Passive Solar Water Heaters: Advantages and Benefits

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

Passive solar water heaters are simple heating systems that utilize free solar heat energy without the use of external energy to transfer hot and cold solar fluids. This is the most popular way of harnessing free solar energy as 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 their simplicity, passive solar water heater systems are more vulnerable than other solar heating systems.

Types of 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 an area with a moderate climate, your family consists of two members, and most 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, a recommendation is to use the thermosyphon system.

Passive Integral Collector Storage System - Batch Water Heater

Batch solar water heaterBatch solar water heater; Photo by EERE [Public domain], via Wikimedia Commons

Integral Collector Storage systems (ICS) 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 need a storage tank. They consist of the ICS collector and piping only.

People have been using batch water heaters for hundreds of years, as they are simple to design and build.

The water storage tank is the solar collector at the same time. It 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, the pressure from the home plumbing moves the hot water from the top of the solar collector/tank as the cold water is pushed to the bottom.

If the solar fluid from the ICS collector is hotter than the temperature setting on the backup unit, then the heater will not activate.

This passive batch system uses 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.

A 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 warmer areas. This passive system is very popular in the southern parts, tropical areas, vacation homes, and recreational facilities where it is used only during the summer. During the rest of the year, especially in cold weather, water should be drained.

Here are the ideas on how to build your own batch solar heater.

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  • 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...


  • Lack of versatility.
  • They are used mainly in warmer areas.
  • When subjected to freezing batch heaters have to be drained.
  • If used in colder areas, the efficiency is very low.
  • A batch collector might damage due to hot conditions.
  • A strong roof is needed as they are heavy.
  • High standby heat loss.

Thermosyphon systems

Thermosyphon solar water heaters are probably the most popular solar heating systems.

According to Wikipedia, "Thermosiphon (or thermosyphon) is a method of passive heat exchange."

It is based on the principle of physics where heated water goes up, cold water goes down, resulting in water or liquid circulation. That is why in thermosyphon type water heaters, the solar storage tank is installed above the solar panels.

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

If you live in a warmer area, you might want to consider a 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 the top of the collector is linked to the top of the collector, transferring the heated water to the tank and from there to the backup heater.

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

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

In thermosyphon systems, a solar storage tank has to be well insulated to reduce the standby heat losses, especially during the night.


  • Simple design.
  • Reliable.
  • Efficient as they are using well-insulated storage tanks. Even during the night, standby heat loss is significantly reduced.
  • No moving parts mean fewer problems.
  • The thermosyphon system is flexible; it can work in warmer and colder areas.
  • Easy troubleshooting.
  • Worry-free maintenance.
  • Cheap.


  • A 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 weighs 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 areas with hard water.

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

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