Switch HVAC systems on and off at variable intervals to save energy while maintaining room conditions. For example, during normal occupancy, the DutyCycle command switches the building's air conditioning and ventilating systems off at variable intervals, provided that required room conditions exist. DutyCycle switches off fans on a preferential basis.
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 necessary to deliver the heat operate at full capacity.
Intermittent operation reduces running time and thereby saves electricity.
This function applies to heating only, cooling only, and combined heating and cooling systems.
Using both NightPurge and DutyCycle in a system can result in command conflicts. You should use switching logic to force NightPurge to override DutyCycle commands.
DutyCycle does not work with fixed off times. The off time duration varies as a function of the system load within limits you define. The following diagram illustrates how the load-corrected off-time duration function operates in DutyCycle.
Fig. 149. Duty cycling function
Upper comfort limit = “In Setpoint” + P1 (setpoint plus comfort range)
Lower comfort limit = “In Setpoint” – P1 (setpoint minus comfort range)
If the minimum zone temperature “In Temp Zone Low” is less than the lower comfort limit or if the maximum zone temperature “In Temp Zone High” is greater than the upper comfort limit, the off time is zero. A zero off time operates the system without interruption to reach the setpoint as quickly as possible.
If the minimum zone temperature “In Temp Zone Low” is equal to the lower comfort limit or if the maximum zone temperature “In Temp Zone High” is equal to the upper comfort limit, DutyCycle cycles the system with minimum off time (P5).
When zone temperature is within the comfort zone limits, DutyCycle continuously calculates off time. Off time is inversely proportional to the deviation of a zone temperature from the setpoint. Off time reaches its maximum (P2) when zone temperature is equal to the setpoint, that is, “In Temp Zone Low” = “In Temp Zone High” = “In Setpoint”. If “In Temp Zone High” is not equal to “In Temp Zone Low” , DutyCycle sets a shorter off time. The following formula defines this relationship (a straight line y with gradient m through a known point P(x0/y0)):
Y = m*(X-X0)+ y0 where m = y/x.
The following example uses this formula to calculate the corresponding equations for the actual off times.
This example describes off time calculation for the three types of systems (heating, cooling, and heating and cooling). Basic requirements for each system type are the same:
These values create the following parameters:
In heating-only systems, DutyCycle calculates off time exclusively on lowest zone temperature “In Temp Zone Low” because heating is only required at the low end of the temperature range.
If temperatures are above the setpoint, the system does not operate and DutyCycle sets off time to the maximum (tmax = P2).
If the minimum zone temperature is less than the lower comfort limit, off time is zero, that is, the system is never cycled off.
When temperature is between the setpoint and the lower comfort limit, DutyCycle calculates heating off-time, toff, ((“Out Fan State” or “Out Fan Speed” = 0) as:
DutyCycle switches the heating system off 14.5 minutes before the end of the cycle.
The following diagram illustrates off-time calculation for heating systems:
Fig. 150. Duty cycling off time for heating
In cooling-only systems, DutyCycle calculates off time exclusively on highest zone temperature “In Temp Zone High” because cooling is only required at the high end of the temperature range.
If temperatures are below the setpoint, the system does not operate and DutyCycle sets off time to the maximum (tmax = P2).
If the maximum zone temperature is greater than the upper comfort limit, off the time is zero, that is, the system is never cycled off.
When temperature is between the setpoint and the upper comfort limit, DutyCycle calculates cooling off-time, toff, (“Out Fan State” or “Out Fan Speed”= 0) as:
The following diagram illustrates off-time calculation for cooling systems:
Fig. 151. Duty cycling off time for cooling
In combined heating/ cooling systems, if the room temperature is outside the comfort range, DutyCycle does not switch off the system.
If room temperature is within the comfort range, DutyCycle 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. DutyCycle selects the lower of the two off times for actual off time duration. For example, the off time for the heating system example is 14,5 minutes and the off time for the cooling system example if 10 minutes 45 seconds. So, DutyCycle switches the heating/cooling system off 10 minutes, 45 seconds before the end of the cycle.
Fig. 152. Duty cycling off time for heating and cooling
Fig. 153. Wire-sheet Icon
Fig. 154. Property Sheet
Fig. 155. Slot Sheet
Table 108. Inputs
Inputs |
Type |
Functional description |
In Temp Zone High |
numeric |
Highest zone temperature The highest zone temperature indicates a need for cooling. For example, “In Temp Zone High” can be a selection of the maximum of all room temperatures in a zone |
In Temp Zone Low |
numeric |
Lowest zone temperature The lowest zone temperature indicates a need for heating. For example, “In Temp Zone Low” can be a selection of the minimum of all room temperatures in a zone |
In Fan Status |
boolean |
Fan status input 1 = off, 2 = fast (for two-speed fans) 3 = slow (for single-speed fans) |
In Setpoint |
numeric |
Setpoint for duty cycling |
In Integral En. |
boolean |
Enables and disables integral control action. When “In Integral En.” is zero, integral action in the master and cascade controllers is disabled and the integral sum is reset |
In Enabled |
boolean |
If the input “In Enabled” is set to false disabled in the slot sheet, the output “Out” will be disabled and is set to the value “0”. |
Table 109. Outputs
Outputs |
Type |
Functional description |
Out Fan State |
boolean |
Single-stage fan speed 1; 0 = off 1 = on |
Out Fan Speed |
boolean |
Two-stage fan speed 2: 0 = slow 1 = fast |
These outputs can also be heating system pumps.
Table 110. Parameter List
Description |
Value Range |
Default Value |
Unit |
Comfort Range DutyCycle uses this value to calculate zone comfort limits around the setpoint |
0…30 K |
3 |
K |
Maximum Off Time |
25…80% |
50% |
% |
Minimum Off Time |
5…50% |
5% |
% |
Cycle Period Select a Cycle period that ensures that the fan motor does not switch on more often than the allowable number of times per hour. |
5…60 min |
30 min |
min |
Cycle Type |
1 = Heating, |
3 = Heating and Cooling |
- |
Two Speed Fan |
false = single speed fan |
true |
- |