DutyCycle

Function

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.


NightPurge and DutyCycle Operation

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.


No Fixed Off Times

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

Off Time CalculationDutyCycle calculates off time by comparing zone temperatures “In Temp Zone High” and “In Temp Zone Low” with the comfort range (P1). DutyCycle calculates the comfort limits around the setpoint “In Setpoint” with the following formulas:

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.


DutyCycle Example

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:

Heating System Off Time Calculation

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


Cooling System Off Time Calculation

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


Heating/ Cooling System Off Time Calculation

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


Inputs and Outputs

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
(use a MAX icon)

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
(use a MIN icon).

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.


Parameter List

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,
2 = Cooling,
3 = Heating and Cooling

3 = Heating and Cooling

-

Two Speed Fan

false = single speed fan
true = 2 speed fans

true

-