Harnessing the Power of the Sun: A Guide to Passive Solar Water Heaters

Are you looking for an eco-friendly and cost-effective way to heat your water?

Look no further than passive solar water heaters!

In this comprehensive guide, we’ll explore the advantages and disadvantages of this technology, including the most popular types: batch and thermosyphon systems.

We’ll also discuss how you can maximize energy efficiency in your home by combining passive solar water heaters with smart home technology.

Discover how you can reduce your carbon footprint and save money on energy bills with this essential guide to passive solar water heating systems.

How Do Passive Solar Water Heaters Work?

Passive solar water heaters utilize the natural circulation of water to transfer solar heat from a solar collector to a storage tank, making them an efficient and cost-effective heating system that harnesses free solar energy without requiring external energy to transfer hot and cold fluids.

This makes them a popular choice for those seeking a simple and reliable way to take advantage of solar power.

These systems can be used for direct domestic water heating or indirectly by circulating the solar heat transfer fluid through a heat exchanger.

While passive solar water heating systems are simple and affordable, they are also more vulnerable to weather conditions and may require more maintenance compared to other systems.

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

If you’re trying to decide between these two systems, consider your specific needs. For example, if you live in an area with a moderate climate, have a family of two, and typically use most of your hot water at the end of the day, a passive batch solar water heater or ICS system may be the best option for you.

Alternatively, if you have more than three people in your household and cannot install an additional solar storage tank near your existing heater, the thermosyphon system may be the better choice.

Types of Passive Solar Water Heater Systems

Passive Integral Collector Storage System – Batch Water Heater

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

Integral Collector Storage systems (ICS) are a type of passive solar water heater that do not require pumps for operation.

They are also known as batch water heaters, and they are the only solar heating systems that do not require a separate storage tank. ICS systems consist of the collector and piping only.

Batch water heaters have been used for hundreds of years, as they are simple to design and build. In this system, the water storage tank doubles as the solar collector, with an inlet pipe connected to the bottom of the tank from the house plumbing and the hot water outlet connected to the backup storage heater from the top of the collector.

When the hot tap is open, 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 a south-oriented insulated glazed box with a tank inside, filled with water, and it is an open-loop system, with the cold domestic water heated directly. However, due to its weight, special care should be taken when installing the batch water heater on the roof.

A batch passive solar water heater is a good option due to its simple and low-cost design, making it an attractive option in warmer areas. This passive system is particularly popular in southern parts, tropical areas, vacation homes, and recreational facilities, where it is used only during the summer. However, during the rest of the year, especially in cold weather, water should be drained.

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


  • Simple design. Batch water heaters have a straightforward design that uses a single tank that serves as both the collector and the storage tank. This design means that there are no external components, making installation and maintenance simple.
  • Low cost. Batch water heaters are relatively inexpensive when compared to other types of solar water heating systems. The simplicity of the design means that they have a lower cost of installation and are less expensive to maintain.
  • Reliable. Because batch water heaters have no moving parts, they are less likely to experience malfunctions or breakdowns. This means that they require less maintenance and have a longer lifespan than more complex solar water heating systems.
  • Can be DIY installed. Batch water heaters can be easily installed by homeowners who have some basic DIY skills. This can save money on installation costs.
  • Effective in mild climates. Batch water heaters are most effective in mild climates where temperatures do not drop below freezing. They work best in areas with a high level of solar radiation, making them ideal for use in locations that receive a lot of sunlight throughout the year.


  • 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 one of the most popular solar heating systems available.

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

It is based on the principle of physics where heated water rises and cold water sinks, 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 a thermosyphon passive system 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 storage tank, transferring the heated water to the tank and from there to the backup heater.

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

If you are located in colder areas where there is a risk of water inside the collectors freezing, the recommendation is to use the passive, indirect closed-loop system filled with an 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 eliminates the problem caused by hard water.

In thermosyphon systems, a solar storage tank should be well insulated to reduce standby heat losses, especially at night.


  • Reliability: Thermosyphon systems are simple and reliable systems that can provide hot water even during power outages or other emergencies. As long as the sun is shining, the system will continue to work.
  • Energy-efficient: These systems are highly energy-efficient, as they don’t rely on electricity or gas to heat water. This means that they can help reduce your energy bills and lower your carbon footprint. Additionally, these systems use well-insulated storage tanks, which minimizes standby heat loss even during the night.
  • No need for a pump: Unlike active solar water heating systems, thermosyphon systems do not require a pump to circulate water or heat transfer fluid. Instead, they rely on natural convection currents to circulate the water, which makes them more energy-efficient and cost-effective.
  • Suitable for remote locations: Thermosyphon systems are ideal for remote locations that do not have access to electricity or where electricity is unreliable. They can be used in off-grid cabins, rural areas, and other remote locations where access to hot water is necessary. They can work in warmer and colder areas.
  • Easy troubleshooting. Their simple design makes them easy to troubleshoot and repair in case of any issues. Additionally, since they have fewer components than other types of water heaters, there are fewer potential failure points, reducing the likelihood of breakdowns or malfunctions.
  • Worry-free maintenance. These passive solar water heaters are generally low-maintenance systems, as they have few moving parts and don’t require regular servicing.
  • Cost-effective: Thermosyphon systems are one of the most affordable ways to heat water, as they don’t require any external energy sources to operate. They rely solely on the sun’s energy, which is free and abundant. Many homeowners with basic DIY skills can install these systems themselves, which can save money on installation costs.


  • 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.

You can’t go wrong with either of the two passive solar water heater types mentioned above. Both systems harness free and environmentally friendly energy, are cost-effective, require minimal maintenance, and are straightforward to install and use.


Passive solar water heaters offer several advantages, but they also have some limitations that homeowners should consider before deciding to install one. Some of the limitations include:

Climate limitations: Passive solar water heating is the most effective in areas with high levels of solar radiation and mild climates. In areas with low levels of sunlight or very cold climates, these systems may not work as efficiently, and homeowners may need to supplement their water heating needs with a backup system.

Dependence on sunlight: As the name suggests, passive solar water heaters rely on sunlight to heat water. This means that they may not work as efficiently on cloudy days or during the winter months when sunlight is scarce.

Initial installation costs: While passive solar water heaters can save homeowners money in the long run, the initial installation costs can be higher than other types of water heating systems. This is especially true if homeowners need to retrofit an existing home to accommodate the new system.

Seasonal variation in performance: Their performance can vary throughout the year. During the summer months, these systems may work very efficiently, but during the winter months, they may not be able to heat water to the desired temperature. Homeowners may need to use a backup system during these times to ensure a reliable supply of hot water.


Passive solar water heaters offer a cost-effective and environmentally-friendly way to heat water using the power of the sun.

As we could see, these systems offer several advantages, including energy efficiency, simple design, and easy troubleshooting, but they also have some limitations.

With simple designs and easy installation, these systems can save homeowners money on energy bills while reducing their carbon footprint. Whether you choose a thermosyphon or batch system, both types of passive solar water heating systems offer unique advantages that make them a popular choice for homeowners looking to make their homes more sustainable.

By harnessing the natural power of the sun, passive solar water heaters are an excellent investment for both your wallet and the planet.


What Is the Main Difference Between Active and Passive Solar Water Heaters?

The main difference between active and passive solar water heaters is the method of circulating water through the system, with active systems requiring external energy and passive systems utilizing natural convection.

Do Passive Solar Water Heaters Work in Winter?

Passive solar water heaters can work in winter, but their performance may be affected by the lower intensity of sunlight and shorter daylight hours. To compensate for this, the systems may require larger solar collectors and storage tanks, as well as additional insulation to prevent heat loss.

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