Night Setback Temperature in Multi-Family Buildings: When It Actually Saves Energy

In most existing buildings, the night setback temperature sits between 16 and 18 degrees Celsius, reached through a reduction of 3 to 5 Kelvin below the daytime temperature. Whether night setback pays off economically across a multi-family housing portfolio depends on the building and on the heat generator installed.

For technical managers at housing companies, the generic advice from a single-family-home guide matters less than a different question: does one identical setback curve applied across an entire portfolio actually fit every building? With heat pumps in particular, the expected savings effect often reverses, because the morning reheating peak costs more energy than was saved the night before.

Between a fixed timer setting and demand-based control, many existing portfolios leave a double-digit percentage of savings potential on the table simply because a rigid setting cannot capture it.

What is the night setback temperature, and how does it differ from a full shutdown?

The setback temperature is the room or flow temperature to which a heating system is reduced during night setback, while the burner and heating circuit pump keep running. In a full shutdown, by contrast, both the burner and the circulation pump switch off completely; the heating controller toggles between the two modes via an ECO/setback parameter or an outdoor temperature threshold.

Technically, night setback works through a time-controlled, parallel downward shift of the heating curve. At the same outdoor temperature, the system then delivers a lower flow temperature and, in turn, a lower room temperature. Two values are set on the central heating controller to achieve this: the setback period, for example 10 p.m. to 5 a.m., and the setback temperature, or the degree of the curve shift.

In practice, a reduction of 3 to 5 Kelvin below the daytime temperature has become the moderate, common standard. That puts the typical setback level at 16 to 18 degrees Celsius. Vaillant recommends not letting the night temperature drop below 16 degrees Celsius, since the risk of mould growth and condensation on cold building components rises below that point.

Setback mode versus shutdown mode: In setback mode, the system keeps running at reduced output and maintains heat distribution throughout the building. In shutdown mode, the system rests completely, which can cause individual apartments to cool down more sharply, particularly where pipework is poorly insulated or on cold nights.

What legal limits does German tenancy law place on night setback in existing buildings?

For rented residential buildings, established German case law requires at least 20 degrees Celsius during the day between 6 a.m. and 11 p.m., and at least 18 degrees Celsius at night between 11 p.m. and 6 a.m. A night setback must never fall below these values at any point, regardless of which setback curve is programmed into the controller.

Section 61 of the German Buildings Energy Act (Gebäudeenergiegesetz, GEG) additionally requires central heating systems to have automatic controls that adjust heat supply based on both outdoor temperature and time. For existing systems without weather-compensated or time-based controls, retrofitting was already mandatory by September 30, 2021, which means a purely manually adjusted heating curve in an existing multi-family building has long since fallen short of the required standard.

How strictly this legal framework is applied in practice is shown by a case from Cologne, where an overly aggressive night setback became a legal problem.

A case from practice: The Cologne Local Court ordered a landlord to set the heating so that 18 degrees Celsius remains reliably reachable throughout the night. A setback between midnight and 6 a.m. had previously left apartments at only 16 to 17 degrees Celsius by morning.

When does night setback in multi-family buildings actually save energy?

Night setback saves roughly 5 to 10 percent of heating energy in classic radiator systems in poorly insulated older buildings. In well-insulated new buildings, the effect is often below 3 percent, because the building barely cools down overnight and correspondingly little heat needs to be supplied afterward.

Insulation and thermal storage mass are the deciding factors. The poorer the insulation and the lower the storage mass, the more a building benefits from setback, because the room temperature drops noticeably overnight and the amount of energy saved is correspondingly large. With good insulation and high building mass, this effect stays small.

The type of heating system matters too. Radiators respond to a changed flow temperature within 1 to 2 hours, while underfloor heating needs 3 to 4 hours for the same effect because of the thermal inertia of the screed. Both the overnight cool-down and the morning reheating happen just as sluggishly with underfloor heating, which is why classic night setback usually achieves little here.

The situation is most critical with heat pumps. Their efficiency drops as the temperature gap between the heat source and the required flow temperature grows, and morning reheating happens to fall right into the coldest hours before sunrise. With a classic, aggressive setback, the expected effect often reverses: instead of saving energy, the system draws more electricity for reheating in the morning than it saved overnight.

The heat pump rule: According to manufacturer guidance, setback for heat pumps should be limited to a maximum of 1 to 2 degrees Celsius, or skipped altogether. Otherwise, the system's seasonal performance factor and coefficient of performance drop noticeably.

Why is one portfolio-wide setback curve on a timer too blunt an approach?

A single setback curve, applied via timer across an entire building portfolio, rarely fits every individual property. Insulation standards and building mass differ from one property to the next, as do the type of heating system and how residents actually use it. A fixed time and a fixed setback value cannot capture these differences.

Current market data shows how wide the gap is between common practice and technical potential. Digital heating monitoring and optimization solutions are, according to a dena/KEDi survey, in use in just 4.4 percent of residential units in German multi-family buildings so far, though adoption is growing by more than 50 percent per year. dena experts estimate that digital, demand-based heating optimization in existing buildings can save 10 to 20 percent of energy without any structural work at all.

A hydraulic balancing exercise and reworked controller parameters often unlock further savings potential that a setback curve alone never captures. This is exactly the ground that heating optimization in existing buildings covers systematically, before the setback temperature of individual buildings even comes into play.

How does a digital building twin determine the right setback temperature automatically?

A digital building twin determines the appropriate setback temperature automatically by combining a building's own consumption data, weather forecasts and usage patterns, instead of relying on a rigid, manually set heating curve. This is exactly what KUGU EOS (Energie-Optimierungssystem) temperature control does: control logic is calculated individually for each building and continuously adjusted, regardless of the manufacturer of the heating system.

An existing building from the 1980s can be controlled energy-efficiently through such a digital twin without any physical changes to the building itself, as the digital heating optimization powered by the digital twin describes in detail. Weather forecasts and usage patterns feed automatically into the control logic, much as the AI-based energy optimization describes as the fastest lever for economic efficiency in existing buildings.

For KUGU EOS temperature control, the manufacturer states an average savings potential of more than 20 percent on energy, cost and CO₂, with 12 percent guaranteed. A pilot project with Gewobag shows what this looks like in daily operation: across 10 properties and 1,055 heating days, it saved 313,039 kWh of energy and €21,915 in energy costs. On top of that, 62.9 tonnes of CO₂ were avoided, an average saving of 23 percent. The project has since been extended to more than 300 properties.

Control instead of guesswork: KUGU VIS (Visuelles-Informationssystem) operational transparency makes the results of this kind of automated control visible. Meter readings, temperature data and consumption values come together in real time, and errors or inefficiencies are flagged without any need for an on-site inspection.

Recurring costs for this kind of predictive, digital control and monitoring can also be passed on as operating costs under Section 7 (2) of the German Heating Costs Ordinance (Heizkostenverordnung). For housing companies, that shifts the economics further in favor of automated temperature control.

From a blanket rule of thumb to building-specific control

The legal minimum of 18 degrees Celsius at night and the technically recommended setback corridor of 16 to 18 degrees Celsius sit only 0 to 2 degrees apart. Anyone hitting that narrow margin using a timer and a rule of thumb is relying on an estimate that can miss depending on weather conditions and building condition.

For a single building, a rough setting might work fine for years. Across a portfolio with varying construction years, varying insulation and varying heat generators, the gap between rough estimate and actual need adds up to a measurable cost block.

The sensible next step for technical managers, then, is to stop maintaining the setback temperature manually building by building, and instead let a digital building twin calculate it continuously and monitor it through an operational transparency solution.

Frequently asked questions about setback temperature in existing buildings

How low is the night temperature legally allowed to drop in rented apartments, according to German case law?

18 degrees Celsius must remain reliably reachable in occupied rooms throughout the night, between 11 p.m. and 6 a.m. During the day, between 6 a.m. and 11 p.m., a minimum of 20 degrees Celsius applies. A night setback must not fall below these limits at any point, not even briefly in the early morning hours.

How long does reheating take after night setback, for radiators versus underfloor heating?

Radiators respond to the increased flow temperature within 1 to 2 hours after setback ends. Underfloor heating needs around 3 to 4 hours to bring rooms back to target temperature, because of the storage mass of the screed. That is why classic night setback has a noticeably weaker effect with underfloor heating.

Why does heat pump efficiency drop with a classic night setback?

A heat pump's efficiency drops because it needs more electricity for the same amount of heat as the temperature gap between the heat source and the required flow temperature grows. Morning reheating also falls into the coldest hours of the day, exactly when that gap is largest. Seasonal performance factor and coefficient of performance both fall noticeably as a result.

Does an older heating system in an existing building have to be retrofitted with weather-compensated controls?

Yes, Section 61 of the German Buildings Energy Act (GEG) requires central heating systems to have automatic controls that adjust heat supply based on outdoor temperature and time. For existing systems without such controls, retrofitting was already mandatory by September 30, 2021. A purely manually set heating curve no longer meets this standard.

Can the cost of automated temperature control be passed on as operating costs?

Yes, recurring costs for predictive, digital control and monitoring of a heating system can be passed on as operating costs under Section 7 (2) of the German Heating Costs Ordinance (Heizkostenverordnung). This applies regardless of whether the solution is manufacturer-bound or manufacturer-independent. For housing companies, this improves the economics compared with purely manual settings.