Heating limit temperature · Glossary

Definition

The heating limit temperature (German: Heizgrenze, also called Heizgrenztemperatur) is the outdoor temperature below which a building must be actively heated in order to maintain the desired indoor temperature. It usually refers to the daily mean outdoor temperature, not a single measurement taken in the afternoon. Above this limit, free heat gains are usually sufficient. These include people, electrical appliances, lighting and solar radiation. In addition, there is the heat stored in the building structure, which bridges short cool periods. The heating limit is therefore not the switching point of an individual heating system, but an energy threshold of the building.

Physical background

Technically, the heating limit lies where heat losses and usable free heat gains balance each other. Losses occur through transmission via the building envelope and through ventilation. They increase roughly in proportion to the temperature difference between inside and outside. In simplified terms, the following applies: heating limit ≈ indoor temperature minus (free gains divided by the building's heat loss coefficient). The better the envelope and the higher the gains, the further the heating limit lies below room temperature.

A calculation example with invented values illustrates this. An unrenovated house loses 250 watts per kelvin of temperature difference. On a daily average it has 1,250 watts of free gains. These then cover a difference of 5 kelvin. At 20 °C indoors, the heating limit is therefore about 15 °C. A well-insulated house loses only 120 watts per kelvin and has 1,000 watts of gains. This covers a good 8 kelvin, and the heating limit is just under 12 °C. If the room temperature is raised to 22 °C, the heating limit shifts upward by around 2 kelvin in both cases. The heating limit therefore also depends on user behaviour.

Standard values and heating degree days

Many simplified methods work with fixed heating limits. In Germany, the G20/15 convention is particularly widespread: 20 °C room temperature and a 15 °C heating limit. The German Weather Service (Deutscher Wetterdienst) publishes degree days according to VDI 3807 Part 1 for the stations of its measurement network. There, a day only counts as a heating day if the daily mean outdoor temperature is below 15 °C. For that day, the difference between 20 °C and the daily mean is then counted. These values serve to compare heating energy consumption on a weather-adjusted basis (see degree day figure and heating degree days).

Other institutions use other conventions. Eurostat also calculates its European heating degree days with a heating limit of 15 °C, but subtracts the daily mean from a reference temperature of 18 °C. Figures from different sources are therefore not directly comparable, even if they have the same name. Anyone comparing values should always check which reference temperature and which heating limit lie behind them.

The choice of limit changes the results. If it is set too high, a building appears to need heating for longer than it actually does. If it is set too low, mild heating days are underestimated. This affects indicators, consumption comparisons and the assessment of efficiency measures. Standard values such as 15 °C are therefore sensible calculation conventions, but not a measured property of a specific building.

Distinction from related terms

The heating limit is often confused with the heating load. The heating load is the thermal output a building needs at a defined, very cold design temperature. Boilers, heat pumps and heating surfaces are sized according to it. The heating limit, by contrast, only states from which outdoor temperature heating demand arises at all, but not how large the output must be on a frosty day.

The heating limit must also be distinguished from the heat demand. Heat demand is a quantity of energy over a period, usually in kilowatt-hours per year. The heating limit helps determine how long the heating season is, but does not replace a demand calculation. Two buildings with the same heating limit can have very different annual consumption. This is due, for example, to insulation standard, ventilation, usage times or hot water demand. Hot water, moreover, is needed all year round, independently of the heating limit.

It is likewise imprecise to equate the heating limit with a thermostat. A thermostat responds to room temperature. The heating limit refers to the outdoor temperature and the energy balance of the building. Many heating controls have a summer-winter changeover or a heating limit function that works together with the heating curve. Such control values are control parameters. They should be adjusted to the building's actual heating limit, but do not automatically match it. If the changeover limit is set too high, the system runs unnecessarily in the transitional season. If it is set too low, rooms remain too cold on cool days.

Typical misinterpretations

Universally valid value: 15 °C is a calculation convention. In renovated buildings, passive houses or buildings with high internal loads, the actual heating limit can be considerably lower. In poorly insulated houses with high ventilation losses or a higher room temperature, it can be higher.
Inference about building quality: Usage also shapes the heating limit. A heavily used, sunny apartment with many appliances behaves differently from a rarely used, shaded house with the same envelope.
Sharp threshold: In practice the transition is gradual. Sun, wind and the thermal inertia of the building mean that on days with the same mean temperature heating is sometimes necessary and sometimes not.

Significance for heat pumps and electricity grids

For the electricity system the term is becoming more important because more and more heat is generated electrically, above all with heat pumps. The heating limit helps determine on how many days of the year these devices must heat. Very cold days determine the maximum heating load and thus possible peaks in electricity demand. The heating limit, by contrast, determines how broadly heating demand is spread over the year. If many heat pumps start up at mild temperatures, different load profiles arise than if well-insulated buildings only need heat at lower temperatures.

In the distribution grid this can be noticeable because heating electricity occurs in spatially concentrated form. Many buildings in a street respond to the same weather, even if their individual heating limits differ. For load forecasts it is therefore important how the heating limits of a building stock are distributed, not just a single standard value.

A better building envelope reduces the annual heat demand, shifts the heating limit downward and shortens the heating season. This reduces operating hours and electricity consumption. The reduction of peak load on very cold days is, however, a separate effect and does not automatically follow from a lower heating limit. For the planning of heat pumps, storage and grid connections, both levels should be considered separately.

Economically, the heating limit acts through consumption volumes. The earlier in autumn and the longer in spring heating takes place, the more energy is needed, even if the output on these days is low. In the transitional season, heat pumps usually work particularly efficiently because of the smaller temperature lift. These days therefore often raise the seasonal performance factor. A lower heating limit saves energy because less heat has to be generated. The machine itself does not become more efficient as a result. In calculation terms, the seasonal performance factor can even fall slightly, because some of the particularly favourable mild operating hours are eliminated.

Placement in planning and assessment

In consumption analyses, energy indicators, subsidy assessments and municipal heat planning, the heating limit usually plays an indirect role. This happens through weather adjustment or model assumptions. For rough comparisons, a standard value such as G20/15 is often sufficient. For renovation decisions, load forecasts or neighbourhood concepts, it should be disclosed which heating limit was assumed and whether it suits the building type.

The heating limit shows that heating demand does not arise only when it freezes. On its own, however, it explains neither energy consumption nor the required system output. For that, insulation standard, ventilation, internal gains, room temperature, heating surfaces, controls and weather patterns must be considered together. The term is useful as a threshold in the building balance: it marks the transition from passively covered heat losses to active heating operation.

Sources

Historische monatliche Gradtage nach VDI 3807 für Deutschland – Datensatzbeschreibung — Deutscher Wetterdienst, Climate Data Center; unbekannt. Datensatzbeschreibung, Abschnitt zur Berechnung
Kostenfreie Gradtagzahlen/Gradtage — Deutscher Wetterdienst; unbekannt. Serviceseite
Heating and cooling degree days – statistics (Statistics Explained) — Eurostat; unbekannt. Abschnitt Methodology

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Edited by eCMS on 12.11.2026 05:05