Hysteresis: Unlock the Secret to Perfect Home Comfort
Hysteresis, sometimes also called “thermal differential” in thermostat manuals, is a key factor in home heating management, especially for those who want to optimize comfort, energy consumption, and the durability of their systems.
Table of Contents
What Is Hysteresis (or Thermal Differential)
Hysteresis is essentially the temperature difference set so that the thermostat decides when to start and stop the heating. Suppose the thermostat is programmed to maintain 20 °C at home and the thermal differential is set to 0.5 °C:
- Setpoint temperature (comfort): 20 °C
- Hysteresis: 0.5 °C
This means:
- If the room temperature drops 0.5 °C below the setpoint (that is, 19.5 °C), the thermostat “calls” the boiler (i.e., it turns it on).
- Once the boiler starts, it will keep running until the indoor temperature returns to 20 °C (or slightly above, depending on the type of thermostat).
In practice, the thermal differential sets the threshold within which the temperature can fluctuate before the thermostat decides to start the boiler again.
Why Hysteresis Is Important
- Thermal comfort: If the hysteresis is too wide, the home will experience significant temperature swings: the heating will turn on rarely, but when it does, it stays on longer. Conversely, if the hysteresis is too narrow (e.g., 0.1 °C), the heating will turn on very frequently, resulting in continuous “on/off” cycles that can be annoying and increase component wear.
- Energy efficiency: A very narrow setting causes frequent, short ignition cycles that, besides wearing out the boiler, can slightly increase energy consumption. On the other hand, an excessively wide hysteresis can let the indoor temperature drop too much, forcing the boiler to run at full capacity for longer periods.
- System durability: A boiler that switches on and off too often (the “on/off” effect) will wear out sooner, increasing maintenance costs and the likelihood of malfunctions.
How Hysteresis Affects the Thermostat’s “Call” to the Boiler
Basically, the thermostat controls the boiler in a binary way:
- Turns on the boiler if the temperature falls below the set threshold.
- Shuts off the boiler when the temperature returns to or exceeds the setpoint.
Hysteresis (thermal differential) specifically prevents excessive “switch on/off” events. Without hysteresis, at the slightest drop below the setpoint, the thermostat would call the boiler; then, with a small increase above the setpoint, it would turn it off again. With a suitable differential, you allow for a small temperature range before the heating system restarts.
How and When to Properly Adjust Hysteresis
- Default value: Most modern thermostats come from the factory with a standard hysteresis of 0.5 °C (half a degree). This value is usually a good compromise between comfort and the number of boiler ignitions.
- If the home has low thermal inertia (e.g., a small apartment or rooms with underfloor heating that heats/cools quickly):
- You may prefer a smaller hysteresis (0.3 °C – 0.4 °C) to keep the temperature more constant, accepting more frequent starts.
- If the home has high thermal inertia (e.g., buildings with thick walls, underfloor heating systems that heat/cool slowly):
- A wider hysteresis (0.5 °C – 0.8 °C) gives the boiler enough time to reach operating conditions without turning on too frequently.
- Consider seasonal HVAC: In summer (for cooling) or in winter (for heating), hysteresis can be adjusted similarly, bearing in mind the different responsiveness of heating and cooling systems.
Tip: Start with the default 0.5 °C, check how often your boiler runs and your comfort level. Increase or decrease by 0.1 °C at a time, and test for at least a few days. This will help you find the best balance.
Boiler Temperature and System Inertia
Aside from hysteresis, the boiler flow temperature (the temperature at which water is heated and sent to radiators or underfloor heating) also affects comfort and energy consumption.
- Boiler set to a very high temperature:
The room heats up faster, but the heat may be distributed unevenly, and in some cases, it can lead to more frequent on/off cycles (especially if hysteresis is narrow). - Boiler with a very low temperature:
The room heats more slowly, causing the boiler to run for longer periods. This can be advantageous in terms of efficiency (especially with condensing boilers) if properly adjusted, but requires more time to reach the desired temperature. - System inertia:
- If you have traditional radiators, they respond relatively quickly, so a medium-high flow temperature and a medium hysteresis (0.5 °C) usually work well.
- If you have an underfloor system (or a very large system), the inertia is high: it takes more time to change the indoor temperature. To avoid frequent restarts, you might keep a wider hysteresis (0.6 °C – 0.8 °C) and a flow temperature that isn’t too high, so the heating occurs gradually.
Frequently Asked Questions about Thermostat Hysteresis
Typically, modern thermostats have a pre-set hysteresis of 0.5 °C (half a degree). This ensures a good compromise between thermal comfort and energy efficiency.
It depends on the characteristics of the home and heating system. If the boiler cycles on/off too often or there are large temperature swings, it may be worth customizing the hysteresis setting.
It is advisable to adjust the hysteresis after observing how your indoor temperatures and boiler cycles behave over a few days. If you notice rapid swings or too many boiler starts, you can lower or raise the thermal differential by 0.1 °C at a time.
Adjusting the boiler flow temperature is especially useful if you want to reduce long-term energy consumption or if the system has high thermal inertia (for example, with underfloor heating). Changing the thermostat controls how frequently the system turns on/off; adjusting the boiler controls the power level and heating speed.
The outdoor sensor helps the boiler (or the control system) modulate the flow temperature based on weather conditions. If the system uses an outdoor sensor, make sure your hysteresis and boiler temperature settings also account for the information provided by the sensor to optimize both comfort and energy usage.
Conclusions
Hysteresis (or thermal differential) in thermostats is often overlooked but is essential for achieving the right balance between thermal comfort, energy savings, and the longevity of your heating system.
The default setting of 0.5 °C works for most situations, but it can be fine-tuned based on the type of home, the system’s thermal inertia, and personal comfort needs. The boiler flow temperature also plays a key role: higher temperatures speed up heat delivery but can increase the risk of frequent on/off cycles; lower temperatures extend heating time but reduce energy consumption and wear.
In short:
- Start with a 0.5 °C hysteresis.
- Observe how many times the thermostat calls the boiler in a day.
- Adjust the hysteresis and the boiler temperature gradually, until you find a combination that offers maximum comfort with fewer on/off cycles.
- Consider your type of heating system (radiators, underfloor heating, etc.) and the home’s insulation.
With a good setup, you’ll not only enjoy a comfortable indoor environment but also reduce energy waste and limit wear on your boiler.
