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How much solar fluid do I need?

The capacity of the solar circuit typically comprises the capacity of the collector manifolds, pipe runs and heat exchangers.  Collector capacities are shown on the collector prices page (here), and typical pipe run volumes can be calculated by knowing that 15mm copper pipe contains about 0.14 litres per metre and 22mm copper pipe around 0.31 litres per metre.  Heat exchanger capacities are usually stated by cylinder manufacturers.  Average domestic installations with 15mm pipework are usually less than 10 litres.

General information about solar antifreeze.

Rule 1 is don't use vehicle antifreeze which is ethylene glycol and can cause kidney failure if ingested.  In the event of a heat exchanger in a hot water cylinder failing, the solar fluid could find its way into domestic hot water. 

It is generally thought that propylene glycol solutions will have a higher boiling point than water, because if neat it boils at around 180C.  However, when mixed as 40:60 PG/water, boiling occurs at 103C.  Pressure has a much greater effect on boiling point.  Eco-nomical 40/60 solar fluid solution boils at 124C at 1bar (gauge*) and 143C at 2.5 bar.

Eco-nomical solar fluid

Eco-nomical solar fluid is ready to use, comprising 40% propylene glycol with high temperature corrosion inhibitor additives tailored to metals found in solar installations (primarily copper, brass and stainless steel), and approximately 60% demineralised water.  It is non toxic and readily bio degradable if spilt.  Freeze protection is provided to -20C.

Temperatures above 170C will cause the PG based solar fluids to age prematurely. and above 200C will degrade the fluid and require its replacement.  In normal operation it is generally recommended to replace the fluid at five yearly intervals.


*Note the difference, often overlooked, between gauge and absolute pressure.  Atmospheric pressure, the pressure of the surrounding air at ground level, is about 1 bar.  Doubling that pressure in a sealed container by using a pump results logically in a pressure of 2 bar.  This is called the absolute pressure.  However, connecting a pressure gauge to the container produces a reading of 1 bar.  This is called gauge pressure, and is the difference between atmospheric and absolute pressure.  Both are used, always be clear which is being discussed.

Conversion factors

1 bar approximately equals

14.5 psi (pounds per square inch)

0.1 MPa, (megaPascals)

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Last modified: 30-06-10