Solar medium: properties and proper handling

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Solar medium transports heat from solar collectors on the roof to the cylinder in the house. It consists of water and antifreeze so the heat transfer medium does not freeze, even in the winter. As the medium is subject to certain signs of ageing, it is important to check the pH value and antifreeze content at regular intervals. Find out here about the properties and specifications for handling solar medium.

Antifreeze in the solar system ensures year-round operation

When the sun shines, the collectors in a solar thermal system heat up. They transfer thermal energy to a medium, which then flows to the cylinder. There, it transfers the energy to heating or DHW via a heat exchanger. The heat transfer medium flows back to the solar collector and the process starts again. 

The solar medium is based on water. If the outside temperatures were to drop below freezing and solar radiation did not occur, the medium would freeze, expand and potentially damage important parts of the system. To prevent this, the solar medium contains an antifreeze. This is usually propylene glycol.

Solar medium: properties change over time

The 1,2-propylene glycol used is mildly flammable, non-toxic, biodegradable and does not require labelling according to EU criteria. The antifreeze for solar systems has a boiling temperature of 188 degrees Celsius and protects against freezing down to -21.5 degrees Celsius at a content of 40 percent by volume.  

As glycol is an organic substance with common signs of ageing, the heat transfer media used by Viessmann, such as Tyfocor LS, contain anti-ageing protection. Alkaline buffering keeps the pH value in the alkaline range for a prolonged period of time and promotes corrosion protection. This in turn has a favourable effect on the service life of the entire system.

Have the antifreeze content and pH value checked during maintenance

Due to normal signs of ageing, it is important to have the heat transfer medium checked as part of the annual heating system maintenance. Heating contractors will check the pH value and the antifreeze content to prevent damage due to corrosion or frost.

The design of the system influences the service life of the solar medium

In smaller systems for DHW heating, heating contractors should change or refill the solar medium every ten years on average. If it is a solar system for central heating backup, this might need to be done earlier. This is caused by downtime without heat extraction in summer. During such periods, the temperature of the heat transfer medium rises so much that cracking or breaking of the molecules can occur. As a result, acid formation increases and pipes and pumps can become clogged.

The Viessmann ThermProtect coating automatically switches off flat-plate collectors and tube collectors when they exceed a certain temperature. This happens regardless of the construction type, design or installation and effectively protects against the medium cracking.

If signs of ageing appear, top up or change the solar medium

If the pH value and antifreeze content are not within the recommended range, a heating contractor must top up the solar medium. The medium that is used depends on the medium already in the system, which can be recognised by means of factors including its colour. Purple Tyfocor G-LS can be mixed with Tyfocor LS, for example. If the additives cannot be combined, the heating contractor will change all of the medium in the solar system.

A heating contractor can help you with these problems

The heat transfer medium mixed with glycol circulates through the solar thermal system in a materially separated manner. It does not mix with either the heating water or the DHW, posing no danger to other systems. If medium has leaked from the solar system or if there is solar medium in the DHW, you should consult a heating contractor. He/she will reliably eliminate leaks and refill the heat transfer medium. In the unlikely event that medium from the solar system gets into the DHW, the heating contractor will flush the system until there is no more residue. In the meantime, refrain from consuming the water.