Basic principle
A heating curve describes the relationship between the outdoor temperature and the supply temperature (the temperature of the water sent to the heating system).
The principle is simple: the colder it is outside, the hotter water is needed to maintain the right indoor temperature. When it's milder outside, a lower water temperature is sufficient.
Example
If it's -15°C outside, the heating system may need a supply temperature of 45°C. When it's +5°C outside, 30°C may be enough.
Key concepts
Slope (the curve)
The slope (often called "the curve" or "slope") describes how steeply the supply temperature changes when the outdoor temperature drops. A higher slope gives more temperature increase per degree of outdoor temperature drop.
- High slope (5-7): The system reacts strongly to temperature changes. Suitable for older houses with radiators.
- Low slope (1-3): Smoother response. Suitable for well-insulated houses and underfloor heating.
Offset (parallel shift)
The offset (also called "parallel shift") moves the entire heating curve up or down. If you increase the offset by 2 degrees, the supply temperature rises by 2 degrees at all outdoor temperatures.
This is the simplest way to adjust if it generally feels too warm or too cold indoors.
Point adjustment (knee adjustment)
Some heating systems offer the ability to adjust the curve at specific outdoor temperatures, often called "knee adjustment" or "point adjustment". This is useful if the curve works well at extreme temperatures but not at intermediate ranges (e.g., around 0°C).
Why is the heating curve important?
Automatic correct temperature
With the correct curve, you don't need to manually adjust thermostats when the weather changes.
Energy efficiency
A heat pump operates more efficiently with lower supply temperatures. Every degree lower on the water temperature saves 2-3% energy.
Even temperature
The system maintains a constant temperature instead of oscillating between too hot and too cold.
Difference from thermostat control
Thermostats and the heating curve complement each other, but they work in different ways:
- The heating curve determines how hot the water produced is, based on the outdoor temperature.
- Thermostats restrict the flow to individual rooms to adjust the temperature locally.
A well-tuned heating curve means the thermostats stay fully open most of the time, which gives the most efficient operation.
Important
If you adjust the curve while the thermostats are closed, you'll get an inaccurate result. Always set thermostats to maximum when fine-tuning the heating curve.
Next steps
Now that you understand the basics, you're ready to start setting up your own heating curve. Read our guide to setting up the heating curve or check the reference tables for recommended values.