In a central heating system, water is the heat transfer medium.

Because the temperatures in a heating system are usually not that high
water is excellent here. In the past, water was converted to pressurized water
steam or so-called high-pressure steam with which temperatures of up to 300 ° C
can be reached.

However, high pressure steam installations have a whole range of drawbacks that we have do not go into this. In addition to water, there are a number of other heat transfer media which are extremely suitable within their temperature range, and these are as an example:

Medium Temperaturerange
Water and steam +20 until +350 ºC
Glycols -10 until +180 ºC
Mineral oils -30 until +250 ºC
Synthetic oils (pressureless) -115 until +400 ºC
Synthetic oils (negative pressure) -40 until +400 ºC
Silicone oils -40 until +400 ºC
Melted salt +160 until +540 ºC
Liquid metals 0 until +850 ºC

This list is not complete but gives a reasonable overview for those who are not familiar with this matter. It should be noted that the use of mineral thermal oils is recommended by almost all manufacturers up to 300 °C or even higher.

What is not mentioned is that the durability at these higher temperatures is considerably limited and gives the chance of increased fouling (scale) in the thermal system. Factors such as flow, viscosity, temperature and turbulence in the liquid play a crucial role in this.

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