Determine if free cooling could be used for AHU operation

Applicable Systems

• Primary: AHU, RTU, UV, MUA

Objective/Method/Outcome

• Objective: Determine if free cooling can be used instead of mechanical cooling.

• Method: Check whether outdoor air temperature is below the setpoint while the system calls for cooling .

• Outcome: An insight is generated if free cooling is available for more than two hours per week.

Logic

The logic flow starts with taking the current supply air temperature and comparing it to a deadband setpoint. It also looks at the outdoor air temperature and sees if it’s below the return air temperature, and in range of ASHRAE standards. If the outdoor air is low enough, the rule looks to see if the damper is closed and the cooling coil valve is open. If the OAT is useable, the cooling coil valve is open, and the AHU is on under these conditions, an insight will be triggered. These conditions must persist for more than 2 hours per week.

IF(SAT < SAT SP){
         IF(OAT < RAT && OAD = Closed) {
                 IF(CCV = Closed){
                         Return:  Free cooling is possible
                   }
            }
   }

Description / Method

Monitors Supply Air Temperature (SAT), Supply Air Temperature Set Point (SAT SP), Outside Air Temperature (OAT), Return Air Temperature (RAT), and Outside Air Damper (OAD) TLs. {AHU}Triggers an insight if the following conditions are true for more than two hours (user-adjustable) a week:

1. SAT is greater than (SAT SP + user-adjustable dead band)
2. OAT is within an acceptable range
3. OAT is less than RAT
4. OAD position is less than 30%
5. CCV position is greater than 10%

Diagnosis / Recommended Action

1. Check is OAD is manually overridden. 1. Review sequence of operation.

Message

Air Handling Unit (AHU) operation could employ free cooling to satisfy Supply Air Temperature requirements. Please ensure that OAD is not manually overridden.

Notes

This logic flow only activates the initialization of the unit. If the temperature falls below the set point a second time, this will purposefully not trigger. However, this rule will activate every time a unit engages. If the unit changes status multiple times per day, a building occupancy trend log may be more suitable to define unit status.Default Values for Constants Rule generates a CTL ‘Weekly_Hours_AHU_Unrealized_Free_Cooling_Opportunity’Default Inputs: Weekly Threshold [hours] = 2 Outside Air Temperature Higher Limit [deg C] = 5 Constant – Outside Air Damper Closed [%] = 30 Constant – Supply Air Temperature Deadband [deg C] = 1 Outside Air Temperature Higher Limit [deg C] = 16 Constant – CCV Open [%] = 10 AHU Schedule = 1

Default Priority

Medium

Inputs

Inputs	Units	Tags	Description
Supply Air Temperature	°C/°F	discharge,air,temp,sensor	Supply Air Temperature
Return Air Temperature	°C/°F	return,air,temperature,cmd	Return Air Temperature
Supply Air Temperature Set Point	°C/°F	discharge,air,temp,sp	Supply Air Temperature Set Point
Cooling Coil Valve	%	cool,valve,water,cmd	Determine if mechanical cooling is being used
Outside Air Temperature	°C/°F	outside,air,temp,sensor	Outside Air Temperature
Outdoor Air Damper Position	%	outdoor,air,damper,cmd	Actual damper position

Constants

Inputs	Units	Default Value	Description
Weekly Threshold	hours	2	Minimum number of hours OAT can be used for free cooling before the insight triggers
Outside Air Temperature Low Limit	°C/°F	5	ASHRAE lower useable OAT limit
Minimum Outside Air Damper Position	%	30	Threshold to determine if Damper is open or closed
Constant – CCV Open	%	10	Lower limit to determine if CCV is actually open
AHU Schedule		1	AHU Off/On schedule. Change this if you have an actual schedule TL for your unit
Constant – Supply Air Temperature Deadband	°C/°F	1	Deadband for Supply Air Temperature
Outside Air Temperature High Limit	°C/°F	16	Upper ASHRAE limit for usable OAT

Output Calculated Trend Logs

Output CTL	Units	Frequency	Description
Weekly_Hours_AHU_Unrealized_Free_Cooling_Opportunity	hours	Weekly	Total weekly hours free cooling could have been used