Temperature Hysteresis Controller

Temperature hysteresis controller keeps the room temperature according to a thermostat: it switches a switching output (boiler relay, thermal actuator, electric heating) on and off around the setpoint, and opens an analog output (0–10 V valve) smoothly within the band. It is enough for ordinary room control — it has no integral that would need tuning. This page describes the behavior from version 2026.1.

Try the hysteresis controller#

Relay: switching on and off#

In heating with the default Plus band, the relay switches on when the temperature drops to the setpoint or below it, and switches off when the temperature exceeds the setpoint plus the hysteresis (0.2 °C by default). In between, it leaves the relay as it is.

• A larger hysteresis means fewer switchings and a larger temperature swing; a smaller one the opposite. For a boiler and a thermal actuator, keep at least 0.2 °C; with a hysteresis of 0, the relay switches at every sensor change of a tenth of a degree.
• The delay of the system adds overshoot to the hysteresis: the relay switches off above the setpoint, but the radiator or the floor is still heating. Underfloor heating with a delay of half an hour overshoots by more than a degree (the Floor on relay scenario) — there, a valve in the band or the PID controller is a better choice.
• Without a sensor value, the controller crosses no threshold: the relay stays in its last state, even when it is on. A sensor outage is therefore guarded by the warning (item 5 of the settings).

Valve: smoothly within the band#

The hysteresis controller does not switch an analog output fully on, but sets it according to the position of the temperature within the band: 100% at the lower edge of the band and below it, 0% at the upper edge and above it, linearly in between. This is control by a P term with a proportional band equal to the width of the band. The band is determined by the hysteresis and the mode:

The temperature settles where the valve gives exactly as much power as the room loses — in cold weather closer to the lower edge, in warm weather closer to the upper edge. A narrower band reduces the deviation, but the valve then reacts more sharply (the Valve in band scenario); with a hysteresis of 0, the valve always gives 50%. When you want no deviation at all, use PID controller: its I term makes up the power the room needs permanently.

Settings#

1. Thermostat — the device with the measured and the set temperature; the controller evaluates it at every change.
2. Heating & Cooling Mode — a fixed mode, or from the season device.
3. Hysteresis range — Range and the band mode (Plus, Minus, Middle) separately for heating and cooling; cooling defaults to Minus. When the season offers both heating and cooling (a season device with both options, or the fixed heating and cooling mode), the cooling setpoint is shifted by “Cooling set point offset in Heating and Cooling mode”.
4. Output devices — a digital output gets the switch-on and switch-off actions. In heating, the output can also watch the floor temperature (“Floor temperature”): above the maximum it switches off; an optional minimum (the room setpoint plus an offset, −1 °C by default) switches it on even when the room is warm. In cooling, the output switches off before the dew point (“Dew point limit”) — this needs a coolant temperature sensor and humidity on the thermostat; without humidity, the protection is turned off.
5. Enable warning when temperature range is exceeded — a new rule has it on for 12–50 °C and sends it to the service group; it watches the thermostat as well as the floor and ceiling sensors.

Hysteresis or PID controller#

The PID controller makes sense when you need the setpoint exactly — supply water behind a mixing valve, a fast system — or when a relay should switch smoothly through PWM (section Relay and PWM).