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Answering your frequently asked questions


How The Solar Water Heater Works

Solar water heaters make use of two natural phenomena for their operation: black objects absorb heat, and hot water rises. It is a simple system, and the only moving part is water.


The absorber panels of the Solahart K and the Solahart L Models.

The performance of Black Chrome is superior and is used in colder climates

Selective surfaces

The type of system installed is dependant on water usage and climate. The simple black painted surface can be improved upon by the use of "selective" surfaces. Instead of black paint, the absorber surface goes through a two stage plating process, first with nickel and

a final coat of Black Chrome. The performance of Black Chrome is superior through its ability to absorb and retain solar energy. The selective surface absorber is used in colder, cloudy climates to compensate for lower levels of solar radiation. ( Fact File 003 describes the function of selective surfaces).

The Solahart L systems are used in areas where water quality is good and freezing does not occur. Recommended where high solar radiation levels occur.


Standard fin and tube absorber design.


In an open circuit system such as Solahart's "L" Series, water circulates through the collectors, is heated by the sun and then stored in the storage cylinder.

Suitable for:

  • High solar radiation areas.
  • Medium to good water quality.

Not suitable for:

  • Frost prone areas.

The Direct Thermosiphon Principle

Imagine a solar absorber filled with cold water. When the sun warms the collector, the hot water in the collector moves upwards into the tank, displacing cold water, which descends to the bottom of the collector. The sun heats the water, and this "circular" path continues. This is known as the "thermosiphon" effect. The greater the temperature difference, the faster the flow of hot water to the storage tank.

When the sun's heat is no longer present, the thermosiphon effect stops, and hot water retained in the storage tank stays hot because of the tank's high density insulation and design.


The Heat Exchanger Thermosiphon Principle

In places where freezing temperatures (0'C; 32'F) occur, a conventional open circuit solar water heater is not suitable due to the expansion of water as it freezes. This will burst the piping within the absorber causing damage which will result in costly replacement not covered by manufacturers warranty. Solahart developed and patented a unique heat exchanger system known as the "jacketed solar water heater".

This design eliminates the potential damage which is possible due to freezing. A jacket is located around the complete circumference of the potable water storage tank and this jacket is connected to the absorber.

The same thermosiphon process occurs, but in this case, the heat is "exchanged" from the closed circuit to the water in the tank. Solahart always specifies the jacketed solar hot water system wherever freezing temperatures occur.

The Solahart J , K and BC XII models use an internal heat exchanger to ensure that the potable water supply is isolated from the absorber plates to prevent freezing and clogging



Solahart's Multiflow(TM) absorber design increases the fluid to surface contract ratio.


HARTGUARD(TM) heat exchange fluid circulates around the closed circuit using the Thermosiphon principle. As it passes around the storage cylinder heat is transferred to the stored water.

Suitable for:

  • Hard water areas.
  • Frost prone area.
  • Low solar radiation.
  • Fast recovery.

Solar hot water system Boosting

The amount of water heated by the sun varies with climate. But even on cloudy, rainy days, there is still some solar energy being collected by the absorbers.

There are some locations around the world where a solar water heater can provide virtually all the user's needs for hot water without the need for boosting. However, for most users, a booster is required to offset the lack of solar radiation on cold and rainy days.

Electric boosters are the most common forms of booster, although the most environmentally friendly and efficient is the Solahart 'Natural Wonder' gas booster.

Whichever type of booster is used, a thermostat automatically controls the use of purchased energy. In the case of the gas booster, the ignition is fully automatic.

With an electric or gas booster, the householder can further manage boosting with an inexpensive time switch to control the booster.


The graphs shown are from Solahart's Solar Contribution Factor(TM) software (called SCF). This is Windows compatible and can be obtained from your Solahart Dealer (or downloaded)


Melbourne, Australia



Amsterdam, Netherlands
The amount of boosting depends on local climate.


Los Angeles, USA

Hot Water Recovery

If all the hot water in a Solahart solar water heater is used, it only takes a short time to reheat.  The time taken depends on if the sun is shining and the type of boosting used.  From a temperature of 20'C (68'F) (feeling cold to touch) an electric booster will take approximately one hour to generate enough hot water to take a hot shower (at 45'C (113'F).  Generally, a gas booster is quicker.  Recovery is quicker still during peak solar conditions (9 a.m. to 3 p.m.).

If hot water is used in the early morning, then there is an opportunity for the sun to reheat the water rather than electric or gas boosting.  This is a less costly and more environmentally friendly approach.


Estrada do Algueirão,  31-1º  Dto.
2725 Algueirão - Portugal
Tel. (351)  21  922  9540 - Fax (351)  21  921  0418
Email:  solahart@digal.pt