cold water most of the time for washing clothes.

and now...

See the Solar Hot Water page


Hot Water System Links

State Rebate Schemes

Vic - Up to $1500 depending on system purchased.

Renewable Energy Certificates

For a new system installed in Australia on or after the 1st of April 2001 which replaces an existing electric hot water system whoose primary source of fuel is from fossil fuel sources (excluding hydro power in Tasmania) it is possible to reduce the effective price of the system by claiming Renewable Energy Certificates (RECs) and selling them to electricity retailers. Speak to an installer for further details, or check out the orer website. Apparently Energex, a retailer in Southern Queensland, will pay up to $855 for the REC's some water heaters will earn over their lifetime.

Review/Comparison of Solar and Heat Pump Systems

See Choice Magazine, Dec 1999 p29, or a web version of the article here.
Vic Approved Rebate Systems List also contains columns listing approximate prices and theortical effeciencies of each approved system.


  • Solar - Enough claims & counter-claims here to sink a ship. On the Solar Hot Water page, I outline some of the reasons why I decided to go with the Solahart system.
    There are a number of different configurations available from different suppliers. There are 'integrated' systems where panels and tank are located on the roof, or tanks can be mounted remotely to the panels, or even sometimes panels can be retrofitted to an existing tank.
    Note: The Solahart site contains a downloadable program to calculate how much of your hot water can be derived from the sun using various Solahart systems.

The 3 main manufacturers : Solahart, Solar Edwards, Beasley
Some other manufacturers: Rheem, Solco, Sola-kleen / Smalls

  • Heat Pump

Solahart - Check out their Sorcerer system.
Quantum - Contact Details:

  • Instantaneous Gas


Last Updated: 02/06/04