4.5 HVAC System Control Requirements

4.5.1    Mandatory Measures

This section covers controls that are mandatory for all system types, including:

•      Heat pump controls for the auxiliary heaters,

•      Zone thermostatic control including special requirements for hotel/motel guest rooms and perimeter systems,

•      Shut-off and setback/setup controls,

•      Infiltration control,

•      Off-hours space isolation

•      Economizer fault detection and diagnostics (FDD), and

•      Control equipment certification.

A.   Heat Pump Controls

Heat pumps with electric resistance supplemental heaters must have controls that limit the operation of the supplemental heater to defrost and as a second stage of heating when the heat pump alone cannot satisfy the load. The most effective solution is to specify an electronic thermostat designed specifically for use with heat pumps. This “anticipatory” thermostat can detect if the heat pump is raising the space temperature during warm-up fast enough to warrant locking out the auxiliary electric resistance heater.

This requirement can also be met using conventional electronic controls with a two-stage thermostat and an outdoor lockout thermostat wired in series with the auxiliary heater. The outdoor thermostat must be set to a temperature where the heat pump capacity is sufficient to warm up the space in a reasonable time (e.g., above 40°F). This conventional control system is depicted schematically in Figure 4-10 below.  Also, as described in the following Sections, heat pump thermostats must also meet the Occupant Controlled Smart Thermostat (OCST) requirements of Reference Joint Appendix JA5.

B.   Zone Thermostatic Controls

Thermostatic controls must be provided for each space-conditioning zone or dwelling unit to control the supply of heating and cooling energy within that zone §120.2(a). The controls must have the following characteristics:

1.   When used to control heating, the thermostatic control must be adjustable down to 55°F or lower.

2.   When used to control cooling, the thermostatic control must be adjustable up to 85°F or higher.

3.   When used to control both heating and cooling, the thermostatic control must be adjustable from 55°F to 85°F and also provide a temperature range or dead band of at least 5°F. When the space temperature is within the dead band, heating and cooling energy must be shut off or reduced to a minimum. A dead band is not required if the thermostat requires a manual changeover between the heating and cooling modes Exception to §120.2(b)3.

4.   For all unitary single zone, air conditioners, heat pumps, and furnaces, §110.2(c) that all thermostats (including residential and nonresidential thermostats) shall have setback capabilities with a minimum of four separate setpoints per 24 hour period; in additions, for nonresidential buildings, §120.2(b)4 requires that thermostatic controls, must comply with the requirements of Reference Joint Appendix JA5 .  This thermostat is also known as the Occupant Controlled Smart Thermostat (OCST), which is capable of responding to demand response signals in the event of grid congestion and shortages during high electrical demand periods.

5.   System with DDC to the zone §110.2(c) are also required to have automatic demand shed controls as described later in this section.

The setpoint may be adjustable either locally or remotely, by continuous adjustment or by selection of sensors.
Figure 4-10 – Proportional Control Zone Thermostat

Thermostats with adjustable setpoints and deadband capability are not required for zones that must have constant temperatures to prevent the degradation of materials, an exempt process, or plants or animals Exception 1 to §120.2(b)4. Included in this category are manufacturing facilities, hospital patient rooms, museums, etc. This does not include computer rooms as the ASHRAE guidelines for data centers and telecom equipment provide a wide range of acceptable temperatures at the inlet to the equipment.

Chapter 12 describes mandated acceptance test requirements for thermostat control for packaged HVAC systems.

C.   Hotel/Motel Guest Rooms and High-Rise Residential Dwellings Thermostats

The Standards require that thermostats in hotel and motel guest rooms have:

1.   Numeric temperature setpoints in °F, and

2.   Setpoint stops that prevent the thermostat from being adjusted outside the normal comfort range (± 5^oF). These stops must be concealed so that they are accessible only to authorized personnel, and

3.   For all unitary single zone, air conditioners, heat pumps, and furnaces, §110.2(c) that all thermostats (including residential and nonresidential thermostats) shall have setback capabilities with a minimum of four separate setpoints per 24 hour period; in additions, for nonresidential buildings, §120.2(b)4 requires that thermostatic controls, must comply with the requirements of Reference Joint Appendix JA5 .  This thermostat is also known as the Occupant Controlled Smart Thermostat (OCST), which is capable of responding to demand response signals in the event of grid congestion and shortages during high electrical demand periods.

The Standards effectively prohibit thermostats having ‘warmer/cooler’ or other labels with no temperature markings in this type of occupancy §120.2(c).

D.   Perimeter Systems Thermostats

Supplemental perimeter heating or cooling systems are sometimes used to augment a space-conditioning system serving both interior and perimeter zones. This is allowed by §120.2(a) Exception, provided controls are incorporated to prevent the two systems from conflicting with each other. If that were the case, then the Standards require that:

1.   The perimeter system must be designed solely to offset envelope heat losses or gains; and

2.   The perimeter system must have at least one thermostatic control for each building orientation of 50 ft. or more; and

3.   The perimeter system is controlled by at least one thermostat located in one of the zones served by the system.

The intent is that all major exposures be controlled by their own thermostat, and that the thermostat be located within the conditioned perimeter zone. Other temperature controls, such as outdoor temperature reset or solar compensated outdoor reset, do not meet these requirements of the Standards.

E.   Shut-off and Temperature Setup/Setback

For specific occupancies and conditions, each space-conditioning system must be provided with controls that can automatically shut off the equipment during unoccupied hours. The control device can be either:

1.   An automatic time switch device must have the same characteristics that lighting devices must have, as described in §110.9. This can be accomplished with a 7-day programmable thermostat with backup capabilities that prevents the device’s schedule for at least 7 days, and time and date for at least 72 hours if the power is lost.

2.   A manual override accessible to the occupants must be included in the control system design either as a part of the control device, or as a separate override control. This override shall allow the system to operate up to four hours during normally unoccupied periods.

3.   An occupancy sensor. Since a building ventilation purge is required prior to normal occupancy §120.1(c)2, an occupancy sensor may be used to control the availability of heating and cooling, but should not be used to control the outdoor ventilation system.

4.   When an automatic time switch is used to control ventilation while occupancy sensors are used simultaneously to control heating and cooling, the controls should be interlocked so that ventilation is provided during off-hours operation.

5.   Where ventilation is provided by operable openings (see discussion on natural ventilation in Section 4.3.1 above) an occupant sensor can be used without interlock.

6.   A 4-hour timer that can be manually operated to start the system. As with occupancy sensors, the same restrictions apply to controlling outdoor air ventilation systems.

F.   When shut down, the controls shall automatically restart the system to maintain:

1.   A setback heating thermostat setpoint, if the system provides mechanical heating. Thermostat setback controls are not required in nonresidential buildings in areas where the Winter Median of Extremes outdoor air temperature is greater than 32°F §120.2(e)2A and Exception.

2.   A setup cooling thermostat setpoint, if the system provides mechanical cooling. Thermostat setup controls are not required in nonresidential buildings in areas where the Summer Design Dry Bulb 0.5 percent temperature is less than 100°F §120.2(e)2B and Exception.

G.   Occupant Sensor Ventilation Coil and Setback:

Multipurpose room less than 1,000 ft², classrooms greater than 750 ft², conference, convention, auditorium and meeting center rooms greater than 750 ft² that do not have processes or operations that generate dusts, fumes, vapors or gasses shall be equipped with occupant sensor(s) to accomplish the following when occupants are not present:

1.   Slightly widen the thermal deadband:  automatically setup the operating cooling temperature set point by 2°F or more and setback the operating heating temperature set point by 2˚F or more; and

2.   Automatically reset the minimum required ventilation rate with an occupant sensor ventilation control device according to § 120.1(c)5:  Occupant Sensor Ventilation Control Devices.

This scenario requires an additional control sequence for built-up VAV systems or a thermostat that can accept an occupancy sensor input and has three scheduling modes (occupied, standby, and unoccupied) for packaged equipment.  A thermostat with three scheduling modes works as follows.  The unoccupied period is scheduled as usual for the normal unoccupied period, e.g. nighttime.  The occupied period is scheduled as usual for the normal occupied period, e.g. daytime.  When the morning warm-up occurs, the thermostat’s occupied schedule is used to establish the heating/cooling temperature setpoints.  Upon completion of the morning warm-up, the standby setpoint schedule on the thermostat is enabled.  This schedule remains in effect until occupancy is sensed (then enabling the occupied setpoint schedule) or until the normally scheduled unoccupied period occurs.  After the period of occupancy ends, e.g. a conference room is vacated, and when the time delay expires as programmed into the occupancy sensor, the standby setpoint schedule on the thermostat is enabled.

The following chart shows an example of how the three scheduling modes might be programmed for a cooling setup of 4^oF and a heating setback of 4^oF.

Example Thermostat Setpoints for Three Modes

H.   Hotel/Motel Guest Room Controls:

Hotel and motel guest rooms shall have captive card key controls, occupancy sensing controls, or automatic controls such that, no longer than 30 minutes after the guest room has been vacated, setpoints are setup at least +5°F (+3°C) in cooling mode and set-down at least -5°F (-3°C) in heating mode.

Figure 4-11 – Shut-Off and Setback Controls Flowchart

These provisions are required by the Standards to reduce the likelihood that shut-off controls will be circumvented to cause equipment to operate continuously during unoccupied hours.

I.    Exceptions for automatic shut-off §120.2(e)1, setback and setup §120.2(e)2 and occupant sensor setback §120.2(e)3 are not required as indicted where:

1.   It can be demonstrated to the satisfaction of the enforcement agency that the system serves an area that must operate continuously Exception to §120.2(e)1, 2 and 3

2.   It can be demonstrated to the satisfaction of the enforcement agency that shutdown, setback, and setup will not result in a decrease in overall building source energy use §120.2(e)1, 2 and 3

3.   Systems have a full load demand less than 2 kW, or 6,826 Btu/h, if they have a readily accessible manual shut-off switch Exception to §120.2(e)1, 2 and 3. Included is the energy consumed within all associated space-conditioning systems including compressors, as well as the energy consumed by any boilers or chillers that are part of the system.

4.   Systems serve hotel/motel guest rooms, if they have a readily accessible manual shut-off switch Exception to §120.2(e) 1 and 2.

5.   The mechanical system serves retail stores and associated malls, restaurants, grocery stores, churches, or theaters equipped with a 7-day programmable timer Exception to §120.2(e) 1.

J.   Infiltration Control

Outdoor air supply and exhaust equipment must incorporate dampers that automatically close when fans shut down. The dampers may either be motorized, or of the gravity type.

Damper control is not required where it can be demonstrated to the satisfaction of the enforcement agency that the space-conditioning system must operate continuously §120.2(f) Exception No. 1. Nor is damper control required on gravity ventilators or other non-electrical equipment, provided that readily accessible manual controls are incorporated §120.2(f) Exception No. 2.

Damper control is also not required at combustion air intakes and shaft vents, or where prohibited by other provisions of law §120.2(f) Exceptions No. 3 and 4. If the designer elects to install dampers or shaft vents to help control stack-induced infiltration, the damper should be motorized and controlled to open in a fire in accordance with applicable fire codes.

K.   Isolation Area Controls

Large space-conditioning systems serving multiple zones may waste considerable quantities of energy by conditioning all zones when only a few zones are occupied. Typically, this occurs during evenings or weekends when only a few people are working. When the total area served by a system exceeds 25,000 ft², the Standards require that the system be designed, installed and controlled with area isolation devices to minimize energy consumption during these times. The requirements are:

1.   The building shall be divided into isolation areas, the area of each not exceeding 25,000 ft². An isolation area may consist of one or more zones.

2.   An isolation area cannot include spaces on different floors.

3.   Each isolation area shall be provided with isolations devices such as valves or dampers that allow the supply of heating or cooling to be setback or shut off independently of other isolation areas.

4.   Each isolation area shall be controlled with an automatic time switch, occupancy sensor, or manual timer. The requirements for these shut-off devices are the same as described previously in §120.2(e)1. As discussed previously for occupancy sensors, a building purge must be incorporated into the control sequences for normally occupied spaces, so occupancy sensors and manual timers are best limited to use in those areas that are intermittently occupied.

Any zones requiring continuous operation do not have to be included in an isolation area.

L.   Isolation of Zonal Systems

Small zonal type systems such as water loop heat pumps or fan coils may be grouped on automatic time switch devices, with control interlocks that start the central plant equipment whenever any isolation area is occupied. The isolation requirements apply to equipment supplying heating and cooling only; central ventilation systems serving zonal type systems do not require these devices.

M.  Isolation of Central Air Systems

Figure 4-12 below depicts four methods of area isolation with a central variable air volume system:

1.   On the lowest floor, programmable DDC boxes can be switched on a separate time schedule for each zone or blocks of zones. When unoccupied, the boxes can be programmed to have zero minimum volume setpoints and unoccupied setback/setup setpoints. Note this form of isolation can be used for sections of a single floor distribution system.

2.   On the second floor, normally closed pneumatic or electric VAV boxes are used to isolate zones or groups of zones. In this scheme the control source (pneumatic air or control power) for each group is switched on a separate control signal from an individual time schedule. Again this form of isolation can be used for sections of a single floor distribution system.

3.   On the third floor isolation is achieved by inserting a single motorized damper on the trunk of the distribution ductwork. With the code requirement for fire/smoke dampers (see next bullet) this method is somewhat obsolete. When applied this method can only control a single trunk duct as a whole. Care must be taken to integrate the motorized damper controls into the fire/life safety system.

4.   On the top floor a combination fire smoke damper is controlled to provide the isolation. Again this control can only be used on a single trunk duct as a whole. Fire/smoke dampers required by code can be used for isolation at virtually no cost provided that they are wired so that the fire life-safety controls take precedence over off-hour controls. (Local fire officials generally allow this dual usage of smoke dampers since it increases the likelihood that the dampers will be in good working order in the event of a fire.)  Note that no isolation devices are required on the return.

Figure 4-12– Isolation Methods for a Central VAV System

N.   Turndown of Central Equipment

Where isolation areas are provided it is critical that the designer design the central systems (fans, pumps, boilers and chillers) to have sufficient stages of capacity or turndown controls to operate stably as required to serve the smallest isolation area on the system. Failure to do so may cause fans to operate in surge, excessive equipment cycling and loss of temperature control. Schemes include:

1.   Application of demand based supply pressure reset for VAV fan systems. This will generally keep variable speed driven fans out of surge and can provide 10:1 turndown.

2.   Use of pony chillers, an additional small chiller to be used at partial load conditions, or unevenly split capacities in chilled water plants. This may be required anyway to serve 24/7 loads.

3.   Unevenly split boiler plants.

O.   Automatic Demand Shed Controls

HVAC systems with DDC to the zone level must be programmed to allow centralized demand shed for non-critical zones as follows:

1.   The controls shall have a capability to remotely setup the operating cooling temperature set points by four degrees or more in all non-critical zones on signal from a centralized contact or software point within an Energy Management Control System (EMCS).

2.   The controls shall be capable of remotely setdown the operating heating temperature set points by four degrees or more in all non-critical zones on signal from a centralized contact or software point within an EMCS.

3.   The controls shall have capabilities to remotely reset the temperatures in all non critical zones to original operating levels on signal from a centralized contact or software point within an EMCS.

4.   The controls shall be programmed to provide an adjustable rate of change for the temperature setup and reset.

5.   The controls shall have the following features:

a.   Disabled. Disabled by authorized facility operators; and

b.   Manual control. Manual control by authorized facility operators to allow adjustment of heating and cooling set points globally from a single point in the EMCS; and

c.   Automatic Demand Shed Control. Upon receipt of a demand response signal, the space-conditioning systems shall conduct a centralized demand shed, as specified in 120.2(h)1 and 120.2(h)2, for non-critical zones during the demand response period.

The Standard defines a critical zone as a zone serving a process where reset of the zone temperature setpoint during a demand shed event might disrupt the process, including but not limited to data centers, telecom/private branch exchange (PBX) rooms, and laboratories.

To comply with this requirement, each non-critical zone temperature control loop will need a switch that adds in an offset on the cooling temperature setpoint on call from a central demand shed signal. A rate of change limiter can either be built into the zone control or into the functional block for the central offset value. The central demand shed signal can be activated either through a global software point or a hardwired digital contact.

This requirement is enhanced with an acceptance test to ensure that the system was programmed as required.

P.   Economizer Fault Detection and Diagnostics

Economizer Fault Detection and Diagnostics (FDD) is a mandatory requirement for all newly installed air-cooled unitary direct-expansion units, with mechanical cooling capacity at AHRI conditions of greater than or equal to 54,000 Btu/hr, and equipped with an economizer.

Where required, the FDD system shall meet the requirements of 120.2(i)2 through 120.2(i)9, as described below. Air-cooled unitary direct expansion units include packaged, split-systems, heat pumps, and variable refrigerant flow (VRF), where the VRF capacity is defined by that of the condensing unit.

1.   The following temperature sensors shall be permanently installed to monitor system operation: outside air, supply air, and return air; and

2.   Temperature sensors shall have an accuracy of ±2°F over the range of 40°F to 80°F; and

3.   Refrigerant pressure sensors, if used, shall have an accuracy of ±3 percent of full scale; and

4.   The controller shall have the capability of displaying the value of each sensor; and

5.   The controller shall provide system status by indicating the following conditions:

a.   Free cooling available

b.   Economizer enabled

c.   Compressor enabled

d.   Heating enabled

e.   Mixed air low limit cycle active

6.   The unit controller shall manually initiate each operating mode so that the operation of compressors, economizers, fans, and heating system can be independently tested and verified; and

7.   Faults shall be reported to a fault management application accessible by day-to-day operating or service personnel, or annunciated locally on zone thermostats; and

8.   The FDD system shall detect the following faults:

a.   Air temperature sensor failure/fault. This failure mode is a malfunctioning air temperature sensor, such as the outside air, discharge air, or return air temperature sensor. This could include mis-calibration, complete failure either through damage to the sensor or its wiring, or failure due to disconnected wiring.

b.   Not economizing when it should. In this case, the economizer should be enabled, but for some reason it’s not providing free cooling. This leads to an unnecessary increase in mechanical cooling energy. Two examples are the economizer high limit setpoint is too low, say 55˚F, or the economizer is stuck closed.

c.   Economizing when it should not. This is opposite to the previous case of not economizing when it should. In this case, conditions are such that the economizer should be at minimum ventilation position but for some reason it is open beyond the correct position. This leads to an unnecessary increase in heating and cooling energy. Two examples are the economizer high limit setpoint is too high, say 82˚F, or the economizer is stuck open.

d.   Damper not modulating. This issue represents a stuck, disconnected, or otherwise inoperable damper that does not modulate open and closed. It is a combination of the previous two faults: not economizing when it should, and economizing when it should not.

e.   Excess outdoor air. This failure mode is the economizer provides an excessive level of ventilation, usually much higher than is needed for design minimum ventilation. It causes an energy penalty during periods when the economizer should not be enabled, that is, during cooling mode when outdoor conditions are higher than the economizer high limit setpoint. During heating mode, excess outdoor air will increase heating energy.

9.   The FDD system shall be certified to the Energy Commission as meeting these requirements 120.2(i)1 through 120.2(i)8 in accordance with Section 100(h): Certification Requirements for Manufactured Equipment, Products, and Devices. That is, the FDD system shall be certified by the manufacturer in a declaration, executed under penalty of perjury under the laws of the State of California, that all the information provided pursuant to the certification is true, complete, accurate and in compliance with all applicable provisions of Part 6.

Q.   Control Equipment Certification

Where used in HVAC systems, occupancy sensors must meet the requirements of §110.9(b)4. These requirements are described in Chapter 5.

Automatic time switches must meet the requirements of §110.9(b)1. These also are described in Chapter 5. When used solely for mechanical controls they are not required to be certified by the Energy Commission. Most standard programmable thermostats and DDC system comply with these requirements. Time controls for HVAC systems must have a readily accessible manual override that can provide up to 4 hours of off-hour control.

CO₂ sensors used in DCV systems used to require certification to and approval by the California Energy Commission. This has been replaced by certification by the manufacture §120.1(c)4F and the acceptance requirements described in Section 4.3.7 Ventilation Requirements.

4.5.2    Prescriptive Requirements    

A.   Space Conditioning Zone Controls

Each space-conditioning zone shall have controls that prevent:

1.   Reheating of air that has been previously cooled by mechanical cooling equipment or an economizer.

2.   Recooling of air that has been previously heated. This does not apply to air returned from heated spaces.

3.   Simultaneous heating and cooling in the same zone, such as mixing or simultaneous supply of air that has been previously mechanically heated and air that has been previously cooled, either by cooling equipment or by economizer systems.

B.   These requirements do not apply to zones having:

1.   VAV controls, as discussed in the following section.

2.   Special pressurization relationships or cross contamination control needs. Laboratories are an example of spaces that might fall in this category.

3.   Site-recovered or site-solar energy providing at least 75 percent of the energy for reheating, or providing warm air in mixing systems.

4.   Specific humidity requirements to satisfy exempt process needs.  Computer rooms are explicitly not covered by this exception.

C.   VAV Zone Controls

To save fan and reheat energy while providing adequate comfort and ventilation, zones served by variable air-volume systems that are designed and controlled to reduce, to a minimum, the volume of reheated, re-cooled, or mixed air are allowed only if the controls meet the following requirements:

1.         For each zone with direct digital controls (DDC):

a.   The volume of primary air that is reheated, re-cooled, or mixed air supply shall not exceed the larger of:

i. 50 percent of the peak primary airflow; or

ii. The design zone outdoor airflow rate per §120.1.

b.   The volume of primary air in the dead band shall not exceed the larger of:

i. 20 percent of the peak primary airflow; or

ii. The design zone outdoor airflow rate per §120.1.

c.   The first stage of heating consists of modulating the zone supply air temperature setpoint up to a maximum setpoint no higher  than 95ºF while the airflow is maintained at the deadband flow rate

d.   The second stage of heating consists of modulating the airflow rate from the deadband flow rate up to the heating maximum flow rate.

2.   For each zone without DDC, the volume of primary air that is reheated, re-cooled, or mixed air supply shall not exceed the larger of the following:

a.   30 percent of the peak primary airflow; or

b.   The design zone outdoor airflow rate per §120.1.

For systems with DDC to the zone level the controls must be able to support two different maximums: one each for heating and cooling.  This control is depicted in Figure 4-13 below. In cooling, this control scheme is similar to a traditional VAV reheat box control. The difference is what occurs in the deadband between heating and cooling and in the heating mode. With traditional VAV control logic, the minimum airflow rate is typically set to the largest rate allowed by code. This airflow rate is supplied to the space in the deadband and heating modes. With the "dual maximum" logic, the minimum rate is the lowest allowed by code (e.g. the minimum ventilation rate) or the minimum rate the controls system can be set to (which is a function of the VAV box velocity pressure sensor amplification factor and the accuracy of the controller to convert the velocity pressure into a digital signal). As the heating demand increases, the dual maximum control first resets the discharge air temperature (typically from the design cold deck temperature up to 85 or 90°F) as a first stage of heating then, if more heat is required, it increases airflow rate up to a “heating” maximum airflow setpoint, which is the same value as what traditional control logic uses as the minimum airflow setpoint. Using this control can save significant fan, reheat and cooling energy while maintaining better ventilation effectiveness as the discharge heating air is controlled to a temperature that will minimize stratification.

This control requires a discharge air sensor and may require a programmable VAV box controller. The discharge air sensor is very useful for diagnosing control and heating system problems even if they are not actively used for control.

Figure 4-13 – Dual-Maximum VAV Box Control Diagram

For systems without DDC to the zone (such as electric or pneumatic thermostats), the airflow that is reheated is limited to a maximum of the larger either 30 percent of the peak primary airflow or the minimum airflow required to ventilate the space.

D.   Economizers    

1.   An economizer must be fully integrated and must be provided for each individual cooling system that has a total mechanical cooling capacity over 54,000 Btu/h. The economizer may be either:

2.   An air economizer capable of modulating outside air and return air dampers to supply 100 percent of the design supply air quantity as outside air; or

3.   A water economizer capable of providing 100 percent of the expected system cooling load at outside air temperatures of 50°F dry-bulb and 45°F wet-bulb and below.

Depicted below in Figure 4-14 is a schematic of an air-side economizer. All air-side economizers have modulating dampers on the return and outdoor air streams. To maintain acceptable building pressure, systems with airside economizer must have provisions to relieve or exhaust air from the building. In Figure 4-14, three common forms of building pressure control are depicted: Option 1 barometric relief, Option 2 a relief fan generally controlled by building static pressure, and Option 3 a return fan often controlled by tracking the supply.

Figure 4-15 depicts an integrated air-side economizer control sequence. On first call for cooling the outdoor air damper is modulated from minimum position to 100 percent outdoor air. As more cooling is required, the damper remains at 100 percent outdoor air as the cooling coil is sequenced on.

Graphics of water-side economizers are presented in Section 4.7 Glossary/Reference at the end of this chapter.

Figure 4-14– Air-Side Economizer Schematic

Figure 4-15 – Typical Air-Side Economizer Control Sequencing

E.   Economizers are not required where:    

1.   Outside air filtration and treatment for the reduction and treatment of unusual outdoor contaminants make compliance infeasible.

2.   Increased overall building TDV energy use results. This may occur where economizers adversely impact other systems, such as humidification, dehumidification or supermarket refrigeration systems.

3.   Systems serving high-rise residential living quarters and hotel/motel guest rooms. Note that these buildings typically have systems smaller than 2,500 cfm, and also have provisions for natural ventilation.

4.   If cooling capacity is less than or equal to 54,000 Btu/h

5.   Where cooling systems have the cooling efficiency that meets or exceeds the cooling efficiency improvement requirements in Table 4-17

6.   Fan systems primarily serving computer room(s).  See Section 140.9 (a) for computer room economizer requirements.

If an economizer is required, it must be designed and equipped with controls that do not increase the building heating energy use during normal operation. This prohibits the application of single-fan dual-duct systems and traditional multizone systems using the Prescriptive Approach of compliance (see Figure 4-17). With these systems the operation of the economizer to pre-cool the air entering the cold deck also precools the air entering the hot deck and thereby increases the heating energy. An exception allows these systems when at least 75 percent of the annual heating is provided by site-recovered or site-solar energy §140.4(e)2A.

The economizer controls must also be fully integrated into the cooling system controls so that the economizer can provide partial cooling even when mechanical cooling is required to meet the remainder of the load §140.4(e)2B.  On packaged units with stand-alone economizers, a two-stage thermostat is necessary to meet this requirement.

The requirement that economizers be designed for concurrent operation is not met by some popular water economizer systems, such as those that use the chilled water system to convey evaporatively-cooled condenser water for “free” cooling. Such systems can provide 100 percent of the cooling load, but when the point is reached where condenser water temperatures cannot be sufficiently cooled by evaporation; the system controls throw the entire load to the mechanical chillers. Because this design cannot allow simultaneous economizer and refrigeration system operation, it does not meet the requirements of this section.  An integrated water-side economizer which uses condenser water to precool the CHWR before it reaches the chillers (typically using a plate-and-frame heat exchanger) can meet this integrated operation requirement

Table 4-17 – Standards Table 140.4-A Economizer Trade-Off Table For Cooling Systems

a If a unit is rated with an IPLV, IEER or SEER, then to eliminate the required air or water economizer, the applicable  minimum cooling efficiency of the HVAC unit must be increased by the percentage shown.  If the HVAC unit is only rated with a full load metric, such as EER or COP cooling, then that metric must be increased by the percentage shown.

F.   Air-Side Economizer High Limit Switches

If an economizer is required by Section 140.4(e)1, and an air economizer is used to meet the requirement, the air side economizer is required to have high-limit shut-off controls that comply with Table 140.4-B of the Standards. This table has four columns:

1.   The first column identifies the high limit control category. There are three categories allowed in this prescriptive requirement: Fixed Dry Bulb; Differential Dry Bulb; and Fixed Enthalpy + Fixed Dry Bulb. 

2.   The second column represents the California climate zone. “All” indicates that this control type complies in every California climate.

3.   The third and fourth columns present the high-limit control setpoints required.

The 2013 Standards eliminated the use of Fixed Enthalpy, Differential Enthalpy and Electronic Enthalpy controls. Research on the accuracy and stability of enthalpy controls led to their elimination (with the exception of use when combined with a fixed dry-bulb sensor).  The enthalpy based controls can be employed if the project uses the performance approach however the performance model will show a penalty due to the inaccuracy of the enthalpy sensors.

Table 4-18 – Standards Table 140.4-B Air Economizer High Limit Shut Off Control Requirements

G.   Air Economizer Construction (§140.4(e)4)

If an economizer is required by Section 140.4(e)1, and an air economizer is used to meet the requirement, then the air economizer, and all return air dampers on any individual cooling fan system that has a total mechanical cooling capacity over 45,000 Btu/hr, shall have the following features:

1.   The requirement for a 5-year factory warranty for the economizer assembly

2.   Certification by the manufacturer that the that the economizer assembly, including but not limited to outdoor air damper, return air damper, drive linkage, and actuator, have been tested and are able to open and close against the rated airflow and pressure of the system after 60,000 damper opening and closing cycles

3.   Economizer outside air and return dampers shall be certified in accordance with AMCA Standard 500 to have a maximum leakage rate of 10 cfm/sf at 1.0 in. w.g.

4.   If the high-limit control uses either a fixed dry-bulb, or fixed enthalpy control, the control shall have an adjustable setpoint.

5.   Economizer sensors shall be calibrated within the following accuracies.

a.   Drybulb and wetbulb temperatures accurate to ±2°F over the range of 40°F to 80°F.

b.   Enthalpy accurate to ±3 Btu/lb over the range of 20 Btu/lb to 36 Btu/lb.

c.   Relative Humidity (RH) accurate to ± 5 percent over the range of 20 percent to 80 percent RH 

6.   Data of sensors used for control of the economizer shall be plotted on a sensor performance curve.

7.   Sensors used for the high limit control shall be located to prevent false readings, e.g. including but not limited to being properly shielded from direct sunlight.

8.   Relief air systems shall be capable of providing 100 percent outside air without over-pressurizing the building.

H.   Air Economizer Compressor Unloading (§140.4(e)5)

New to the 2013 Standards are requirements for minimum compressor unloading for DX units with air-side economizers:

4.   Unit controls shall have mechanical capacity controls interlocked with economizer controls such that the economizer is at 100 percent open position when mechanical cooling is on and does not begin to close until the leaving air temperature is less than 45°F. 

5.   Direct Expansion (DX) units that control the capacity of the mechanical cooling directly based on occupied  space temperature shall have a minimum of 2 stages of mechanical cooling capacity, per the following effective dates: 

a.   ≥ 75,000 Btu/hr – Effective 1/1/2014

b.   ≥ 65,000 Btu/hr – Effective 1/1/2016

6.   DX units not within the scope of Section 140.4(e)5.B, such as those that control space temperature by modulating the airflow to the space, shall (i) comply with the requirements in Table 140.4-C, and (ii) shall have controls that do not false load the mechanical cooling system by limiting or disabling the economizer or by any other means, such as hot gas bypass, except at the lowest stage of mechanical cooling capacity.

Table 4-19 – Standards Table 140.4-C Direct Expansion (DX) Unit Requirements For Cooling Stages And Compressor Displacement

Chapter 12, Acceptance Requirements, describe mandated acceptance test requirements for economizers.

To reduce the time required to perform the economizer acceptance test, factory calibration and a calibration certificate of economizer control sensors (outdoor air temperature, return air temperature, etc.)

Figure 4-16 – Economizer Flowchart

Figure 4-17– Single-Fan Dual-Duct System

I.    VAV Supply Fan Controls      

Both single and multiple zone systems are required to have VAV supply based on Standard Table 140.4-D.  This table has four columns: cooling system type (chilled water or DX); Fan motor size (used for chilled water systems); Cooling Size (used for DX systems); and effective date.  As of the effective date chilled water units with a total supply fan horsepower greater than the fan motor size limit are required to be VAV.  Similarly for DX systems as of the effective date units with a nominal rated cooling capacity greater than the threshold cooling capacity limit are required to be VAV.  The VAV requirements for supply fans are as follows:

1.     Single zone systems (where the fans are controlled directly by the space thermostat) shall have a minimum of 2 stages of fan speed with no more than 66 percent speed when operating on stage 1 while drawing no more than 40 percent full fan power when running at 66 percent speed. 

2.     All systems with air-side economizers to satisfy 140.4(e) regardless of size or date are required to have a minimum of 2 speeds of fan control during economizer operation. 

3.     Multiple zone systems shall limit the fan motor demand to no more than 30 percent of design wattage at 50 percent design air volume.

Variable speed drives can be used to meet any of these three requirements.

Actual fan part load performance, available from the fan manufacturer, should be used to test for compliance with item 3 above. Figure 4-18 shows typical performance curves for different types of fans. As can be seen, both air foil fans and backward inclined fans using either discharge dampers or inlet vanes consume more than 30 percent power at 50 percent flow when static pressure set point is one-third of total design static pressure using certified manufacturer’s test data. These fans will not normally comply with these requirements unless a variable speed drive is used.

VAV fan systems that don’t have DDC to the zone level are required to have the static pressure sensor located in a position such that the control setpoint is ≤1/3 of the design static pressure of the fan. For systems without static pressure reset the further the sensor is from the fan the more energy will be saved. For systems with multiple duct branches in the distribution you must provide separate sensors in each branch and control the fan to satisfy the sensor with the greatest demand. When locating sensors, care should be taken to have at least one sensor between the fan and all operable dampers (e.g. at the bottom of a supply shaft riser before the floor fire/smoke damper) to prevent loss of fan static pressure control.

For systems with DDC to the zone level the sensor(s) may be anywhere in the distribution system and the duct static pressure setpoint must be reset by the zone demand. Typically this is done by one of the following methods:

1.   Controlling so that the most open VAV box damper is 95 percent open.

2.   Using a “trim and respond” algorithm to continually reduce the pressure until one or more zones indicate that they are unable to maintain airflow rate setpoints.

3.   Other methods that dynamically reduce duct static pressure setpoint as low

as possible while maintaining adequate pressure at the VAV box zone(s) of

greatest demand.

Reset of supply pressure by demand not only saves energy but it also protects fans from operation in surge at low loads. Chapter 12, Acceptance Requirements, describes mandated acceptance test requirements for VAV system fan control.

Figure 4-18 – VAV Fan Performance Curve

Table 4-20 – Standards Table 140.4-D Effective Dates For Fan Control Systems

J.   Supply-Air Temperature Reset Control    

Mechanical space-conditioning systems supplying heated or cooled air to multiple zones must include controls that automatically reset the supply-air temperature in response to representative building loads, or to outdoor air temperature. The controls must be capable of resetting the supply-air temperature by at least 25 percent of the difference between the design supply-air temperature and the design room air temperature.

For example, if the design supply temperature is 55°F and the design room temperature is 75°F, then the difference is 20°F, and 25 percent is 5°F. Therefore, the controls must be capable of resetting the supply temperature from 55°F to 60°F.

Air distribution zones that are likely to have constant loads, such as interior zones, shall have airflow rates designed to meet the load at the fully reset temperature. Otherwise, these zones may prevent the controls from fully resetting the temperature, or will unnecessarily limit the hours when the reset can be used.

Supply air reset is required for VAV reheat systems even if they have VSD fan controls. The recommended control sequence is to lead with supply temperature setpoint reset in cool weather where reheat might dominate the equation and to keep the chillers off as long as possible, then return to a fixed low setpoint in warmer weather when the chillers are likely to be on. During reset, employ a demand-based control that uses the warmest supply air temperature that satisfies all of the zones in cooling.

This sequence is described as follows: during occupied mode, the setpoint is reset from
T-min (53°F) when the outdoor air temperature is 70°F and above, proportionally up to
T-max when the outdoor air temperature is 65°F and below. T-max shall range from 55°F to 65°F and shall be the output of a slow reverse-acting proportional-integral (PI) loop that maintains the cooling loop of the zone served by the system with the highest cooling loop at a setpoint of 90 percent. See Figure 4-19.

Supply temperature reset is also required for constant volume systems with reheat justified on the basis of special zone pressurization relationships or cross-contamination control needs.

Supply-air temperature reset is not required when:

1.   The zone(s) must have specific humidity levels required to meet exempt process needs.  Computer rooms cannot use this exception; or

2.   Where it can be demonstrated to the satisfaction of the enforcement agency that supply air reset would increase overall building energy use; or

3.   The space-conditioning zone has controls that prevent reheating and recooling and simultaneously provide heating and cooling to the same zone; or

4.   75 percent of the energy for reheating is from site-recovered or site solar energy source; or

5.         The zone has a peak supply air quantity of 300 cfm or less.

Figure 4-19 – Energy Efficient Supply Air Temperature Reset Control for VAV Systems

Recommended Supply Air Temperature Reset Method

K.   Heat Rejection Fan Control    

When the fans on cooling towers, closed-circuit fluid coolers, air-cooled condensers and evaporative condensers are powered by a fan motor or 7.5 hp or larger, the system must be capable of operating at 2/3 of full speed or less and have controls that automatically change the fan speed to control the leaving fluid temperature or condensing temperature or pressure of the heat rejection device.  Fan speed control are exempt when:

1.   Fans powered by motors smaller than 7.5 hp.

2.   Heat rejection devices included as an integral part of the equipment 'listed in the Standards Tables 110.2-A through 110.2-I. This includes unitary air-conditioners, unitary heat pumps, packaged chillers and packaged terminal heat pumps.

3.   Condenser fans serving multiple refrigerant circuits or flooded condensers.

4.   Up to 1/3 of the fans on a condenser or tower with multiple fans where the lead fans comply with the speed control requirement.

Where applicable, 2-speed motors, pony motors or variable speed drives can be used to comply with this requirement.

L.   Design of Hydronic Systems for Variable Flow    

Hot water and chilled water systems are required to be designed for variable flow. Variable flow is provided by using 2-way control valves. The Standards only require that flow is reduced to the greater of 50 percent design flow (or less) or the minimum flow required by the equipment manufacturer for operation of the central plant equipment. There are two exceptions for this requirement:

1.   Systems that include no more than three control valves, and

2.   Systems having a total pump system power less than or equal to
1.5 hp

It is not necessary for each individual pump to meet the variable flow requirement of §140.4(k)1; these requirements can be met by varying the total flow for the entire pumping system in the plant. Strategies that can be used to meet these requirements include but are not limited to variable frequency drives on pumps and staging of the pumps.

It should be noted that the primary loop on a primary/secondary or primary/secondary/tertiary system could be designed for constant flow even if the secondary or tertiary loop serves more than 3 control valves. This is allowed because the primary loop does not directly serve any coil control valves. However the secondary (and tertiary loops) of these systems must be designed for variable flow if they have 4 or more control valves.

The flow limitations are provided for primary-only variable flow chilled water systems where a minimum flow is typically required to keep a chiller on-line. In these systems minimum flow can be provided with either a bypass with a control valve or some 3-way valves to ensure minimum flow at all times. The system with a bypass valve is more efficient as it only provides bypass when absolutely required to keep the plant on line.

For hot water systems application of slant-tube or bent tube boilers will provide the greatest flow turndown. Typically copper fin tube boilers require a higher minimum flow.

M.  Isolation for Chillers and Boilers

Plants with multiple chillers or boilers are required to provide either isolation valves or dedicated pumps and check valves to ensure that flow will only go through the chillers or boilers that are staged on. Chillers that are piped-in series for the purpose of increased temperature differential shall be considered as one chiller.

N.   Chilled and Hot Water Reset

Similar to the requirements for supply air temperature reset, chilled and hot water systems that have a design capacity > 500,000 Btu/h are required to provide controls to reset the hot or cold water temperature setpoints as a function of building loads or the outdoor air temperature. This reset can be achieved either using a direct indication of demand (usually cooling or heating valve position) or an indirect indication of demand (typically outdoor air temperature). On systems with DDC controls reset using valve position is recommended.

There is an exception to this requirement for hydronic systems that are designed for variable flow complying with §140.4(k)1.

O.   Isolation Valves for Water-Loop Heat Pump Systems

Water circulation systems serving water-cooled air conditioner and hydronic heat pump systems that have a design circulation pump brake horsepower >5 bhp are required to be provided with 2-way isolation valves that close whenever the compressor is off. These systems are also required to be provided with the variable speed drives and pressure controls described in the following section.

Although this is not required on central tenant condenser water systems (for water-cooled AC units and HPs) it is a good idea to provide the 2-way isolation valves on these systems as well. 'In addition to providing pump energy savings, these 2-way valves can double as head-pressure control valves to allow aggressive condenser water reset for energy savings in chilled water plants that are also cooled by the towers.

P.   VSDs for Pumps Serving Variable Flow Systems    

Variable Flow Controls - Pumps on variable flow systems that have a design circulation pump brake horsepower > 5 bhp are required to have either variable speed drives or a different control that will result in pump motor demand of no more than 30 percent of design wattage at 50 percent of design water flow.

Pressure Sensor Location and Setpoint

1.   For systems without direct digital control of individual coils reporting to the central control panel, differential pressure must be measured at  the most remote heat exchanger or the heat exchanger requiring the most pressure. This includes chilled water systems, condenser water systems serving water-cooled air conditioning (AC) loads and water-loop heat pump systems.

2.   For systems with direct digital control of individual coils with a central control panel, the static pressure set point must be reset based on the valve requiring the most pressure and the setpoint shall be no less than 80 percent open. The pressure sensor(s) may be mounted anywhere.

Exceptions are provided for hot-water systems and condenser water systems that only serve water-cooled chillers. The hot water systems are exempted because the heat from the added pumping energy of the pump riding the curve provides a beneficial heat that reduces the boiler use. This reduces the benefit from the reduced pumping energy.

Q.   Hydronic Heat Pump (WLHP) Controls    

Hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection and heat addition must have controls that are capable of providing a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition by the central devices. Exceptions are provided where a system loop temperature optimization controller is used to determine the most efficient operating temperature based on real-time conditions of demand and capacity, dead bands of less than 20°F shall be allowed.

4.5.3    Acceptance Requirements

A.   There are a number of acceptance requirements related to control systems. These include:

1.   Automatic time switch control devices.

2.   Constant volume package unit.

3.   Air-side economizers.

4.   VAV supply fan controls.

5.   Hydronic system controls.

These tests are described in Chapter 12, Acceptance Requirements, as well as the Reference Nonresidential Appendix NA7.