Heating curve · Glossary

Definition

The heating curve, also called the heating characteristic, describes in a weather-compensated heating control the relationship between outdoor temperature and target flow temperature. It determines how warm the heating water should be as it flows from the heat generator into radiators, underfloor heating or other heat emitters, so that the rooms reach the desired temperature in any weather. When it is cold outside, the curve specifies a higher flow temperature; in mild weather, a lower one. The outdoor temperature is usually measured by an outdoor sensor on the north side of the building. Some controllers also take into account the room temperature (room influence) or a damped average of the outdoor temperature.

The heating curve is not a property of the heat generator alone, but a setting for the entire heating system. The building envelope, the size and type of the heat emitters, heat distribution, control and user behaviour all interact in it. The same appliance can operate economically with a well-set curve and consume unnecessarily much energy with an unfavourable setting.

Slope, level and heating limit

A heating curve usually has two main adjustable parameters. The slope, also called gradient, indicates how strongly the flow temperature rises when the outdoor temperature falls. A poorly insulated building with small radiators generally needs a steeper curve than a well-insulated building with large heat emitters or underfloor heating. The level, also called parallel shift, raises or lowers the entire curve. If it is raised, flow temperatures rise at every outdoor temperature; if it is lowered, the system runs cooler overall. Many controllers also have a heating limit or summer/winter changeover, above which space heating switches off, as well as upper and lower limits for the flow temperature. More on this under heating limit.

An example illustrates this: with a flat curve for underfloor heating, the controller may demand a flow temperature of perhaps 35 degrees Celsius at minus 10 degrees Celsius outdoors, and only about 27 degrees at plus 10 degrees. A steep curve for small radiators in an unrenovated old building may demand 60 degrees or more at the same cold. The numerical values for slope and level are defined differently depending on the manufacturer and therefore cannot be compared directly between controllers of different brands. What matters is the course of the curve shown on the controller display or in the documentation.

Distinction from related variables

The heating curve directly controls only the flow. To be distinguished from it are the room temperature as the result in the room and the return temperature, at which the water flows back to the generator after releasing its heat. The difference between flow and return, the spread, together with the volume flow shows how much heat the heat emitters have released. All three temperatures are important for efficiency and control.

The heating curve is often confused with the heating load. The heating load is a power, usually given in kilowatts. It describes how much heat per unit of time a building needs at the standard outdoor temperature to stay warm. The heating curve, by contrast, is a control relationship between two temperatures. A high heating load can make high flow temperatures necessary, but does not have to if the heat emitters are large enough. Conversely, a building with a moderate heating load can be poorly set and therefore run with an excessive flow temperature.

Thermostatic valves and room controllers are also often overestimated. They do not determine the temperature level at which the system fundamentally operates. They only throttle the heat flow in individual rooms. If the heating curve is set too high, the valves do protect against overheating, but they do not eliminate the cause. The system then generates heat at too high a temperature level and subsequently throttles it down again.

Significance for heat pumps

For a heat pump, the heating curve carries particular weight because electricity demand depends closely on the temperature lift, that is, the difference between the temperature of the heat source and the required heating level. An air-to-water heat pump operates considerably less efficiently with cold outdoor air and a high flow temperature than with mild air and a low flow temperature. An unnecessarily high heating curve therefore generally lowers the seasonal performance factor. Large radiators, surface heating systems, hydraulic balancing and the lowest suitable curve possible can make the same appliance considerably more efficient.

Field measurements show that heat pumps also frequently manage with moderate flow temperatures in existing buildings. In the WP-QS im Bestand project, Fraunhofer ISE measured 77 heat pumps in single- to three-family houses over four years. The seasonal performance factors ranged between 2.6 and 5.4. Air-to-water systems achieved an average of 3.4, ground-source systems 4.3. The wide range makes clear how strongly design and setting influence operation. However, it cannot be attributed to the heating curve alone.

Significance for condensing boilers

The heating curve also matters for condensing boilers, although for a different reason. A boiler only uses the condensing effect if the water vapour in the flue gas condenses. For this, the return temperature must be below the dew point of the flue gases, roughly 55 degrees Celsius for natural gas. A curve that is too high usually also raises the return temperature and thus weakens the condensing effect. This brings no more comfort, but it does bring higher fuel consumption.

Setting in practice

The suitable heating curve cannot be reliably read from the data sheet. It is found step by step. The starting point is the condition of the building, the heat emitters and the desired room temperatures. Then one observes how the building reacts at different outdoor temperatures. The following rules of thumb help here:

Too warm in all weather: the level is probably too high and can be lowered.
Too cool only in cold weather, but comfortable in the transitional season: the slope is probably too low.
Too warm only in the transitional season, suitable in cold weather: a steeper curve with a lower level can help.
Make changes in small steps and wait several days each time, if possible with open thermostatic valves in the reference room.

Patience is necessary because buildings react slowly owing to their thermal inertia, and individual days can be distorted by sun, wind or internal heat sources. If the curve is too low, rooms do not become warm enough in cold weather or take a long time to heat up. However, the cause does not always lie in the curve. Air in the heating circuit, missing hydraulic balancing, heat emitters that are too small, incorrectly set pumps or an unfavourably placed sensor can create the same impression. Anyone who then merely raises the curve covers up the problem with a higher temperature and pays for it with permanently higher consumption. It is technically cleaner to first check the cause of the undersupply.

Diagnostic tool for heat pump suitability

The heating curve shows that a building's suitability for a heat pump is not a yes-or-no question, but a question of degree. If a house needs a 60-degree flow temperature on the coldest days, that is unfavourable for a heat pump. If 45 degrees is sufficient after radiator replacement, hydraulic balancing or insulation, the assessment turns out differently. A practical test in an existing building: gradually lower the curve of the existing boiler in winter and observe at which flow temperature the rooms become too cool. This reveals which temperature level the building actually requires in real operation.

Legal framework in Germany

In Germany, building energy law prescribes a heating system check for older water-based heating systems that are not heat pumps. This applies only in buildings with at least six dwellings or other independent units of use (§ 60b). Systems installed before 1 October 2009 must be checked by 30 September 2027. Later systems are due within one year after the end of 15 years of operation. The check is intended, among other things, to clarify whether the control parameters are energetically optimised and how the flow temperature can be lowered. As a possible measure, the law expressly mentions lowering the flow temperature or optimising the heating curve in the case of gross misadjustments. If the check reveals a need for optimisation, the measures must be implemented within one year. This obligation does not apply to single- and two-family houses.

For heat pumps installed in such larger buildings since 1 January 2024, an operational check under § 60a applies instead. The version of the GEG valid until July 2026 expressly mentioned the setting of the heating curve. The Gebäudeenergiegesetz (Building Energy Act) was comprehensively amended by an act promulgated in the Bundesgesetzblatt (Federal Law Gazette) of 28 July 2026 and has since been called the Gebäudemodernisierungsgesetz (GModG, Building Modernisation Act). § 60a was newly worded in the process. According to the new wording, the operational check covers the settings and components that are decisive for the operation, function and efficiency of the heat pump. From a technical standpoint, this includes the heating curve, even if it may no longer be listed individually in the law. Anyone who wants to check specific obligations should consult the currently consolidated version.

Significance for the electricity system and flexibility

Through the electrification of heat supply, the heating curve also becomes more important for the electricity system. A curve that is too high increases the electricity demand of heat pumps, especially on cold days. Then many systems are running at the same time anyway and the electrical load is high. Low, suitable heating curves therefore reduce peak loads and relieve distribution grids and electricity generation.

The heating curve is also linked to flexibility. A building with low flow temperatures, sufficient heat emitters and thermal storage mass can shift heating times without comfort immediately suffering. This facilitates grid-serving or electricity-price-oriented heating. An excessive curve reduces this room for manoeuvre because the system is operating at a high temperature level anyway and is more likely to cycle or overheat. Flexibility thus arises not through digital control alone, but also through the building's physical ability to absorb, store and slowly release heat efficiently.

Distributed responsibility

Several actors are responsible for the heating curve. Manufacturers supply appliances and control concepts, specialist companies design and set up systems. Owners decide on the building envelope, heat emitters and investments, users on room temperatures, ventilation and thermostats. Energy suppliers and grid operators see only total consumption, not the cause in the control. A wrong heating curve therefore triggers no fault message, but causes silent, ongoing excess consumption. Those who clearly distinguish energy demand, heating output, flow temperature and control do not wrongly blame poor system coordination on the heat generator, the building or the users alone.

Sources

Wärmepumpen heizen auch im Altbau klimafreundlich (Presseinformation) — Fraunhofer-Institut für Solare Energiesysteme ISE; 2025-11-03. Seite 1, Abschnitt Effizienz
Abschlussbericht WP-QS im Bestand — Fraunhofer-Institut für Solare Energiesysteme ISE; unbekannt. Zusammenfassung, S. 7
§ 60b GEG – Prüfung und Optimierung älterer Heizungsanlagen — LexMea (Wiedergabe des Gesetzestextes); unbekannt. § 60b Abs. 1, 2 und Optimierungspflicht
Bundesgesetzblatt Teil I 2026 Nr. 226 — Bundesministerium der Justiz (Bundesgesetzblatt); 2026-07-28. Neufassung § 60a; Bußgeldvorschriften zu § 60a und § 60b
Gebäudeenergiegesetz – § 60a Prüfung und Optimierung von Wärmepumpen (bisherige Fassung) — Bundesministerium der Justiz / gesetze-im-internet.de; unbekannt. § 60a Abs. 1 und 2

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Edited by eCMS on 12.11.2026 05:05