Two-point control with hysteresis is easy to setup and can be used with regular switching
actuators. However, it is also sub-optimal in terms of power consumption and comfort, due to
the typical fluctuations of the room temperature around the setpoint which arise as a result of
the simple control algorithm.
Time
Setpoint
ON
OFF
ON
ON
ON
OFF
OFF
OFF
Se
upper hyst.
Setpoint -
lower hyst.
2.4.5.2
P-I Control with Continuous Output
Proportional-Integral (P-I) Control is a more complex control algorithm where the thermostat
constantly measures the difference between the current room temperature and the desired
setpoint (the
error term
), and computes an appropriate
correction value
based on the current
difference, and also on the previous history of the system. This correction value (e.g. 0-100%) is
periodically sent to a proportional HVAC actuator device.
When configuring a thermostat to use P-I Control, the following parameters must be consid-
ered:
•
Cycle time:
The control algorithm periodically compares the current room temperature
with the desired setpoint, and computes the correction values to be applied. The cycle
time determines how often this is done. Longer cycle times (typically 15-20 minutes)
should be chosen for HVAC systems with high thermal inertia, such as radiant floor
heating, and shorter cycle times (typically 10-15 minutes) for HVAC systems with low
thermal inertia, such as as air convection heating and cooling.
•
Proportional band:
The proportional band (measured in degrees) determines the mag-
nitude of the output value relative to the error term. For example, a proportional band of
5ºC means that the output will be set to 100% when the error (the difference between the
2.4
Thermostats
36
