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The temperature level of a heating system describes the target system temperature of the heating system at full load. Example: The heating system runs at full load at a standard outdoor temperature of -14 °C with the following system temperature: flow = 75 °C, return = 55 °C, room temperature = 20 °C, system temperature: 75/55/20.
Typical system temperatures for hot water heating systems at different room temperatures are 75/65 °C, 75/55 °C, 70/50 °C or 55/45 °C.
Influencing factors include the heat generator, the radiator size, the room heating load or the minimum flue gas temperature for fuel boilers.
For heat pumps, lower system temperatures with a low spread are generally aimed for, as heat pumps then run particularly efficiently. It is important that only the flow temperature and the room temperature can be stored in the heat generator. Ideally, the return temperature should then be set automatically. This can be achieved by hydraulic balancing, for example.
Below you will find an overview of common system temperatures for heating systems with different heat generators.
| Heat generator | Flow | Return flow | Spread | System temperatures |
| Low temperature boiler | 75 °C | 55 °C | 20 K | 75/55 |
| Condensing boiler | 50 – 75 °C | 35 – 55 °C | 15 – 20 K | 50/35, 60/40, 75/55 |
| Heat pump | 35 – 55 °C | 30 – 45 °C | 5 – 10 K | 35/30, 45/35, 55/45 |
