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Solar Hot Water and the ‘Thermosiphon’ Effect

solar panels on house roof

Hot water is an essential part of modern life. We rely on it to keep us clean, healthy and comfortable, and expect it to be ready to go at the turn of a knob or push of a button. Our reliance on hot water can make a high energy bill feel like a personal attack or cause a malfunctioning hot water heater to ruin our day. With solar water, you can enjoy your hot water in peace knowing that the world’s biggest energy source, the sun, will keep your hot water in good supply while costing you less. 

Depending on the climate you live in, solar hot water can provide up to 90% of your hot water for no cost, without producing greenhouse gas emissions. Heating water can be responsible for more than 30% of your household energy costs, as often the system is running continuously, heating dishwashers, running washing machines and filling bathtubs.

Along with the regular savings on your hot water usage, having solar water in your home can make you eligible for solar energy government rebates.

Along with these savings, solar hot water systems tend to have a longer life expectancy than traditional hot water systems. Thus, solar hot water systems have improved over time to become Australian weather-resistant, have high storage and even seek the sun to make their heating more efficient. Read on to discover why solar hot water is the most efficient choice for your home.

What is solar hot water?

A solar hot water heater uses the power of the sun to generate heat for your water.

This power is captured through collectors on the roof of your home and converted into a transfer fluid before moving to a series of pipes known as the heat exchangers in the solar water storage tank.

The heat is then transferred to your water, ready for use in your home.

A solar hot water heater takes up very little room on your rooftop and will store unused energy for later use, which makes it an incredibly efficient system overall. Installing solar hot water is the equivalent energy savings of taking a car off the road every year, and it is a great way to reduce both your carbon footprint and the cost of running your household. 


Rheem hot water solar panels
Solar panels from Rheem


Panels & Collectors

The panels and collectors of your system can make all the difference to the quality and reliability of your hot water.

Some of the most popular options include:

  • Antifreeze Panels – A mixture of glycol and water flows through a jacketed lining and the panels to keep the water hot in frosty conditions. 
  • Selective Surfaces Panels– used to increase absorption and reduce loss of solar energy 
  • Evacuated Tube Collectors – using a solar coating liquid and a natural vacuum between two fused tubes of glass, an evacuated tube design creates excellent insulation against heat loss 
  • Flat Plate Collector – one of the most common types of solar systems, a plate on the roof is heated by the sun before fluid within the copper pipes absorbs the fluid and is used to heat your water 


Two Main System Types

There are two main types of solar hot water systems: lo-line and hi-line solar.

Lo-line systems pull cold water from the storage tank to be heated via panels on the roof, before circulating back into the tank.

A hi-line system is entirely on the roof and utilises an innovative process called the thermosiphon effect to heat the water.


The Thermosiphon Difference… 

The thermosiphon effect is the process of hot water rising and cold water falling in the solar panel and tank.

Mitigating the need for mechanical components, fewer things can go wrong with a hi-line system, as the technology relies on the principle that hot water rises. The water moves based on its temperature, becoming less dense with heat, causing it to rise into the collector. The storage tank is installed above the collector, welcoming the water without pumps or controllers. 


a diagram of the thermosiphon effect on a rooftop
A diagram of the thermosiphon effect on a rooftop


In the above image, the thermosiphon process is used to support roof-mounted solar collectors through the following steps:

  1. The flat panel collectors absorb the energy from the sun from their optimal position on the rooftop, ready to transfer into heat for the water. 
  2. The water increases in density, allowing it to rise through the pipes into the system 
  3. The cool pipes transfer the cold water along the opposite side, ending up in the collector. 


A thermosiphon system has some distinct advantages, including:

  • By eliminating the need for a pump, your thermosiphon system will simplify and streamline the process to your solar hot water. 
  • They are more reliable and require less maintenance with fewer moving parts to care for. 
  • A lower energy cost than a heat pump that requires energy to run
  • Thermosiphon systems can resist extreme weather conditions with a few minor precautions 
  • The insulation in a thermosiphon tank can aid in conserving heat and preventing freeze over 
  • Thermosiphon tanks can be installed under the roof of the home to add an extra layer of weather protection 


When installing a thermosiphon system, there are a few factors to consider, including:

  • The system itself is quite heavy and unsuitable for roofs that aren’t verified for structural integrity 
  • The installation process can be expensive and intricate 
  • It is difficult to hook up with hybrid systems using gas or electricity, so only suitable for solar systems 

With these considerations in mind, a thermosiphon system can provide a reliable, cost-effective and low maintenance method of keeping your home’s hot water flowing for years to come. 


Solar hot water system
An example of a complete solar hot water system


Other solar heating options…

Storage Tank 

The current most common type of water heater in Australia, a storage tank method is usually used with an electric system, however, it can be supported by a solar hot water system. Evacuated tube or panel type systems are usually best with a storage tank, and the tank will top up the energy levels if the sun is having an off day. 

Heat Pump

Heat pump systems work similarly to an air conditioner or a refrigerator and use electricity to move heat from the air to the water. 

Instantaneous/Continuous Flow 

This type of water heater limits your ability to maximise the potential of your solar power. You would have to limit most of your hot water usage to the middle of the day, making morning and evening showers inefficient. 


Make the most efficient choice with a solar thermosiphon system…

With many Australian homes feeling the strain of high energy bills, the time has never been more appropriate or more urgent to switch to more sustainable choices. If environmental consciousness plus a low energy bill is what you’re seeking in your next hot water system move – there is no option better than a solar hot water system using the thermosiphon effect.

A great example of this is the Rheem Solar Hot Water System Range, which features marine-grade stainless steel tanks, high-efficiency panels, and has both hi-line and lo-line options. 




As Australia’s largest online retailer of hot water systems and hot water heaters, we are dedicated to bringing comprehensive solutions to your household’s hot water. With over 30 years of experience serving the Australian public, we are committed to excellent customer service, affordable prices and an extensive range to meet your hot water needs. Not only do we stock the most efficient and modern solar solutions, but we guarantee the reputability and quality of each brand that we sell. 

The major brands in our range include Rinnai, Bosch, Dux, Rheem, Vulcan, Aquamax, Quantum, Evoheat, and Stiebel Eltron. We have solutions in all shapes, sizes, and capacities to meet your needs. 

Visit our online store today, or contact our team to discuss your installation, servicing and repairs. With our depth of knowledge, we can make your switch to solar as simple as can be. 

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