While heat pumps may be relatively new in the world of hot water heating, heat pumps have been around for a long time in the form of fridges and freezers.

They work by ‘pumping’ heat from one place to another, using a compressor and a refrigerant. In the case of your fridge it is pumping the heat from inside the cabinet to the room outside. In the case of a heat pump hot water system they are pumping the heat from the outside air, which has been heated by the sun, into the water storage tank.

They are very efficient at heating because instead of converting electricity (or gas) directly into heat it uses a much smaller amount of energy to move heat from elsewhere. In South Australia we mostly use air-sourced heat pumps which suits our temperate climate.

System efficiency is measured as COP, or coefficient of performance, a ratio of energy consumed to energy provided. Higher is better. A good heat pump unit will achieve a COP of 4+, which means they use around 75% less energy than an electric hot water system. When they are paired with a solar PV system to provide the electricity they can be 100% solar powered, compared to a solar hot water system that requires significant boosting in the Winter.

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Solar hot water works by absorbing sunlight and using it to heat water. The sun is free and renewable energy with typical Adelaide climates achieving around 80% of the households hot water needs from the suns rays. Shortfalls are covered automatically with gas or electricity to boost the water on overcast days when no solar gain is available.

The two main types of solar collectors are flat plate or evacuated tube.

Flat plate systems are pipes in an insulated box with glass at the front. While simple and cheap, it is difficult to insulate the pipes against heat loss while still exposing them to the sun. this reduces the efficiency in colder weather.

Evacuated tubes work like a thermos, with two layers of glass with the air evacuated from between them. The inside is coated to very efficiently absorb sunlight, but the heat can’t easily escape back through the vacuum. This makes them more efficient than flat panels when the air is cold.

As the sun shines on a collector panel, the water in the pipes inside the collector becomes hot. This heated water is circulated up the collector and out through a pipe to the storage tank. Cooler water from the bottom of the tank is then returned to the bottom of the collector, replacing the warmer water.

Some systems don’t heat the water directly but instead heat a fluid similar to antifreeze used in vehicle cooling systems. This fluid flows in a closed loop and transfers the collected heat to the water in the tank via a heat exchanger.

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Gas water heater covers three main product types, gas storage water heaters (GSWH), instantaneous or continuous flow gas water heaters (IGWH) and solar water heaters with instantaneous gas boosting (SWHs). Gas water heaters make up approximately 48 per cent of water heaters in Australia

The most efficient Gas hot water system is a natural gas 5 star continuous flow hot water system. These instantaneous gas hot water systems, also known as continuous flow, don’t have storage tanks and heat water only when required. In comparison to a 3 star storage hot water system, energy savings for these systems will on average exceed $400 per year. With rising gas prices this difference is set to grow further.
All gas water heaters are regulated to meet minimum energy performance standards (MEPS).

Instantaneous Gas water systems heat water by a gas burner as it flows through a coiled pipe called a heat exchanger. The gas burner starts when a hot water tap is turned on. It only heats the amount of water that is required instead of continuously heating a full tank. As they don’t store any hot water, there is no heat loss from a tank and so can have energy and cost savings.

To allow enough time for heating, the water is slowed down as it flows through the heat exchanger. This means that instantaneous units supply hot water at a lower pressure than storage systems. They can typically deliver hot water at flow rates per minute of between 10 litres and 30 litres, depending on the model.

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