Activating night setback on a heating system
Night setback is a method of reducing the heating output at a specific time. In technical jargon, it is therefore also called reduced operation. This is because night setback can also be activated during the day, in which case "day setback" would be the correct term to use. Settings for night setback are performed directly on the heating control unit, usually under "Time program". Those who do not have access to this are able to control the heating output of the heat generator – albeit indirectly – either manually or via programmable thermostats. This is because changing the thermostat only controls the flow rate of the heating water, not the actual heating output of the heat generator.
Night setback in detail
Night setback allows system owners to influence the heating curve – also referred to as the heating characteristic curve – directly. Activating the function shifts the set room temperatures downwards so that the heat generator heats less. In concrete terms, this means that, for the selected period of reduced operation, the room is heated less than during the rest of the day. You can read about how the heating curve works in the guide to "Setting the heating curve".
Division into several time phases
For night setback to work, the heating operation must be divided up into several sections, known as time phases. Depending on the heating system, up to eight time phases can be selected. In addition to room heating, time programs can also be set for DHW heating and for the DHW circulation pump (if installed).
Here's an example with four time phases:
- Time phase 1: 06:45 to 12:15 with normal room temperature (approx. 20 degrees Celsius)
- Time phase 2: 13:00 to 18:00 with reduced room temperature (approx. 16 degrees Celsius)
- Time phase 3: 18:15 to 22:15 with normal room temperature
- Time phase 4: 22:15 to 06:30 with reduced room temperature
Important: between the time phases, the room is usually heated to a reduced temperature. However, systems with heat pumps may work differently. Please refer to the operating instructions supplied with your heating system to find out how it behaves between the time phases.
Advantages and disadvantages of night setback
By temporarily reducing the heating output, system owners can, in theory, save on heating costs. But only in theory. Because in practice, the potential for savings depends on many factors. This includes, among other things, the energy status of the building in question and the type of radiator and heating system.
Buildings react differently to night setback
Due to its thermal mass, a well insulated house reacts very sluggishly, only losing heat slowly. If night setback is activated at 9 p.m., for example, the effect is usually not noticeable until several hours later. In some cases, not until the following morning. By comparison, unmodernised buildings cool down much more quickly, which reduces the theoretical savings potential of a night setback. At the same time, the rooms have to be heated up again the following day. In a best case scenario, the amount of energy saved by night setback should be greater than that needed to heat the rooms back up again. However, the exact opposite can occur.
As a general rule, buildings have different storage masses and therefore different intervals apply. When activating night setback, this factor must not be ignored.
Not every system combination is suitable for night setback
Another factor influencing night setback is the heating system used and its radiators. Oil and gas boilers can heat rooms to the required temperature within a short time. However, to do so they need high flow temperatures and therefore a lot of energy. Heat pumps can get by with low flow temperatures. Compared to gas and oil heating, however, they are sluggish and need a long advance runtime. This same goes for underfloor heating, which is not actually a heating system but a large-area radiator. It only absorbs heat slowly and then releases it into the rooms even if night setback is already activated. Whether system owners really save energy with night setback can only be answered on an individual basis.
Temperature must never drop below the minimum
Even if night setback does deliver the outcome required, it's important that system owners do not overdo it. Because under no circumstances should the temperature fall below the limit of 15 to 16 degrees Celsius for an extended period of time. Otherwise, this increases the risk of moisture condensing on the cold walls. Vulnerable areas such as the corners of rooms can quickly become damp and thus provide the ideal breeding ground for mould.
How to activate night setback
Once aware of the advantages and disadvantages of setback, the user can set about activating night setback in their heating system. The function is pre-installed in almost all Viessmann heating systems.
The navigation route from the main menu to "Time program" may look different depending on the control unit. As a rule, you can access the time program via "Menu" → "Heating" → "Heating circuit" → "Time program". For the correct procedure, please refer to the operating instructions supplied.