4.3 Ventilation Requirements    

All of the ventilation requirements are mandatory measures. Some measures require acceptance testing, which is addressed in Section 4.3.12.

Within a building, all enclosed spaces that are normally used by humans must be continuously ventilated during occupied hours with outdoor air, using either natural or mechanical ventilation §120.1(a)1 An exception is provided to §120.1(a)1 for refrigerated warehouses or other buildings or spaces that are not normally used for human occupancy or work.

The standards allow for ventilation to use transfer air as long as it doesn’t have any “unusual sources of indoor air contaminants” and “the outdoor air that is supplied to all spaces combined, is sufficient to meet the requirements of Section120.1(b)2 for each space individually ( see exception to §120.1(b)2). Good practice dictates that sources of contaminants be isolated and controlled with local exhaust.  The designation and treatment of such spaces is subject to the designer’s discretion. Spaces needing special consideration include:

•      Commercial and coin-operated dry cleaners

•      Bars and cocktail lounges

•      Smoking lounges and other designated smoking areas

•      Beauty and barbershops

•      Auto repair workshops

•      Print shops, graphic arts studios and other spaces where solvents are used in a process

•      Copy rooms, laser printer rooms or other rooms where it is expected that equipment may generate heavy concentrations of ozone or other contaminants

•      Blueprint machines

“Spaces normally used by humans” refers to spaces where people can be reasonably expected to remain for an extended period of time. Spaces where occupancy will be brief and intermittent, and that do not have any unusual sources of air contaminants, do not need to be directly ventilated. For example:

•      A closet does not need to be ventilated, provided it is not normally occupied.

•      A storeroom that is only infrequently or briefly occupied does not require ventilation. However, a storeroom that can be expected to be occupied for extended periods for clean-up or inventory must be ventilated, preferably with systems controlled by a local switch so that the ventilation system operates only when the space is occupied.

“Continuously ventilated during occupied hours” implies that the design ventilation must be provided throughout the entire occupied period. This means that VAV systems must provide the code-required ventilation over their full range of operating supply airflow. Some means of dynamically controlling the minimum ventilation air must be provided.

4.3.1    Natural Ventilation    

Natural outdoor ventilation may be provided for spaces where all normally occupied areas of the space are within a specific distance from an operable wall or roof opening through which outdoor air can flow. This distance is 20 ft. for most spaces and 25 ft. for hotel/motel guestrooms and high-rise residential spaces. The sum of the operable open areas must total at least 5 percent of the floor area of each space that is naturally ventilated. The openings must also be readily accessible to the occupants of the space at all times.

Airflow through the openings must come directly from the outdoors; air may not flow through any intermediate spaces such as other occupied spaces, unconditioned spaces, corridors, or atriums. High windows or operable skylights need to have a control mechanism accessible from the floor.

4.3.2    Mechanical Ventilation    

Mechanical outdoor ventilation must be provided for all spaces normally occupied that are not naturally ventilated. The Standards require that a space conditioning system provide outdoor air equal to or exceeding the ventilation rates required for each of the spaces that it serves. At the space, the required ventilation can be provided either directly through supply air or indirectly through transfer of air from the plenum or an adjacent space. The required minimum ventilation airflow at the space can be provided by an equal quantity of supply or transfer air. At the air-handling unit, the minimum outside air must be the sum of the ventilation requirements of each of the spaces that it serves. The designer may specify higher outside air ventilation rates based on the owner’s preference or specific ventilation needs associated with the space. However, specifying more ventilation air than the minimum allowable ventilation rates increases energy consumption and electrical peak demand and increases the costs of operating the HVAC equipment. Thus the designer should have a compelling reason to specify higher design minimum outside air rates than the calculated minimum outside air requirements in the Standards.

In the 2013 revision to the Standards the minimum OSA provided is required to be within 10 percent of the calculated minimum for both VAV and constant volume units.

A.   In summary:

1.   Ventilation compliance at the space is satisfied by providing supply and/or transfer air (exception to §120.1(b)2).

2.   Ventilation compliance at the unit is satisfied by providing, at minimum, the outdoor air that represents the sum of the ventilation requirements at each space that it serves.

B.   For each space requiring mechanical ventilation the ventilation rates must be the greater of either:

1.   The conditioned floor area of the space, multiplied by the applicable minimum ventilation rate from the Standards in Table 120.1-A (Table 4-12). This provides dilution for the building-borne contaminants like off-gassing of paints and carpets.

Table 4-12 – (Standards Table 120.1-A) Minimum Ventilation Rates

2.   15 cfm per person, multiplied by the expected number of occupants. For spaces with fixed seating (such as a theater or auditorium), the expected number of occupants is the number of fixed seats. For spaces without fixed seating, the expected number of occupants is assumed to be no less than one-half that determined for egress purposes in the California Building Code (CBC). The Standards specify the minimum outdoor ventilation rate to which the system must be designed. If desired, the designer may, with documentation, elect to provide more ventilation air. For example, the design outdoor ventilation rate may be determined using the procedures described in ASHRAE 62, provided the resulting outdoor air quantities are no less than required by the Standards.

Table 4-12 shows the typical maximum occupant loads for various building uses (upon which minimum ventilation calculations are based). This provides dilution for the occupant-borne contaminants (or bioeffluents) like body odor and germs.

Table 4-13 summarizes the combination of these two rates for typical spaces.

As previously stated, each space-conditioning system must provide outdoor ventilation air as follows.

1.   For a space-conditioning system serving a single space, the required system outdoor airflow is equal to the design outdoor ventilation rate of the space.

2.   For a space-conditioning system serving multiple spaces, the required outdoor air quantity delivered by the space-conditioning system must be not less than the sum of the required outdoor ventilation rate to each space. The Standards do not require that each space actually receive its calculated outdoor air quantity (§120.1(b)2 Exception). Instead, the actual supply to any given space may be any combination of recirculated air, outdoor air, or air transferred directly from other spaces, provided:

a.   The total amount of outdoor air delivered by the space-conditioning system(s) to all spaces is at least as large as the sum of the space design quantities

b.   Each space always receives a supply airflow, including recirculated air and/or transfer air, no less than the calculated outdoor ventilation rate

c.   When using transfer air, none of the spaces from which air is transferred has any unusual sources of contaminants

Table 4-13 – CBC Occupant Densities (ft² /person)
Source Table 1004.1.1 of the California Building Code

C.   Where:

Floor Area, Gross. The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, closets, the thickness of interior walls, columns or other features. The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above. The gross floor area shall not include shafts with no openings or interior courts.

Floor Area, Net. The actual occupied area not including unoccupied accessory areas such as corridors, stairways, toilet rooms, mechanical rooms and closets.

Table 4-14 – Required Minimum Ventilation Rate per Occupancy

4.3.3    Direct Air Transfer    

The Standards allow air to be directly transferred from other spaces in order to meet a part of the ventilation supply to a space, provided the total outdoor quantity required by all spaces served by the building’s ventilation system is supplied by the mechanical systems. This method can be used for any space, but is particularly applicable to conference rooms, toilet rooms, and other rooms that have high ventilation requirements. Transfer air must be free from any unusual contaminants, and as such should not be taken directly from rooms where such sources of contaminants are anticipated. It is typically taken from the return plenum or directly from an adjacent space.

Air may be transferred using any method that ensures a positive airflow. Examples include dedicated transfer fans, exhaust fans and fan-powered VAV boxes. A system having a ducted return may be balanced so that air naturally transfers into the space. Exhaust fans serving the space may discharge directly outdoors, or into a return plenum. Transfer systems should be designed to minimize recirculation of transfer air back into the space; duct work should be arranged to separate the transfer air intake and return points.

When each space in a two-space building is served by a separate constant volume system, the calculation and application of ventilation rate is straightforward, and each space will always receive its design outdoor air quantity. However, a central system serving both spaces does not deliver the design outdoor air quantity to each space. Instead, one space receives more than its allotted share, and the other less. This is because the training room has a higher design outdoor ventilation rate and/or a lower cooling load relative to the other space.

4.3.4    Distribution of Outdoor Air to Zonal Units    

When a return plenum is used to distribute outside air to a zonal heating or cooling unit, the outside air supply must be connected either:

A.   Within 5 ft. of the unit; or

B.   Within 15 ft. of the unit, with the air directed substantially toward the unit, and with a discharge velocity of at least 500 ft. per minute.

Water source heat pumps and fan coils are the most common application of this configuration. The unit fans should be controlled to run continuously during occupancy in order for the ventilation air to be circulated to the occupied space.

A central space-conditioning system(s) augmented by a few zonal units for spot conditioning may use transfer air from spaces served by the central system. A direct source of outdoor air is not required for each zonal unit. Similarly, transfer air may be used in buildings having central interior space-conditioning systems with outdoor air, and zonal units on the perimeter (without outdoor air).

While not required, the Standards recommend that sources of unusual contaminants be controlled through the use of containment systems that capture the contaminants and discharge them directly outdoors. Such systems may include exhaust hoods, fume hoods, small space exhausts and differential pressure control between spaces. The designer is advised to consult ASHRAE standards or other publications for guidance in this subject.

4.3.5    Ventilation System Operation and Controls    

A.   Outdoor Ventilation Air and VAV Systems

Except for systems employing Energy Commission-certified demand controlled ventilation (DCV) devices or space occupancy sensors, the Standards require that the minimum rate of outdoor air calculated per §120.1(b)2 be provided to each space at all times when the space is normally occupied §120.1(c)1. For spaces served by variable air volume (VAV) systems, this means that the minimum supply setting of each VAV box should be no less than the design outdoor ventilation rate calculated for the space, unless transfer air is used. If transfer air is used, the minimum box position, plus the transfer air, must meet the minimum ventilation rate. If transfer air is not used, the box must be controlled so that the minimum required airflow is maintained at all times (unless demand controlled ventilation or occupant sensor are employed).

The design outdoor ventilation rate at the system level must always be maintained when the space is occupied, even when the fan has modulated to its minimum capacity §120.1(c)1. Section 4.3.12 describes mandated acceptance test requirements for outside air ventilation in VAV air handling systems. In these tests, the minimum outside air in VAV systems will be measured both at full flow and with all boxes at minimum position.

Figure 4-2 shows a typical VAV system. In standard practice, the testing and balancing (TAB) contractor sets the minimum position setting for the outdoor air damper during construction. It is set under the conditions of design airflow for the system, and remains in the same position throughout the full range of system operation. Does this meet code? The answer is no. As the system airflow drops, so will the pressure in the mixed air plenum. A fixed position on the minimum outdoor air damper will produce a varying outdoor airflow. As depicted in Figure 4-2, this effect will be approximately linear (in other words, outdoor air airflow will drop directly in proportion to the supply airflow).

The following paragraphs present several methods used to dynamically control the minimum outdoor air in VAV systems, which are described in detail below.

Regardless of how the minimum ventilation is controlled, care should be taken to reduce the amount of outdoor air provided when the system is operating during the weekend or after hours with only a fraction of the zones active. §120.2(g) requires provision of “isolation zones” of 25,000 ft² or less. This can be provided by having the VAV boxes return to fully closed when their associated zone is in unoccupied mode. When a space or group of spaces is returned to occupied mode (e.g. through off-hour scheduling or a janitor’s override), only the boxes serving those zones need to be active. During this partial occupancy the ventilation air can be reduced to the requirements of those zones that are active. If all zones are of the same occupancy type (e.g. private offices), simply assign a floor area to each isolation zone and prorate the minimum ventilation area by the ratio of the sum of the floor areas presently active divided by the sum of all the floor areas served by the HVAC system.

Figure 4-2 – VAV Reheat System with a Fixed Minimum Outdoor Air Damper Setpoint

B.   Fixed Minimum Damper Setpoint

This method does not comply with the Standards; the airflow at a fixed minimum damper position will vary with the pressure in the mixed air plenum
(see Figure 4-2).  It is explicitly prohibited in 120.1(e)2.

C.   Dual Minimum Setpoint Design

This method complies with the Standards. An inexpensive enhancement to the fixed damper setpoint design is the dual minimum setpoint design, commonly used on some packaged AC units. The minimum damper position is set proportionally based on fan speed or airflow between a setpoint determined when the fan is at full speed (or airflow) and minimum speed (or airflow). This method complies with the letter of the Standards but is not accurate over the entire range of airflow rates and when there are wind or stack effect pressure fluctuations. But with DDC, this design has very low costs.

D.   Energy Balance Method

The energy balance method (Figure 4-3) uses temperature sensors in the outside, as well as return and mixed air plenums to determine the percentage of outdoor air in the supply air stream. The outdoor airflow is then calculated using the equations shown in Figure 4-3. This method requires an airflow monitoring station on the supply fan.

While technically feasible, it may be difficult to meet the outside air acceptance requirements with this approach because:

1.   It is difficult to accurately measure the mixed air temperature, which is critical to the success of this strategy. Even with an averaging type bulb, most mixing plenums have some stratification or horizontal separation between the outside and mixed airstreams.

2.   Even with the best installation, high accuracy sensors, and field calibration of the sensors, the equation for percent outdoor air will become inaccurate as the return air temperature approaches the outdoor air temperature. When they are equal, this equation predicts an infinite percentage outdoor air.

3.   The accuracy of the airflow monitoring station is likely to be low at low supply airflows.

4.   The denominator of the calculation amplifies sensor inaccuracy as the return air temperature approaches the outdoor air temperature.

Figure 4-3 – Energy Balance Method of Controlling Minimum Outdoor Air

E.   Return Fan Tracking

This method is also technically feasible, but will likely not meet the acceptance requirements because the cumulative error of the two airflow measurements can be large, particularly at low supply/return airflow rates. It only works theoretically when the minimum outdoor air rate equals the rate of air required to maintain building pressurization (the difference between supply air and return air rates). Return fan tracking (Figure 4-4) uses airflow monitoring stations on both the supply and return fans. The theory behind this is that the difference between the supply and return fans has to be made up by outdoor air, and controlling the flow of return air forces more ventilation into the building. Several problems occur with this method:

1.   The relative accuracy of airflow monitoring stations is poor, particularly at low airflows;

2.   The cost of airflow monitoring stations;

3.   It will cause building pressurization problems unless the ventilation air is equal to the desired building exfiltration plus the building exhaust.

ASHRAE research has also demonstrated that in some cases this arrangement can cause outdoor air to be drawn into the system through the exhaust dampers due to negative pressures at the return fan discharge.

Figure 4-4 – Return Fan Tracking

F.   Airflow Measurement of the Entire Outdoor Air Inlet

Again, this method is technically feasible but will likely not meet the acceptance requirements depending on the airflow measurement technology. Most airflow sensors will not be accurate to a 5-15 percent turndown (the normal commercial ventilation range). Controlling the outdoor air damper by direct measurement with an airflow monitoring station (Figure 4-5) can be an unreliable method. Its success relies on the turndown accuracy of the airflow monitoring station. Depending on the loads in a building, the ventilation airflow can be between 5 and 15 percent of the design airflow. If the outdoor airflow sensor is sized for the design flow for the airside economizer, this method has to have an airflow monitoring station that can turn down to the minimum ventilation flow (between 5 and 15 percent). Of the different types available, only a hot-wire anemometer array is likely to have this low-flow accuracy while traditional pitot arrays will not. One advantage of this approach is that it provides outdoor airflow readings under all operating conditions, not just when on minimum outdoor air. For highest accuracy, provide a damper and outdoor air sensor for the minimum ventilation air that is separate from the economizer outdoor air intake.

Figure 4-5 – Airflow Measurement of 100% Outdoor Air

G.   Injection Fan Method

This method complies with the Standards, but it is expensive and may require additional space. Note that an airflow sensor and damper are required since fan airflow rate will vary as mixed air plenum pressure varies. The injection fan method (Figure 4-6) uses a separate outdoor air inlet and fan sized for the minimum ventilation airflow. This inlet contains an airflow monitoring station, and a fan with capacity control (e.g., discharge damper; VFD), which is modulated as required to achieve the desired ventilation rate. The discharge damper is recommended since a damper must be provided anyway to shut off the intake when the AHU is off, and also to prevent excess outdoor air intake when the mixed air plenum is very negative under peak conditions. (The fan is operating against a negative differential pressure and thus cannot stop flow just by slowing or stopping the fan.) This method works, but the cost is high and often requires additional space for the injection fan assembly.

Figure 4-6 – Injection Fan with Dedicated Minimum Outdoor Air Damper

H.   Dedicated Minimum Ventilation Damper with Pressure Control

This approach is low cost and takes little space. It can be accurate if the differential setpoint corresponding to the minimum outdoor air rate is properly set in the field. An inexpensive but effective design uses a minimum ventilation damper with differential pressure control (Figure 4-7). In this method, the economizer damper is broken into two pieces: a small two position damper controlled for minimum ventilation air and a larger, modulating, maximum outdoor air damper that is used in economizer mode. A differential pressure transducer is placed across the minimum outdoor air damper. During start-up, the air balancer opens the minimum outside air (OA) damper and return air damper, closes the economizer OA damper, runs the supply fan at design airflow, measures the OA airflow (using a hand-held velometer) and adjusts the minimum OA damper position until the OA airflow equals the design minimum OA airflow. The linkages on the minimum OA damper are then adjusted so that the current position is the “full open” actuator position. At this point the design pressure (DP) across the minimum OA damper is measured. This value becomes the DP setpoint. The principle used here is that airflow is constant across a fixed orifice (the open damper) at fixed DP.

As the supply fan modulates when the economizer is off, the return air damper is controlled to maintain the DP setpoint across the minimum ventilation damper.

The main downside to this method is the complexity of controls and the potential problems determining the DP setpoint in the field. It is often difficult to measure the outdoor air rate due to turbulence and space constraints.

Figure 4-7 – Minimum Outdoor Air Damper with Pressure Control

4.3.6    Pre-Occupancy Purge

Since many indoor air pollutants are out-gassed from the building materials and furnishings, the Standards require that buildings having a scheduled operation be purged before occupancy §120.1(c)2. Immediately prior to occupancy, outdoor ventilation must be provided in an amount equal to the lesser of:

1.   The minimum required ventilation rate for 1 hour; or

2.   3 complete air changes.

Either criteria can be used to comply with the Standards. 3 complete air changes means an amount of ventilation air equal to 3 times the volume of the occupied space. This air may be introduced at any rate provided for and allowed by the system, so that the actual purge period may be less than an hour.

A pre-occupancy purge is not required for buildings or spaces that are not occupied on a scheduled basis, such as storage rooms. Also, a purge is not required for spaces provided with natural ventilation.

Where pre-occupancy purge is required, it does not have to be coincident with morning warm-up (or cool-down). The simplest means to integrate the two controls is to simply schedule the system to be occupied one hour prior to the actual time of anticipated occupancy. This allows the optimal start, warm-up or pull-down routines to bring the spaces up to (or down to) desired temperatures before opening the outdoor air damper for ventilation. This will reduce the required system heating capacity and ensure that the spaces will be at the desired temperatures and fully purged at the start of occupancy.

Figure 4-8 – Pre-Occupancy Purge Flowchart

4.3.7    Demand Controlled Ventilation and Occupant Sensor Ventilation Control Devices    

Demand controlled ventilation (DCV) systems reduce the amount of ventilation supply air in response to a measured level of carbon dioxide (CO₂) in the breathing zone. The Standards only permit CO₂ sensors for the purpose of meeting this requirement; VOC and so-called “IAQ” sensors are not approved as alternative devices to meet this requirement. The Standards only permit DCV systems to vary the ventilation component that corresponds to occupant bioeffluents (this is basis for the 15 cfm/person portion of the ventilation requirement). The purpose of CO₂ sensors is to track occupancy in a space; however, there are many factors that must be considered when designing a DCV system. There is often a lag time in the detection of occupancy through the build-up of CO₂. This lag time may be increased by any factors that affect mixing, such as short circuiting of supply air or inadequate air circulation, as well as sensor placement and sensor accuracy. Build-up of odors, bioeffluents, and other health concerns may also lag changes in occupancy; therefore, the designers must be careful to specify CO₂ based DCV systems that are designed to provide adequate ventilation to the space by ensuring proper mixing, avoiding short circuiting, and proper placement and calibration of the sensors.

The Standards requires the use of DVC systems for spaces with all of the following characteristics:

1.   Served by single zone units with any controls or multiple zone systems with Direct Digital Controls (DDC) to the zone level, and

2.   Has a design occupancy of 40 ft²/person or smaller (for areas without fixed seating where the design density for egress purposes in the CBC is 40 ft²/person or smaller), and

3.   Has an air economizer

There are five exceptions to this requirement:

1.   The following spaces are permitted to use DCV but are not required to: classrooms, call centers, office spaces served by multiple zone systems that are continuously occupied during normal business hours with occupant density greater than 25 people per 1000 ft² per §120.1(b)2B (Table 4-13 Table 4-14), healthcare facilities and medical buildings, and public areas of social services buildings.

2.   Where the space exhaust is greater than the required ventilation rate minus 0.2 cfm/ft².

3.   DCV devices are not allowed in the following spaces: Spaces that have processes or operations that generate dusts, fumes, mists, vapors, or gases and are not provided with local exhaust ventilation, such as indoor operation of internal combustion engines or areas designated for unvented food service preparation, or beauty salons.

4.   Spaces with an area of less than 150 ft², or a design occupancy of less than 10 people per §120.1(b)2B (Table 4-13 and Table 4-14 above).

5.   Spaces less than 1500 ft² that comply with §120.1(c)5 Occupant Sensor Ventilation Control Devices.

The spaces 'listed in Exception 1 are exempted either due to concerns about equipment maintenance practices (schools and public buildings) or concerns about high levels of pathogens (social service buildings, medical buildings, healthcare facilities and to some extent classrooms). The second exception relates to the fact that spaces with high exhaust requirements won’t be able to provide sufficient turndown to justify the cost of the DCV controls. An example of this is a restaurant seating area where the seating area air is used as make-up air for the kitchen hood exhaust. The third exception recognizes that some spaces may need additional ventilation due to contaminants that are not occupant borne. It addresses spaces like theater stages where theatrical fog may be used or movie theater lobbies where unvented popcorn machines may be emitting odors and vapors into the space in either case justifying the need for higher ventilation rates. DCV devices shall not be installed in spaces included in Exception 3. The fourth exception recognizes the fact that DCV devices may not be cost effective in small spaces such as a 15 ft x 10 ft conference room or spaces with only a few occupants at design conditions.  The fifth exception allows an occupant sensor to reduce the amount of ventilation supply air in a vacant room.

Although not required, the Standards permit design professionals to apply DCV on any intermittently occupied spaces served by either single-zone or multiple-zone equipment. §120.1(b)2 requires a minimum of 15 CFM of outdoor air per person times the expected number of occupants; however, it must be noted that these are minimum ventilation levels and the designers may specify higher ventilation levels if there are health related concerns that warrant higher ventilation rates.

CO₂ based DCV is based on two principles:

1.   Several studies (Berg-Munch et al. 1986, Cain et al. 1983, Fanger 1983 and 1988, Iwashita et al. 1990, Rasmussen et al. 1985) concluded that about 15 cfm of outdoor air ventilation per person will control human body odor such that roughly 80 percent of unadapted persons (visitors) will find the odor to be at an acceptable level. These studies are the basis of the 15 cfm/person rate required by these Standards and most building codes. This ventilation rate can be roughly equated to CO₂ concentration using the following steady-state equation.

where V is the ventilation rate per person,  is the CO2 generation rate per person, Cin,ss is the steady-state value of the indoor CO2 concentration, and Cout is the outdoor concentration. At the rate of CO2 generated by adults at typical activity levels in offices, 15 cfm/person equates to a differential CO2 concentration (indoor minus outdoor) of approximately 700 ppm.

2.   The same level of odor acceptability was found to occur at 700 ppm differential CO₂ concentration even for spaces that were not at equilibrium (Berg-Munch et al. 1986, Fanger 1983, Rasmussen et al. 1985), and the correlation was not strongly dependent on the level of physical activity. This suggests that while CO₂ concentration may not track the number of occupants when spaces are not at steady-state, it does track the concentration of bioeffluents that determine people’s perception of air quality. It also suggests that odorous bioeffluents are generated at approximately the same rate as CO₂.

Hence as activity level and bioeffluent generation rate increases (in the equation above), the rate of outdoor air required to provide acceptable air quality (V) increases proportionally, resulting in the same differential CO₂ concentration.

Note that CO₂ concentration only tracks indoor contaminants that are generated by occupants themselves and, to a lesser extent, their activities. It will not track other pollutants, particularly volatile organic compounds (VOCs) that off-gas from furnishings and building materials. Hence, where permitted or required by the Standards, demand controlled ventilation systems cannot reduce the outdoor air ventilation rate below the floor rate 'listed in Standards Table 120.1-A (typically 0.15 cfm/ft²) during normally occupied times.

DCV systems save energy if the occupancy varies significantly over time. Hence they are most cost effective when applied to densely occupied spaces like auditoriums, conference rooms, lounges or theaters. Because DCV systems must maintain the floor ventilation rate 'listed in Standards Table 120.1-A, they will not be applicable to sparsely occupied buildings such as offices where the floor rate always exceeds the minimum rate required by the occupants (see Table 4-14).

Where DCV is employed (whether mandated or not) the controls must meet all of the following requirements:

1.   Sensors must be provided in each room served by the system that has a design occupancy of 40 ft²/person or less, with no less than one sensor per 10,000 ft² of floor space. When a zone or a space is served by more than one sensor, signal from any sensor indicating that CO₂ is near or at the setpoint within a space, must trigger an increase in ventilation to the space. This requirement ensures that the space is adequately ventilated in case a sensor malfunctions. Design professional should ensure that sensors are placed throughout a large space, so that all areas are monitored by a sensor.

2.   The CO₂ sensors must be located in the breathing zone (between 3 and 6 ft. above the floor or at the anticipated height of the occupant’s head). Sensors in return air ducts are not allowed since they can result in under-ventilation due to CO₂ measurement error caused by short-circuiting of supply air into return grilles and leakage of outdoor air (or return air from other spaces) into return air ducts.

3.   The ventilation must be maintained that will result in a concentration of CO₂ at or below 600 ppm above the ambient level. The ambient levels can either be assumed to be 400 ppm or dynamically measured by a sensor that is installed within four feet of the outdoor air intake. At 400 ppm outside CO₂ concentration, the resulting DCV CO₂ setpoint would be 1000 ppm. (Note that a 600 ppm differential is less than the 700 ppm that corresponds to the 15 cfm/person ventilation rate. This provides a margin of safety against sensor error, and because 1000 ppm CO₂ is a commonly recognized guideline value and referenced in earlier versions of ASHRAE Standard 62.)

4.   Regardless of the CO₂ sensor’s reading, the system is not required to provide more than the minimum ventilation rate required by §120.1(b). This prevents a faulty sensor reading from causing a system to provide more than the code required ventilation for system without DCV control. This high limit can be implemented in the controls.

5.   The system shall always provide a minimum ventilation of the sum of the Standards Table 120.1-A values for all rooms with DCV and §120.1(b)2 (Table 4-13of this 'manual) for all other spaces served by the system. This is a low limit setting that must be implemented in the controls.

6.   The CO₂ sensors must be factory-certified to have an accuracy within plus or minus 75 ppm at 600 and 1000 ppm concentration when measured at sea level and 25°C (77°F), factory calibrated or calibrated at start-up, and certified by the manufacturer to require calibration no more frequently than once every 5 years. A number of manufacturers have “self calibrating” sensors now that either adjust to ambient levels during unoccupied times or adjust to the decrease in sensor bulb output through use of dual sources or dual sensors. For all systems, the manufacturers of sensors must provide a document to installers that their sensors meet these requirements. The installer must make this certification information available to the builder, building inspectors and, if specific sensors are specified on the plans, to plan checkers.

7.   When a sensor failure is detected, the system must provide a signal to reset the system to provide the minimum quantity of outside air levels required by §120.1(b)2 to the zone(s) serviced by the sensor at all times that the zone is occupied. This requirement ensures that the space is adequately ventilated in case a sensor malfunctions. A sensor that provides a high CO₂ signal on sensor failure will comply with this requirement.

8.   For systems that are equipped with DDC to the zone level, the CO₂ sensor(s) reading for each zone must be displayed continuously, and recorded. The energy management control system (EMCS) may be used to display and record the sensors’ readings. The display(s) must be readily available to maintenance staff so they can monitor the systems performance.

New in the 2013 version of the Standards is the use of occupant sensor ventilation control devices §120.1(c)5.  These are mandated for multipurpose rooms less than 1000 ft² ; classrooms over 750 ft²; and 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(by reference to §120.2(e)3).  They are also an alternate method of compliance for spaces mandated to have DCV that are less than 1,500 ft² (Exception 5 to 120.1(c)3). 

There are a few spaces where it appears that both DCV and occupant sensor ventilation controls are mandated (e.g. auditoriums greater than 750 ft²).  Exception 1 to §120.1(c)5 exempts occupant sensor ventilation controls if DCV is implemented as required by §120.1(c)4.

Where occupant sensor ventilation control devices are employed (whether mandated or not) the controls must meet all of the following requirements:

A.         Sensors must meet the requirements of §110.9(b)4 and shall have suitable coverage to detect occupants in the entire space.

B.         Sensors that are used for lighting can be used for ventilation as well as long as the ventilation system is controlled directly from the occupant sensor and is not subject to lighting overrides.

C.        If a terminal unit serves several enclosed spaces, each space shall have its own occupant sensor and all sensors must indicate lack of occupancy before the zone airflow is cut off.

D.        The occupant sensor override shall be disabled during preoccupancy purge (i.e. the terminal unit and central ventilation shall be active regardless of occupant status).

E.         Supply fans on systems with all zones provided with occupant sensor ventilation control devices can cycle off if all zones are vacant provided that minimum ventilation to all zones is provided as follows:

F.         For spaces with a design occupant density greater than or equal to 25 people per 1000 ft² (40 square foot or less per person); 25 percent of the rate 'listed in Table 120.1-A: Minimum Ventilation Rates.

To implement the last provision the supply fan on the unit serving the zones would have to cycle on for at least 15 minutes of every hour with the outside air damper at or above minimum position.

Section 4.3.7 describes mandated acceptance test requirements for DCV and occupant sensor ventilation control systems.

Fan cycling per §120.1(c)5E

While §120.1(c)1 requires that ventilation be continuous during normally occupied hours when the space is usually occupied, Exception No. 2 allows the ventilation to be disrupted for not more than 30 minutes at a time. In this case the ventilation rate during the time the system is ventilating must be increased so the average rate over the hour is equal to the required rate.

It’s important to review any related ventilation and fan cycling requirements in Title 8, which is the Division of Occupational Safety and Health (Cal/OSHA) regulations.  Section 5142 specifies the operational requirements related to HVAC minimum ventilation.  It states:

Operation:

1.     The HVAC system shall be maintained and operated to provide at least the quantity of outdoor air required by the State Building Standards Code, Title 24, Part 2, California Administrative Code, in effect at the time the building permit was issued.

2.     The HVAC system shall be operated continuously during working hours except:

A.   During scheduled maintenance and emergency repairs;

B.   During periods not exceeding a total of 90 hours per calendar year when a serving electric utility by contractual arrangement requests its customers to decrease electrical power demand; or

C.   During periods for which the employer can demonstrate that the quantity of outdoor air supplied by nonmechanical means meets the outdoor air supply rate required by (a)(1) of this Section. The employer must have available a record of calculations and/or measurements substantiating that the required outdoor air supply rate is satisfied by infiltration and/or by a nonmechanically driven outdoor air supply system.

Title 8 Section 5142(a)(1) refers to Title 24, Part 2 (the California Building Code) for the minimum ventilation requirements.  Section 1203 in the California Building Code specifies the ventilation requirements, but simply refers to the California Mechanical Code, which is Title 24, Part 4.

Chapter 4 the California Mechanical Code specifies the ventilation requirements.  Section 402.3 states, “The system shall operate so that all rooms and spaces are continuously provided with the required ventilation rate while occupied.”  Section 403.5.1 states, “Ventilation systems shall be designed to be capable of providing the required ventilation rates in the breathing zone whenever the zones served by the system are occupied, including all full and part-load conditions.”  The required ventilation rates are thus not required whenever the zones are unoccupied.  This section affirms that ventilation fans may be turned off during unoccupied periods.  'In 'addition, Section 403.6 states, “The system shall be permitted to be designed to vary the design outdoor air intake flow or the space or zone airflow as operating conditions change.”  This provides further validation to fan cycling as operating conditions change between occupied and unoccupied.  A vacant zone has no workers present and is thus not subject to working hour’s requirements until the zone is actually occupied by a worker.  Finally, Table 4-1 in Title 24, Part 4, states; “Ventilation air supply requirements for occupancies regulated by the California Energy Commission are found in the California Energy Code.”  Thus, it refers to Title 24, Part 6 as the authority on ventilation.

Title 8 Section 5142(a)(2) states, “The HVAC system shall be operated continuously during working hours.”  This regulation does not indicate that the airflow, cooling, or heating needs to be continuous.  If the HVAC system is designed to maintain average ventilation with a fan cycling algorithm, and is active in that mode, providing average ventilation air as required during working hours, it is considered to be operating continuously per its mode and sequence.  During unoccupied periods, the HVAC system is turned off except for setback and it no longer operates continuously.  During the occupied period, occupant sensors or CO2 sensors in the space provide continuous monitoring and the sequence is operating, cycling the fan and dampers as needed to maintain the ventilation during the occupied period.  The HVAC system is operating with the purpose of providing ventilation, heating, and cooling continuously during the working hours.  The heater, air conditioner, fans, and dampers all cycle on and off subject to their system controls to meet the requirements during the working hours.

Exceptions A, B, and C to Title 8 Section 5142(a)(2) all refer to a complete system shutdown where the required ventilation is not maintained.

4.3.8    Fan Cycling    

While §120.1(c)1 requires that ventilation be continuous during normally occupied hours, Exception No. 2 allows the ventilation to be disrupted for not more than 30 minutes at a time. In this case the ventilation rate during the time the system is ventilating must be increased so the average rate over the hour is equal to the required rate.

This restriction limits the duty cycling of fans by energy management systems to not more than 30 minutes at a time. 'In 'addition, when a space-conditioning system that also provides ventilation is controlled by a thermostat incorporating a fan “On/Auto” switch, the switch should be set to the “On” position. Otherwise, during mild conditions, the fan may be off the majority of the time.

4.3.9    Variable Air Volume (VAV) Changeover Systems    

Some VAV systems provide conditioned supply air, either heated or cooled, through a single set of ducting. These systems are called VAV changeover systems or, perhaps more commonly, variable volume and temperature (VVT™) systems, named after a control system distributed by Carrier Corp. In the event that heating is needed in some spaces at the same time that cooling is needed in others, the system must alternate between supplying heated and cooled air. When the supply air is heated, for example, the spaces requiring cooling are isolated (cut off) by the VAV dampers and must wait until the system switches back to cooling mode. In the meantime, they are generally not supplied with ventilation air.

Systems of this type may not meet the ventilation requirements if improperly applied. Where changeover systems span multiple orientations the designer must make control provisions to ensure that no zone is shut off for more than 30 at a time and that ventilation rates are increased during the remaining time to compensate. Alternatively, minimum damper position or airflow setpoints can be set for each zone to maintain supply air rates, but this can result in temperature control problems since warm air will be supplied to spaces that require cooling, and vice versa. Changeover systems that are applied to a common building orientation (e.g., all east or all interior) are generally the most successful since zones will usually have similar loads, allowing minimum airflow rates to be maintained without causing temperature control problems.

4.3.10  Adjustment of Ventilation Rate    

§120.1(b) specifies the minimum required outdoor ventilation rate, but does not restrict the maximum. However, if the designer elects to have the space-conditioning system operate at a ventilation rate higher than the rate required by the Standards, then the Standards require that the space-conditioning system must be adjustable so that in the future the ventilation rate can be reduced to the amount required by the Standards or the rate required for make-up of exhaust systems that are required for a process, for control of odors, or for the removal of contaminants within the space §120.1(e).

In other words, a system can be designed to supply higher than minimum outside air volumes provided dampers or fan speed can be adjusted to allow no more than the minimum volume if, at a later time, someone decides it is desirable. The Standards preclude a system designed for 100 percent outdoor air, with no provision for any return air, unless the supply air quantity can be adjusted to be equal to the designed minimum outdoor air volume. The intent is to prevent systems from being designed that will permanently over-ventilate spaces.

4.3.11  Miscellaneous Dampers

Dampers should not be installed on combustion air intakes, or where prohibited by other provisions of law §120.2(f) Exception Nos. 3 & 4. If the designer elects to install dampers on shaft vents to help control stack-induced infiltration, the damper should be motorized and controlled to open in accordance with applicable fire codes.

4.3.12  Acceptance Requirements

The Standards have acceptance test requirements for:

•      Ventilation quantities at design airflow for constant volume systems §120.5(a)1 and NA7.5.1.2.

•      Ventilation quantities at design and minimum airflow for VAV systems §120.5(a)1 and NA7.5.1.1.

•      Ventilation system time controls §120.5(a)2 and NA7.5.2.

•      Demand controlled ventilation systems §120.5(a)5 and NA7.5.5.

These test requirements are described in Chapter 12 and the Reference Nonresidential Appendix NA7.5. They are described in brief in the following paragraphs.

A.   Ventilation Airflow

Ventilation airflow has to be certified to be measured within 10 percent of the design airflow quantities at two points of operation: full design supply airflow (all systems) and (for VAV systems) at airflow with all VAV boxes at or near minimum position.

If airflow monitoring stations are provided, they can be used for these measurements.

B.   Ventilation System Time Controls and Preoccupancy Purge

Programming for preoccupancy purge and HVAC schedules are checked and certified as part of the acceptance requirements. The sequences are also required to be identified by specification section paragraph number (or drawing sheet number) in the compliance forms.

C.   Demand Controlled Ventilation System

Demand controlled ventilation systems are checked for compliance with sensor location, calibration (either factory certificate or field validation) and tested for system response with both a high signal (produced by a certified calibration test gas applied to the sensor) and low signal (by increasing the setpoint above the ambient level). A certificate of acceptance must be provided to the enforcement agency that the demand control ventilation system meets the Acceptance Requirements for Code Compliance. The certificate of acceptance must include certification from the manufacturers of sensor devices that they will meet the requirements of §120.1(c)4F and that they will provide a signal that indicates the CO₂ level in the range required by §120.1(c)4, certification from the controls manufacturer that they respond to the type of signal that the installed sensors supply and that they can be calibrated to the CO₂ levels specified in §120.1(c)4, and that the CO₂ sensors have an accuracy of within plus or minus 75 ppm at 600 and 1,000 ppm concentrations, and require calibration no more frequently than once every 5 years.