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Heat Transfer Fluids

Heat transfer fluids transmit the heat from solar collectors to the water for the home. The heat transfer fluid flows through the collector to the water storage tank, passing on the thermal energy by way of a heat exchanger.

There are many potential heat transfer fluids. The right fluid depends on the type of solar hot water heating system and the climate. Factors to consider are: freezing point, boiling point, and viscosity (which determines how much energy is needed to pump the fluid). at transfer fluids used in solar hot water systems are non-toxic. The most common transfer fluids are:


Different heat transfer fluids are suitable for different climates.


  1. non-toxic, easy to pump, inexpensive and efficient

  2. not suitable for extreme heat or cold due to it’s high freezing point and low boiling point

  3. water with a high mineral content can lead to corrosion in the pipes

Propylene Glycol 

  1. known as anti-freeze in solar thermal systems

  2. works in freezing temperatures

  3. must be replaced every 3-5 years

  4. requires additional components

  5. non-toxic


  1. higher boiling point

  2. works in freezing temperatures

  3. requires addition components

Hydrocarbon Oils

  1. need more energy to pump than water

  2. low freezing point

  3. some types are toxic, requiring a double-walled, closed-loop heat exchanger


  1. low freezing point and high boiling point

  2. non-toxic and non-corrosive

  3. leak very easily

  4. require more energy to pump


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