Setting the heating curve: how to find the right setting

The heating curve is the most important setting on your heat pump, and you can change it yourself. It determines how warm the heating water becomes at which outdoor temperature. If it is set too high, the system heats with a higher temperature than necessary, and with a heat pump, this costs electricity every day.

This article shows you how to find your way to the right setting. Why the flow temperature makes such a difference is explained in detail in the article on Flow Temperature.

What the heating curve does

A sensor on the outside wall measures the outdoor temperature. The control system looks up the matching flow temperature in the heating curve and sets it. The colder it is outside, the warmer the heating water must be. Energieberatung Niederösterreich (Energy Advisory Service of Lower Austria) calls this weather-compensated control.

For it to work well, the curve must suit your house: its insulation, your radiators or underfloor heating, and the room temperature you want.

Slope and parallel shift

On almost every control system, you will find two adjustment options. The names differ depending on the device, but the effect is the same.

The slope (also inclination or gradient) determines how much the flow temperature rises when it gets colder outside. A steep curve reacts strongly, a flat one weakly. The eNu shows this with two examples: with a slope of around 1.5, which is often set as a factory default for radiators, the flow temperature is around 77 °C when it is minus 15 degrees outside. With a slope of 0.4, typical for underfloor heating or a well-insulated house with large radiators, it is only around 40 °C in the same cold conditions.

The parallel shift (also level or offset) shifts the entire curve up or down. It changes the flow temperature by approximately the same amount at any outdoor temperature.

The rule of thumb for choosing which setting to adjust:

• If it is consistently too warm or too cool for you in all weathers, the parallel shift is usually the issue.
• If it is fine on mild days, but gets too cold or too warm during frosty weather, it is more likely due to the slope.

Which values are right for you cannot be read from a table. They depend on the house and can only be found by trial and error.

Why finding the right setting is worth the effort

According to eNu, the heating curve is set too steep on up to 85 per cent of heating systems. The reason is understandable: anyone commissioning a system wants to avoid complaints about cold rooms and prefers to set it with a safety margin.

With a heat pump, this margin costs a noticeable amount. klimaaktiv puts the savings at 2 to 2.5 per cent less electricity consumption per degree of lower flow temperature, while eNu cites 2.5 to 4 per cent. Both organisations recommend setting the curve as flat as possible. klimaaktiv also points out that lower flow temperatures also benefit the lifespan of the heat pump.

Proceed step by step

1. Note down the initial values. Take a photo of the control screen or write down the slope and parallel shift. That way, you can go back at any time.

2. Open the valves. Turn the thermostatic valves in the living areas to the setting that corresponds to your desired temperature, and leave them there. According to the eNu, setting 3 usually represents about 20 °C, setting 4 about 22 °C. Make sure that no furniture or curtains cover the radiators.

3. Start during the heating season. On mild autumn days, the system runs very little, so you will hardly notice anything. It only becomes meaningful when it is really cold outside.

4. Change in small steps. Set the curve one step flatter or slightly lower. Only adjust one thing at a time, otherwise you won't know afterwards what worked.

5. Wait. The eNu recommends observing for two to three days after each change. A house reacts slowly, especially with underfloor heating. If you readjust on the same evening, you will end up going in circles.

6. Repeat until the first room becomes too cool. Then go back one step. That is the limit of what your heating surfaces can achieve.

7. Write down the result. The values you find should be kept in your records, along with the commissioning report.

The room thermostats: leave open

Thermostatic valves and the heating curve work against each other if the curve is too high. The valve then constantly throttles to prevent the room from overheating. The eNu describes the typical sign: the radiator repeatedly gets cooler in between. The valve has then stopped the flow and "masks" a heating curve that is set too high.

The valves should only intervene when additional heat enters the room, for example from the sun or a stove. In normal operation, the heating curve regulates the system. So set the valves to a fixed level and do not adjust them daily.

If you have a room control unit or a room sensor that affects the heat pump, ask your installer how this room influence is set. Otherwise, you will be regulating in two places at the same time.

When individual rooms do not play along

If one room remains cold while the rest of the house is too warm, this is rarely due to the heating curve. Often, hydraulic balancing (hydraulischer Abgleich) is missing, or the radiator in that room is too small. The eNu mentions unevenly warm rooms and different heating speeds as typical signs.

Making the heating curve steeper because of a single room is the most expensive way. It is better to talk to your installer about balancing or a larger radiator in that specific room.

When to get help

• The control system displays an error message.
• You cannot find the setting or it is locked.
• Even with a steeper curve, the house does not get warm during frosty weather.

The eNu recommends having the setting of the heating curve explained to you during the initial briefing. You can read more about the numbers you should keep an eye on after installation in the article Nach dem Einbau (After Installation). Whether your setting actually saves electricity can be seen from the seasonal performance factor (Jahresarbeitszahl), which you can calculate yourself.