Passive Solar Water Heater

How thermosyphon and batch passive solar hot water heater work - advantages and disadvantages

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Check out the advantages and disadvantages of the passive solar water heaters including the most popular types, batch and thermosyphon systems.

Passive solar water heater is a simple heating system that utilize the free heat energy from the sun, and requiring no external energy to transfer hot and cold solar fluids.

Passive solar hot water heater is the most popular way of harnessing the free sun energy; they are the least expensive, simple and reliable.

Such a system uses the principle of physics (heated water rises to the upper part of the tank) to transfer the heat from the solar collector to the storage tank. Passive systems can be used for direct domestic water heating or indirectly by circulating the solar heat transfer fluid through the heat exchanger.

Because of its simplicity, passive solar water heater systems are more vulnerable than other solar heating systems.

Types of the passive solar water heater systems

Two passive solar water heater systems are the most popular today:

  • Integral Collector Storage System

  • Thermosyphon solar heater system

How to choose between these two passive solar hot water systems?

If you live in the area with a temperate climate, you have up to two family members, and most of the hot water you are using is at the end of the day, you want a passive batch solar water heater or ICS system.

If there are more than three people in your home and you cannot install an additional solar storage tank near the existing heater, recommendation is to use the thermosyphon system.

Passive Integral Collector Storage System - Batch Water Heater

batch water heater

Integral Collector Storage systems are passive as they don't require pumps for the operation. They are also called batch water heaters and they are the only solar heating systems that do not require the storage tank. They consist of the ICS collector and piping only.

Solar storage tank is the solar collector at the same, has an inlet pipe connected to the bottom of the tank from the house plumbing and from the top of the collector hot water is usually connected to the backup storage heater. Whenever you open the hot tap, pressure from the home plumbing moves the hot water from the top of the solar collector/tank as the cold water is pushed into the bottom.

If the solar fluid from the ICS collector is hotter than the temperature setting on the back-up unit than heater will not activate.

These passive solar water heater systems are integral as the big cylindrical storage tank is used as the solar collector. People are using the batch water heater for hundreds of years, as they are simple to design and build. This passive batch system is using the south oriented insulated glazed box with a tank inside, filled with water.

This is also an open-loop system as the cold domestic water is heated directly. Due to its weight, special care should be taken if installing the batch water heater on the roof, which is the best spot.

Batch passive solar water heater is the best option for your money due to its simple and low cost design, and if you live in the warmer areas. This passive system is very popular in southern parts, tropical areas and in vacation homes and recreational facilities where it is used only during the summer while during the rest of the year should be drained.

We recommend: How to build your own batch solar heater.


  • One of the simplest solar heating systems.

  • Batch systems are very easy to maintain.

  • No mineral build-up when working with the hard water.

  • No moving parts, controllers, antifreeze...

emergency plumber


  • Lack of versatility.

  • They are used mainly in the warmer areas.

  • Batch heater when subjected to freezing has to be drained.

  • If used in colder areas, the efficiency is very low.

  • Batch collector might damage due to hot conditions.

  • Strong roof needed as they are heavy.

  • High stand-by losses


passive solar water heater thermosyphonThermosyphon solar water heaters are probably the most popular solar heating systems.

Thermosyphon work is based on the principle of physics where heated water rises, so the solar storage tank, has to be installed above the solar panels.

The main components of the thermosyphon passive systems are solar storage tank, panels, pipes and valves.

If you live in the warmer area you might want to consider direct, open-loop thermosiphon heating system, where domestic water is heated directly inside the flat plate solar collectors or panels. Usually, the bottom of the storage tank is connected to the bottom of the collector while top of the collector is connected to the top of the collector, transferring the heated water to the tank and from there to the backup heater.

The main disadvantage of these passive thermosyphon systems is that they are vulnerable on conditions like hard water, as the flat plate collectors are constructed of the small pipes, which can easily become clogged. One of the solutions is to use the softener.

If you are located in the colder areas, where there is a danger that water inside the collectors can freeze up, recommendation is to use the passive indirect closed-loop system filled with antifreeze solution, usually propylene glycol. Heat exchanger inside or outside the heater is used to transfer the heat from the solar fluid to the domestic water. This is what eliminates the problem caused by the hard water.

Solar storage tank in thermosyphon systems has to be well insulated to reduce the stand-by heat losses, especially during the night.


  • Simple by design.

  • Reliable.

  • Efficient as they are using well-insulated storage tanks. Even during the night, stand-by heat loss is significantly reduced.

  • No moving parts mean fewer problems.

  • Thermosyphon system is flexible; it can work in warmer and colder areas.

  • Easy troubleshooting.

  • Worry-free maintenance.

  • Cheap.


  • Pump might be needed for colder areas to move the water and prevent freezing.

  • They are heavy. When installed on the roof, the storage tank full of water, weights many gallons, so you might have to put some reinforcement on the roof.

  • The tubing inside the solar collectors is subjected to mineral build-up due to its small size in the areas with the hard water.

Either one you choose from the above two passive solar water heaters you won't mistake, as both of them utilize a free and "green" energy, are affordable, maintenance free, easy to install and use.

Pictures of solar heaters are courtesy of and Wikipedia

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