Duty cycling
Duty cycling switches air-handling units on and off at variable intervals to save energy while maintaining room conditions. A requirement for intermittent operation is that the systems have adequate performance reserves, especially during the transitional seasons. In general, such systems run at partial load when heating and cooling, while the pumps and fans operate at full capacity.
Intermittent operation reduces running time and thereby saves electrical energy.
Heating system off time calculation
In heating-only systems, duty cycling calculates the off time exclusively on the lowest zone temperature (room temperature setpoint - 1/2 comfort zone) because heating is only required at the low end of the temperature range. If the room temperature is above the setpoint, the system does not operate and duty cycling sets off time to the maximum value DUC-02.
If the room temperature is less than the lower comfort limit (room temperature setpoint - 1/2-comfort zone), off time is zero, that is, the system is never cycled off.
When temperature is between the setpoint and the lower comfort limit, duty cycling calculates heating off time between the minimum and maximum times with a linear relationship:
Cooling system off time calculation
In cooling-only systems, duty cycling calculates the off time based on the room temperature.
If the room temperature is below the setpoint, the system does not operate and duty cycling sets off time to the maximum DUC-02.
If the room temperature is greater than the upper comfort limit, the off time is zero and the system never cycles off.
When the room temperature is between the setpoint and the upper comfort limit, duty cycling calculates the cooling off time with a linear relationship between the minimum and maximum off times.
Heating/ cooling system off time calculation
In combined heating/cooling systems, if room temperature is outside the comfort range, duty cycling does not switch off the system.
If the room temperature is within the comfort range, duty cycling calculates two off times. One calculation is the same as for heating-only systems. The other calculation is the same as for cooling-only systems. Duty cycling selected the lower of the two off times for actual off time duration.
For example, the off time for the heating system is 14 minutes and the off time for the cooling system is 10 minutes, duty cycling switches the heating/ cooling system off 10 minutes before the end of the cycle.
Description |
Default |
DUC-01: duty cycle; 1=enable |
0 |
DUC-02: duty cycling comfort zone |
3 K |
DUC-03: maximum off time (% of duty cycle period) |
50 % |
DUC-04: minimum off time (% of duty cycle period) |
10 % |
DUC-05: duty cycle period |
30 min |
DUC-06: duty cycling mode 1 = heating |
3 |