Dual Maximum VAV Box Controls

ASHRAE Golden Gate Chapter November 13, 2014 Dual Maximum VAV Box Controls Steven T. Taylor, PE FASHRAE Taylor Engineering LLC staylor@taylor engineering.com

Agenda VAV Box basics Sizing VAV Boxes How low can you go? VAV Box control logic Conventional VAV box control logic Dual Maximum control logic Non-complying dual maximum control logic Determining setpoints How well does Dual Maximum logic work? ASHRAE RP-1515 Results

VAV Box Sizing Oversized box Less pressure drop Less noise Higher box min more reheat, higher fan energy Cost more Undersized box Higher pressure drop More noise Lower box min less reheat, lower fan energy

VAV Box Sizing: LCC Analysis Compared TP from 0.3 to 0.8 Low rise office building Sensitivity Analysis Climate Loads operating schedules Utility rates Load calc (aggressive, conservative) 8 bit versus 10 bit A/D converter SAT (50 F-60 F), SAT reset, SP reset TP RESULT: Size VAV Boxes for ~0.5 TP SP VP

VAV Box Sizing: Sample Calc. Nominal size Inlet dia. (in.) Outlet width (in.) Outlet height (in.) SP (in. w.g.)* VP (in. w.g.) TP (in. w.g.) Max CFM Radiated NC* 4 4 12 8 0.08 0.42 0.50 230 21 5 5 12 8 0.15 0.35 0.50 333 20 6 6 12 8 0.24 0.25 0.49 425 21 7 7 12 10 0.25 0.25 0.50 580 20 8 8 12 10 0.33 0.17 0.50 675 22 9 9 14 13 0.27 0.23 0.50 930 17 10 10 14 13 0.32 0.18 0.50 1100 19 12 12 16 15 0.32 0.17 0.49 1560 19 14 14 20 18 0.31 0.19 0.50 2130 18 16 16 24 18 0.32 0.18 0.50 2730 22 *From selection software using ARI 885-95 and assuming inlet SP = 1.5 and outlet SP = 0.25

What is the lowest minimum? Function of FPM DDC Controller: What s the lowest controllable velocity pressure (VP) signal? DDC velocity pressure transducer accuracy A/D converter resolution (bits) VAV Box: Amplification factor, F Inlet size, D min 4005 VP F min CFM min FPM min D 4 2

Why Not Just Look in the VAV Box Catalog? Equates to ~0.03 minimum VP and about 30% of design CFM for typical box selections

Why Not Just Look in the VAV Box Still too high Catalog? Good Advice! NEVER use Box manufacturer s minimums!

Controllable VP Minimum Some manufacturers list the range in catalog Some you have to ask Most available from ASHRAE RP 1353 and PG&E research projects (see references) Almost all controllers ± 10% at 0.003 Specify allowable setpoint 0.004

Typical DDC Performance Flow Error [cfm] 30 20 10 0-10 -20-30 -40 A Nailor VAV Box B Titus VAV Box 0 100 200 300 400 500 600 700 800 Reference Flow [cfm] Highly accurate down to about 50 CFM (0.003 )

Flow Probe Amplification 10000 C F M 1000 100 10 I n l e t S i z e 22 inch 16 inch 14 inch 12 inch 10 inch 9 inch 8 inch 7 inch 6 inch 5 inch 4 inch 0.01 0.1 1 Flow Probe Velocity Pressure Signal (Inches W.G.) K 7000 3665 2806 2062 1432 1160 916 702 515 358 229 C F M @ O n e I n c h S i g n a l F 4005A K F = amplification factor K = actual flow in CFM at flow probe output of 1.0 w.c. A = is the nominal inlet area in ft 2 2

Typical Flow Probe Performance 2000 1000 Velocity [fpm ] 500 200 100 50.001.005.01.05 Flow Grid Pressure [iwc].1.5 1 F slope ~ 2.6 Specify 2.0

Sample Controllable Minimum FPM CFM min 4005 min FPM min VP F D 4 min 2 170 4005 0.004 2.3 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%

VAV Reheat Control

Q: Why do some buildings use 3-5 times as much energy as others? 2,500 Boiler Input (Btu/sf) 2,000 1,500 1,000 500 Site #1 Site #2 0 Mar-02 Apr-02 May-02 Jun-02 Jul-02 A: Reheat

Conventional VAV Control

Conventional VAV Box Minimum Setpoint No less than larger of: Minimum ventilation rate Per Title 24 Controller minimum Not an issue for conventional logic see discussion above Limit dumping Not an issue see RP-1515 results below Limit stratification No more than 20 F above space temperature (~ 95 F) per Standard 90.1 No more than larger of: 30% of cooling maximum Per Title 24/Standard 90.1. Now only allowed for non-ddc controls Minimum ventilation rate

Title 24 VAV Control Requirement for zones with DDC

Dual Maximum Control per Title 24 and Standard 90.1 for DDC

Dual Maximum Logic in Action Start heating, increase DAT setpoint At 50% Heat, start increasing airflow

Sort of Dual Maximum Control (Found in some configurable controllers) T-24

Sort of Dual Maximum Control (Found in some configurable controllers) Title 24

VAV Box Dual Maximum Control Setpoints per Title 24 and Standard 90.1 Minimum: 1. No less than larger of: Minimum ventilation rate for the zone Controller minimum 2. No more than 20% of cooling maximum Heating Maximum: 1. No less than larger of: Minimum Limit stratification - 20 F SAT above space temperature ( ~90 F to 95 F) 2. No more than 50% of cooling maximum MARK INLET DESIGN CFM SIZE COOL MIN HEAT VR 101 12 1035 135 260 VR 102 10 810 90 230 VR 103 6 210 50 50 Always use Option 1 above Do not use code maximum just because it s legal! Avoid using %-of-cooling-maximum setpoints boxes are usually oversized!

What about Standard 62.1 Multiple Spaces compliance? Oakland Office - VAV System with Outdoor Air Economizer

How Well Does Dual Max Logic Actually Work? RP-1515 results

RP 1515, "Thermal and Air Quality Acceptability in Buildings that Reduce Energy by Reducing Minimum Airflow from Overhead Diffusers" Objectives Measure energy savings & validate simulations Identify comfort issues that may occur at low flow Funding California Energy Commission - PIER ASHRAE UC Berkeley - Center for the Built Environment Research Team UC Berkeley Taylor Engineering Price Industries Method Field Study in 7 buildings Background survey Right now survey matched to zone trends Energy monitoring Laboratory Study Air distribution for various diffuser types

Yahoo! Sunnyvale Campus 1073 Zones 3700 Occupants Plaque face diffusers

800 Ferry Building 22 Zones Perforated Diffuser with blades in face

Measured flow fractions: Yahoo campus Warm Season All Occupied Hours Cool Season All Occupied Hours Density 0.00 0.05 0.10 0.15 Low Minimum 30% Minimum Density 0.00 0.05 0.10 0.15 Low Minimum 30% Minimum 0 20 40 60 80 100 0 20 40 60 80 100 Flow Fraction [%] Flow Fraction [%]

Total Electricity Usage

Energy Savings

Occupant Comfort Survey Questions When the logic changed from conventional (High minimum) to Dual Maximum (Low minimum): Did occupants become less comfortable? Was their sense of air movement stronger due to dumping? Approach 3 surveys: Yahoo warm and cool seasons 800 Ferry building warm season 3 4 weeks of surveying each season Switched between high and low minimum operation in the middle of each survey period Surveys administered 3 times/day About 10,000 responses received

"How satisfied are you with the temperature in your workspace? HIGH min flow rate LOW min flow rate 30.0% 249 % dissatisfied of people 25.0% 20.0% 15.0% 10.0% 5.0% 463 1408 1793 681 766 0.0% 800 Ferry Building Yahoo! Cool season Yahoo! Warm season % Dissatisfied people HIGH Conventional LOW Dual Maximum 800 Ferry Building 27.3% 12.5% Yahoo! cool season 8.7% 9.4% Yahoo! warm season 20.1% 10.3%

Thermal sensation distribution (Yahoo! warm season)

Loads are very low! 0.2 0.5 1 CFM/FT 2 at 20 F T 140 Zones, 2 buildings, 1 warm month (Sept)

What happens when load is less than airflow minimum? Conventional Logic Actual Required CFM Dual Max Logic

Air temperature (800 Ferry building) High flow: average Tair = 71.2ºF Low flow: average Tair = 73.3ºF 85 HIGH minimum flow rate LOW minimum flow rate 80 75 70 65 60 09/22 09/23 09/26 09/27 09/28 09/29 09/30 10/03 10/04 10/05 10/06 10/07 10/10 10/11 10/12 10/13 10/14 10/17 10/18 zone temperature [F] 10/19 10/20 N Mean sd 14 27 23 35 45 36 35 41 47 34 65 23 30 51 45 64 25 37 67 27 48 72 72 71 71 72 71 71 71 70 71 71 72 73 74 74 73 74 74 75 75 73 1.4 2 1.8 1.5 1.6 1.5 1.4 1.8 1.6 1.5 2 0.89 1.6 1.5 1.1 1.9 1.1 1.4 1.3 1.2 2.1 Dates

How about Drafts? (800 Ferry building)

Perceived air quality

CBE background survey results

Price Lab Tests Perforated diffusers with blades in the neck, Cooling Mode. Results: Negligible impact on ADPI all near 1 Negligible impact on ACE all near 1

Dual Maximum VAV Box Logic Summary Title 24 requires Dual Maximum logic for VAV zones with DDC Setpoints Set minimum and heating-maximum setpoints as low as possible Minimum allowed by controls is seldom a factor if box properly sized Do not use maximum allowed by T-24 just because it is legal Cost impact Requires discharge temperature sensor This is nice for diagnostics anyway Usually requires programmable zone controller Configurable controller programming should soon catch up Performance Thermal comfort improves with low flow operation Dumping & draft are not an issue at low flow Energy savings are significant and similar to simulation predictions

Bibliography Taylor S., Stein J.; Sizing VAV Boxes, ASHRAE Journal, March 2004 Dickerhoff D., Stein J.; Stability and Accuracy of VAV Terminal Units at Low Flow, PG&E 0514, http://www.etcc-ca.com/reports/stability-and-accuracyvav-terminal-units-low-flow Lui R., Wen J.; Stability and Accuracy of VAV Box Control at Low Flows, ASHRAE 1353-RP Taylor S. Dual Maximum VAV Box Logic, ASHRAE Journal, December 2012

Questions? Steve Taylor staylor@taylor-engineering.com