current temperature and the desired setpoint) reaches 5ºC. If the difference is only 1ºC
(20% of the proportional band), the output will be set to 20%, and so on.
•
Integral time:
The P-I control algorithm not only considers the current value of the error
term at any given time, but also the “history” of the system; that is, the past values of
the error term. This makes it possible to react faster to changes in ambient conditions
that impact the room temperature. Short integral times allow for fast reactions to such
changes, but a too short integral time introduces a risk of permanent oscillations around
the setpoint. Long integral times eliminate the risk of oscillations, but the reaction to
external changes will be slower.
Note
Every time the desired temperature setpoint changes, the current cycle is interrupted,
the new values are processed, and a new cycle is started. This allows the system to reach
its steady state more quickly.
If the P-I control parameters are properly adjusted, the control algorithm may be able to ef-
ficiently control all common types of heating and cooling systems, thus making the room
temperature control work as fast as possible and without deviation. However, tuning of the P-I
control parameters is not straightforward, and even small changes to the parameters values can
result in noticeable changes to the control performance: An incorrectly configured system may
be too slow, needing a long time to reach the desired temperature setpoint, or it may fluctuate
above or below the setpoint.
For this reason, a set of
predefined parameters
that correspond to the most common types of
heating and cooling systems are available and can be easily selected, as shown in the following
table. For expert users, the option of entering custom parameter values is also provided.
Application
(heating)
Proportional band
(K)
Integral time
(min)
Recommended
cycle time (min)
Warm water heating
5
150
15
Floor heating
5
240
15-20
Electric heating
4
100
10-15
Convection heating
4
90
10-15
Application
(cooling)
Proportional band
(K)
Integral time
(min)
Recommended
cycle time (min)
Cooling ceiling
4
240
15-20
Convection cooling
4
90
10-15
2.4
Thermostats
37
