dual maximum vav box control logic - taylor engineering · the title 24/90.1 language shown above...

16 AS HRAE Jou rna l ash rae .o rg D e c e m b e r 2 0 1 2

Most variable air volume (VAV) systems use the same VAV box control

logic that was used when the system first became popular in the

1970s. But with modern direct digital controls (DDC), zone control logic can

be much more sophisticated and higher performing. This article discusses one

such strategy dubbed “dual maximum” VAV box control logic and shows

how this control logic improves energy efficiency and occupant comfort.

ASHRAE Standard 90.12 (Section 6.5.2.1.1) does not allow the sup-ply air temperature to exceed 20°F (11.1°C) above the space tempera-ture to avoid excessive stratification and short circuiting. To maintain a zone air distribution effectiveness of 1.0 for ceiling supply/return systems, ASHRAE Standard 62.1 requires that the supply air tempera-ture be no more than 15°F (8.3°C) above space temperature. So for typical overhead systems, the sup-ply air temperature is limited to 85°F to 90°F (29°C to 32°C), as-suming a 70°F (21°C) space heating setpoint temperature.

c. The lowest setpoint allowed by the VAV box controls. With pneumatic controls, the minimum setpoint

About the AuthorsSteven T. Taylor, P.E., and Jeff Stein, P.E., are principals, and Gwelen Paliaga and Hwakong Cheng, P.E., are senior engineers at Taylor Engi-neering in Alameda, Calif.

By Steven T. Taylor, P.E., Fellow ASHRAE; Jeff Stein, P.E., Member ASHRAE; Gwelen Paliaga, Member ASHRAE; and Hwakong Cheng, P.E., Member ASHRAE

Dual Maximum VAV Box Control Logic

Conventional VAV Box LogicVAV boxes with reheat coils were

traditionally controlled using the con-trol logic shown in Figure 1. The sup-ply airflow setpoint is reset from the zone maximum airflow setpoint when the space is at full cooling proportion-ally down the zone minimum when no cooling is required. This minimum air-flow rate is maintained as the space tem-perature falls through the deadband into heating mode. The hot water valve then opens (or electric heat stages or modu-

lates up) to maintain the space at the heating setpoint until it is fully open (or all electric heat stages are on). The mini-mum airflow setpoint with this logic is determined by the following:1. No less than the larger of:

a. The zone minimum outdoor air rate, e.g., that required by code or Standard 62.1.1

b. The amount of air required to heat the space at a supply air temperature appropriate for the application. For overhead supply and return systems,

This article was published in ASHRAE Journal, December 2012. Copyright 2012 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

Dec ember 2012 ASHRAE Jou rna l 17

is quite high. But with modern direct digital controls, the minimum can be very low, as discussed in more detail later.

2. No more than 30% of the cooling maximum airflow set-point per the prescriptive requirements of ASHRAE Standard 90.1-2010 (Section 6.5.2.1) and California’s Title 24-2010 Energy Standards3 for pneumatic controls. This fairly low setpoint significantly limits the use of VAV reheat terminals in zones with high heating loads, such as exterior zones with large windows in cold climates. For these zones, other systems that do not rely on the cooling airflow for heating must be used, such as fan-powered VAV boxes, radiant heat, or convectors.

This logic results in a large amount of reheat energy, shown in Figure 1 by the purple area. For a given supply air tempera-ture, the lower the minimum airflow setpoint, the lower the reheat energy, which is why the energy standards referenced above limit this value.

Dual Maximum VAV Box LogicThe previous conventional logic still applies to zones that

do not have DDC, but the 2008 version of Title 24 and an addendum proposed to ASHRAE Standard 90.14 require that VAV boxes with DDC provide what has been dubbed “dual maximum logic.” Here is the language from Title 24-2013 (the Standard 90.1 addendum has similar language):

A. For each zone with direct digital controls (DDC): i. The volume of primary air that is reheated, re-cooled, or mixed air supply shall not exceed the larger of: a. Fifty percent of the peak primary airflow; or b. The design zone outdoor airflow rate per Section 120.1

[which is the minimum ventilation section]. ii. The volume of primary air in the deadband shall not ex-ceed the larger of:

a. Twenty percent of the peak primary airflow; or b. The design zone outdoor airflow rate per Section 120.1.

iii. The first stage of heating consists of modulating the zone supply air temperature setpoint up to a maximum setpoint no higher than 95°F (35°C) while the airflow is maintained at the dead band flow rate. iv. The second stage of heating consists of modulating the airflow rate from the dead band flow rate up to the heating maximum flow rate. The logic this language requires is shown in Figure 2. The

term “dual maximum logic” comes from the fact that there are now two maximum airflow setpoints: one for heating in addi-tion to the one for cooling.

With dual maximum logic, the minimum airflow setpoint is determined as follows:

1. No less than the larger of:a. The zone minimum outdoor air rate. Meeting venti-

lation requirements with low minimums is discussed further below.

b. The lowest setpoint allowed by the VAV box controls. With modern DDC, the controllable minimum for most DDC manufacturers is usually not an issue be-cause it is normally below the ventilation requirement.

Minimum controllable setpoints are generally much lower than those published by VAV box manufactur-ers, which are based on very conservative assumptions regarding the capability of the digital controller that is seldom provided with the VAV box. For a detailed discussion of how to determine the lowest controllable setpoint for a VAV box and controller, see the Journal article “Sizing VAV Boxes.”5 Table 1 shows typical VAV box performance with a velocity pressure probe with a 2.3 amplification factor and a digital controller capable of controlling airflow to 0.004 in. w.g. (1 Pa) velocity pressure reading. The minimum controllable setpoint is about 8% of the design maximum airflow rate if the box is selected at 0.5 in. w.g. (125 Pa) pressure drop with a two-row hot water coil, up to about 12% if the design airflow rate is at a value just above the maximum of the next smallest box size. A reasonable rule-of-thumb is to assume the minimum can be no lower than 10% of the cooling maximum. Another approach is for designers to ignore this constraint in their VAV box schedules and rely on the controls vendor to determine if any sched-uled minimum setpoint is not possible and adjust the setpoint accordingly.

Figure 1: Conventional VAV reheat control diagram.

Figure 2: Dual maximum VAV reheat control diagram.

Reheat Valve Position

Minimum Airflow

Setpoint

Heating Loop Cooling LoopDeadBand

Airflow Setpoint

Maximum Airflow

Setpoint

Supply Air Temperature Setpoint (Requires

Discharge Temperature Sensor)

Minimum Airflow

Setpoint

Heating Loop Cooling LoopDeadBand

Airflow Setpoint

Maximum Heating Airflow

Setpoint

Maximum Supply

AirTemperature

Maximum Cooling Airflow Setpoint


35°C) for overhead supply/return systems as discussed earlier.

2. No more than 50% of the cooling maximum airflow set-point. This is a larger percentage than the 30% allowed using conventional logic which allows VAV reheat terminals to be used in more applications. This does result in more reheat at high heating loads, but the lower minimum airflow setpoint (a maximum of 20% vs. 30% with the conventional control logic) offsets this by reducing reheat at lower heating loads. Since most zones spend far more time at low heating loads than they do at high heating loads, the dual maximums approach will result in overall reheat savings.

To optimize the design, engineers should resist the shortcut to simply specify 20% minimum and 50% heating maximum setpoints. These are the highest setpoints allowed by the ener-gy standards and higher than necessary for most applications. Rather, designers should use these values as limits only as they are intended and determine the actual setpoints based on the ventilation requirement (minimum airflow setpoint) and heating requirements (heating maximum airflow setpoint).

To implement the dual maximum logic shown in Figure 2 will require the following controls:

A fully programmable DDC zone controller, or a con-figurable controller with the dual maximum logic already in-stalled. Configurable controllers have fixed control logic; only setpoints can be adjusted. Dual maximum logic is not new (the authors have specified it since the late 1990s) but it is not yet common enough that many configurable controllers include it. Rather, they use conventional logic or one of the two logic diagrams shown in Figure 3. Neither of these meets the re-quirements of the Title 24/90.1 language shown earlier based on simulations that showed they used more energy with a heat-ing maximum setpoint of 50% than conventional logic did at a 30% minimum. Prior to specifying or installing a VAV con-troller, the designer should verify that the controller is capable of providing the control logic shown in Figure 2 (Page 17).

A supply air temperature sensor. This is needed to ensure that the supply air temperature does not get too hot and result in stratification, as would likely occur when the zone is heat-ing at the low minimum airflow rate. As shown in Figure 2, the zone heating control loop does not modulate the hot water valve directly. Instead, cascading logic is used: the loop deter-mines the supply air temperature setpoint and then the hot wa-

ter valve is modulated to maintain that setpoint. The supply air temperature sensor (typically costing less than $100) required for this logic can also be used for diagnosing failed hot water control valves and other system faults.

For electric resistance heat, a modulating controller (e.g., silicon controlled rectifier) and an electronic airflow sensor capable of sensing very low airflow rates and limiting heater capacity according to the available airflow. Electric heat with step controls generally cannot meet the requirements of the Title 24/90.1 language shown above because of high mini-mum airflow rates required to close safety switches.

Figure 4 shows trend data from a VAV zone programmed with dual maximum logic. As the zone temperature drops be-low the heating setpoint (third row), the heating PID loop out-

Box Inlet Diameter

Maximum cfm at 0.5 in. w.g.

Pressure Drop

Minimum cfm at 0.004 in. w.g. Sensor Reading

Minimum Ratio at Highest Maximum

(%)

Minimum Ratio at Lowest Maximum

(%)

6 425 33 7.8% –

8 715 58 8.1% 13.6%

10 1,100 91 8.3% 12.7%

12 1,560 130 8.3% 11.8%

14 2,130 177 8.3% 11.3%

16 2,730 232 8.5% 10.9%

Table 1: Typical VAV box and controller performance.

Figure 3: Control diagrams for example configurable controllers.

2. No more than 20% of the cooling maximum airflow setpoint.

The heating maximum airflow set-point must be:

1. No less than the larger of:a. The minimum airflow rate deter-

mined above.b. The amount of air required to heat

the space at a supply air tempera-ture appropriate for the applica-tion or as limited by Title 24/90.1, typically 85°F to 95°F (29°C to


Heating Loop Cooling LoopDead

Band

Airflow Setpoint


Minimum Airflow

Setpoint


Setpoint


Minimum Airflow

Setpoint

Heating Loop Cooling LoopDead

Band

Airflow Setpoint


Setpoint


www.info.hotims.com/41646-1


put (Htg%) starts to increase (second row). As it does, the sup-ply air temperature setpoint is increased from 55°F (13°C) up to a maximum of 85°F (29°C) (first row). The hot water valve (second row) is modulated to maintain supply air temperature at setpoint (first row). The valve loop is not very stable because of the low airflow rate and the changing setpoint, but this is not perceptible to occupants in the space. (Note that conventional logic without the benefit of supply air temperature feedback would have opened the valve equal to the zone PID loop output value and heated the air well above the 85°F to 90°F (29°C to 32°C) needed to limit stratification.) At 50% heating PID loop output (Htg%), the primary airflow setpoint (“Cold Duct CFM”) starts to increase from the 400 cfm (189 L/s) mini-mum (20% in this case) up to the heating maximum of 1,000 cfm (472 L/s) (50%) before the zone temperature starts to rise above the heating setpoint and the loop output starts to fall.

In addition to ensuring that the supply air temperature is never too warm, another advantage of controlling the hot water valve off of supply air temperature is that the hot water system is self-balancing even during transients such as warm-up, presuming a two-way valve variable flow system is used in accordance with Section 6.5.4.1 of Standard 90.1. The control valves will never supply more than the design hot water flow due to the supply air temperature feedback without the need for balancing valves or balancing labor. With standard valve control off of space temper-ature, all valves go full wide open during warm-up, possibly re-sulting in flow imbalance unless the system is manually balanced.

Dual Maximum Performance: RP-1515Figure 5 shows data collected for a large office complex in

Sunnyvale, Calif., for ASHRAE Research Project 15156 where VAV boxes initially used conventional logic with 30% mini-mums for several months after which the logic was changed to

Figure 5: RP-1515 data for dual maximum logic vs. conven-tional logic (warm season data).

Frac

tion

of

Tim

e

0.15

0.10

0.05

0.00

0 20 40 60 80 100Percent of Design Airflow

Dual Maximum LogicConventional Logic

Figure 4: Trend data from a VAV zone programmed with dual maximum logic.

9080706050

10080604020

0

7372717069

2,0001,000

0

8:30 8:45 9:00 9:15 9:30 9:45 10:00 10:15 10:30 10:45

8:30 8:45 9:00 9:15 9:30 9:45 10:00 10:15 10:30 10:45

8:30 8:45 9:00 9:15 9:30 9:45 10:00 10:15 10:30 10:45

8:30 8:45 9:00 9:15 9:30 9:45 10:00 10:15 10:30 10:45

DATDAT Setpoint

Heating %Cooling %Reheat Valve

Cooling Setpoint Heating Setpoint Zone Temperature

Cold Duct cfm

Per

cent

Tem

per

atur

e (°

F)cf

mTe

mp

erat

ure

(°F)

dual maximum logic with minimums set to the California Title 24 minimum ventilation rate. Even in warm weather, most of the zones operate at very low loads and, therefore, low airflow rates.

The fact that zones are at minimum airflow even in warm weather also explains a surprising result from RP-1515: com-fort improved when dual maximum logic was installed, from 77% acceptance to 88% acceptance in one building. Surveys (Figure 6) show that the lower acceptance with conventional logic is primary due to cold complaints caused by the high minimums “pushing” the zone down to the heating setpoint (e.g., 70°F [21°C]) even in warm weather when occupants are likely wearing summer apparel. Per ASHRAE Standard 55,7 occupants are likely to find temperatures below about 74°F (23°C) too cool when wearing lightweight summer clothing.

Energy savings from dual maximum logic are from re-duced fan power, a small savings in mechanical cooling


reduced. RP-1515 will measure air change effectiveness at low airflow fractions in laboratory mockup with a single occupant and heating load produced by a controllable cold wall. The tests are incomplete at this time but we do not expect low airflow rates to significantly impact zone ventilation effectiveness in heating mode.

3. Meeting ventilation requirements. Complying with ventila-tion code in California is easy; the code simply requires that overall building outdoor air rates be maintained and that each space be supplied with the minimum ventilation rate without regard to the fraction of outdoor air being supplied—outdoor air and return air are equally acceptable. This contrasts with ASHRAE Standard 62.1 which requires a more sophisticated approach using the so-called “multiple spaces equations” that track dilution from multiple airflow paths for each space. Research projects such as RP-127610 have shown that the multiple spaces approach in Standard 62.1 is

Figure 6: RP-1515 comfort survey, “How do you rate your thermal sensation right now?”

Figure 7: RP-1515 fan and cooling power vs. OA temperature.energy, and reduced natural gas use due to reduced reheat energy. Figure 7 shows measured AC unit power (fan and DX cooling) for an RP-1515 building while Figure 8 shows weather normalized annual savings for five buildings devel-oped using correlations from the RP-1515 measurements and typical annual weather.

Potential Issues with Low Minimum Airflow RatesConcerns regarding the low minimum airflow rates inherent

with dual maximum logic include:1. Low Air Diffusion Performance Index (ADPI) and dif-

fuser dumping causing draft complaints. Bauman8 using an instrumented test chamber with segmented thermal manne-quins found that acceptable ADPI could be maintained at 25% of design flow. RP-1515 used a laboratory mockup similar to Bauman’s to test performance at low rates. Tests of perforated diffusers with blades in the neck show negligible difference in ADPI between 80% and 18% of design flow (all close or

Po

wer

Co

nsum

ptio

n (k

W)

200

150

100

50

0

30 40 50 60 70 80 90

Outside Air Temperature (°F)

Dual Maximum LogicConventional Logic

Figure 8: RP-1515 annual energy savings.

equal to 100% ADPI), more uniform temperature at lower flow, and lower air speeds in the occupied region at lower flow (Figure 9). RP-1515 field studies showed improved thermal comfort and no increase in draft complaints. These tests indicate that declining diffuser per-formance at low airflow rates is not an issue with respect to occupant comfort. Tests were not run below 18% of design flow due to test lab limitations but the researchers fully expect the conclusions also apply to flow rates as low as 10% of design flow.

2. Low ventilation effectiveness. Most studies have shown that supply of cold air from the ceiling results in fully mixed spaces (air change effectiveness ~ 1.0) regardless of diffuser selection and airflow rates. Fisk9 found that to be the case even at 25% flow in cooling mode and found that effectiveness actually increased as percentage airflow rate was

4.54.03.53.02.52.01.51.00.50.0

Ann

ual F

an &

Co

olin

g

Ele

ctri

city

Use

kW

h/f

t2

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Ann

ual H

eatin

g G

as U

se

Ther

ms/

ft2

Electricity Savings (Fan & Cooling)

Gas Savings (Reheat)

Building 1 Building 2 Building 3 Building 4 Building 5

Building 1 Building 2 Building 3 Building 4 Building 5

Conventional LogicDual Maximum Logic

60

50

40

30

20

10

0

Per

cent

of

Tota

l Vo

tes Conventional Logic

Dual Maximum Logic40%

18%

(–3)Cold

(–2)Cool

(–1)Slightly

Cool

(0)Neutral

(1)SlightlyWarm

(2)Warm

(3)Hot

Thermal Sensation

Dec ember 2012 ASHRAE Jou rna l 23

valid. California’s simplified approach probably “works” from an indoor air quality perspective because California rates are typically larger than Standard 62.1 rates, about 30% larger for typical offices, for example. Unfortunately, it is difficult to use the Standard 62.1 multiple spaces equations because there are myriad assumptions required and the results for both overall outdoor air rate and zone minimum airflow rates can vary significantly depending on these assumptions. Some designers are extremely conservative and as-sume, for instance, that all zones are at their design airflow except for the “critical” zone which is fully occupied yet somehow at its minimum airflow rate. A more realistic approach is to use simula-tions to more accurately track airflow rates delivered by the zone control logic relative to the requirements for ventilation.

Figure 10 shows the results of a DOE-2.2 simulation of a typical office building in Oakland, Calif., served by a VAV air handler with an outdoor air economizer. The y-axis is the ratio of the air handler outdoor air rate to that required by Standard 62.1 (Vot) using the multiple spaces equation.

Figure 11 shows the same results without an air economizer. The building model was extremely detailed. For instance, each room was provided with multiple realistic occupancy and inter-nal load schedules that varied from one zone to another, day to day, and week to week. Occupancy sensors were modeled so that lights were shut off in unoccupied spaces. Minimum airflow rates were modeled as the larger of the California code ventilation re-quirement for offices or the controllable VAV box minimums.

Conference rooms were modeled with CO2 sensors that would raise this minimum rate as required to provide 15 cfm/person (7 L/s per person), the California code occupant com-ponent ventilation requirement for offices. (CO2 demand con-trolled ventilation [DCV] is required for these densely occupied spaces by Title 24 and Standard 90.1.) The results of the hourly analysis were exported to a spreadsheet where every hour was tested for compliance with Standard 62.1 using the multiple spaces equation, including assuming a 0.8 zone air distribution effectiveness when the zones were in heating mode.

The results indicate that when the AHU has an outdoor air economizer, outdoor air rates met Standard 62.1 requirements for every hour and the average annual outdoor air supplied was 364% of the Standard 62.1 rate. However, without an economiz-er there were 151 hours (4% of the total occupied hours) that did not comply with Standard 62.1 but the average annual outdoor air rate was 168% larger than Standard 62.1 rates. To provide full Standard 62.1 compliance without an economizer, the AHU outdoor air rate would have to increase by about 5% above Title 24 minimum rates in this example. The results suggest the fol-lowing approach to ensuring ventilation rates are met:

• Minimum zone airflow rates should be no lower than the Title 24 building rate of 0.15 cfm/ft2 (0.76 L/s·m2).

• CO2 DCV should be used on all densely occupied spaces to allow the occupant ventilation rate component to be dy-namically determined.

• Outdoor air economizers should ensure Standard 62.1 compliance as well as improve energy performance and thus are strongly encouraged.

Another control option is to dynamically reset zone mini-mum airflow setpoints based on air-handling system outdoor air supply, e.g., reducing minimums when the AHU is sup-plying 100% outdoor air in economizer mode and increasing minimums as outdoor air percentage falls when not in econo-

Figure 9: RP-1515 laboratory measured air speed at 42 in. (1 m) above floor. Note: Diffuser located in center of 20 ft (6 m) wide room.

Air

Sp

eed

(fp

m)

45

40

35

30

25

20

15

10

5

010 8 6 4 2 1 3 5 7 9

Distance from Diffuser (ft)

80% Flow33% Flow18% Flow

Figure 10: Simulated outdoor air rate with economizer vs. Standard 62.1 using multiple spaces equations.

Rat

io o

f A

HU

OA

Rat

e to

Std

. 62.

1 R

equi

red

OA

Rat

e (S

td 6

2.1

V ot =

1.0

) 12

10

8

6

4

2

030 40 50 60 70 80 90 100


Figure 11: Simulated outdoor air rate without economizer vs. Standard 62.1 using multiple spaces equations.


Rat

io o

f A

HU

OA

Rat

e to

Std

. 62.

1 R

equi

red

OA

Rat

e (S

td 6

2.1

V ot =

1.0

) 12

10

8

6

4

2

030 40 50 60 70 80 90 100


mizer mode. This will be the subject of a future article.

ConclusionsThe benefits of dual maximum logic over conventional logic

include: • Lower fan energy, lower reheat energy, and lower cooling

energy use; • Improved thermal comfort by not pushing zone tempera-

ture to heating setpoints during the cooling season; • Reduced stratification due to supply air temperature con-

trol, and • Self-balancing of hot water systems with two-way valves.

Disadvantages include: • Added cost of a discharge temperature sensor (very minor

and also useful for diagnostics); • Added cost for more complex control programming (a

one-time cost for the control vendor); and • Difficulty in determining zone minimum airflow setpoints

and air handler minimum outdoor airflow setpoints that meet Standard 62.1 (not an issue under Title 24 ventilation code).

RP-1515 results show that low minimum airflow setpoints rates improve air distribution performance and occupant com-fort, results supported by the authors’ more than 10 years of successful experience using dual maximum logic in Califor-nia. To ensure Standard 62.1 compliance at low minimum rates, we recommend using system simulations (rather than conservative assumptions in spreadsheets) to determine zone

minimum setpoints along with CO2 DCV and outdoor air economizers.

References1. ANSI/ASHRAE Standard 62.1-2010, Ventilation for Acceptable

Indoor Air Quality.2. ANSI/ASHRAE Standard 90.1-2010, Energy Standard for Build-

ings Except Low-Rise Residential Buildings.3. Building Energy Efficiency Standards for Residential and Non-

residential Buildings. 2008. Title 24, California Code of Regulations, Part 6. California Energy Commission.

4. Standard 90.1-2010, Addendum ck.5. Taylor, S., J. Stein. 2004. “Sizing VAV boxes.” ASHRAE Journal

46(3).6. Center for the Built Environment, Taylor Engineering, Price

Industries. 2013. ASHRAE RP 1515, “Thermal and air quality accept-ability in buildings that reduce energy by reducing minimum airflow from overhead diffusers.” In progress; report expected January 2013.

7. ASHRAE Standard 55-2010, Thermal Environmental Conditions for Human Occupancy. 8. Bauman, F., C. Huizenga, T. Xu, T. Akimoto. 1995. “Thermal

Comfort With a Variable Air Volume (VAV) System.” Center for Environmental Design Research, University of California, Berkeley. 9. Fisk, W.J., D. Faulkner, D. Sullivan, F.S. Bauman. 1997. “Air

change effectiveness and pollutant removal efficiency during adverse conditions.” Indoor Air 7:55-63. 10. Yuill, D., G. Yuill, A. Coward. 2007. ASHRAE RP 1276: “A

Study of Multiple Space Effects on Ventilation System Efficiency in Standard 62.1-2004 and Experimental Validation of the Multiple Spaces Equation.”

dual maximum vav box control logic - taylor engineering · the title 24/90.1 language shown above...

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