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
•
•

VAV Box control logic
•
•
•
•

Sizing VAV Boxes
How low can you go?
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
•
•
•
•
•
•
•

TP  SP  VP
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
RESULT: Size VAV Boxes for ~0.5” ∆TP
VAV Box Sizing: Sample Calc.
Outlet Outlet
Max Radiated
Inlet
∆SP
∆VP
∆TP
width height
CFM
NC*
dia.
(in.
(in.
(in.
(in.)
(in.)
(in.)
w.g.)*
w.g.)
w.g.)
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
Nominal
size
What is the lowest minimum?
•
Function of
–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
FPM min
VPmin
 4005
F
CFM min
 D 2 

 FPM min 
 4 
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
Catalog?
Still too high
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
30
Nailor VAV Box
“A”
Titus VAV Box
“B”
Flow Error [cfm]
20
10
0
-10
-20
-30
-40
0
100
200
300
400
500
Reference Flow [cfm]
Highly accurate down to about 50 CFM (0.003”)
600
700
800
Flow Probe Amplification
K
10000
7000
3665
2806
C
F
M
@
2062
1432
1160
916
22 inch
1000
I
n
l
e
t
C
F
M
100
S
i
z
e
702
16 inch
14 inch
515
12 inch
358
10 inch
9 inch
8 inch
229
7 inch
6 inch
I
n
c
h
S
i
g
n
a
l
 4005 A 
F 

 K 
2
5 inch
F = amplification factor
K = actual flow in CFM at flow probe
output of 1.0” w.c.
A = is the nominal inlet area in ft2
4 inch
10
0.01
O
n
e
0.1
Flow Probe Velocity Pressure Signal (Inches W.G.)
1
Typical Flow Probe Performance
2000
V e lo c ity [fp m ]
1000
500
200
100
50
.001
.005 .01
.05
Flow Grid Pressure [iwc]
F  slope ~ 2 . 6
.1
.5
1
Specify ≥2.0
Sample Controllable Minimum
VPmin
FPM min  4005
F
 D 2 

CFM min  FPM min 
 4 
Box Inlet Diameter
6
8
10
12
14
16
0.004
170  4005
2.3
Minimum Maximum CFM at 0.004 Minimum Ratio Minimum Ratio CFM at 0.5 at lowest at Highest in.w.g. sensor in.w.g. Maximum, %
Maximum, %
reading
pressure drop
425
33
7.8%
‐
715
58
8.1%
13.6%
1,100
91
8.3%
12.7%
1,560
130
8.3%
11.8%
2,130
177
8.3%
11.3%
2,730
232
8.5%
10.9%
VAV Reheat Control
Q: Why do some buildings use 3 -5
times as much energy as others?
Boiler Input (Btu/sf)
2,500
Site #1
Site #2
2,000
1,500
1,000
500
0
Mar-02
A: Reheat
Apr-02
May-02
Jun-02
Jul-02
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:
MARK
VR‐101
VR‐102
VR‐103
INLET SIZE
12
10
6
COOL
1035
810
210
DESIGN CFM
MIN HEAT
135
260
90
230
50
50
 Minimum
 Limit stratification - ≤20°F SAT above space temperature (≤~90°F to 95°F)
2. No more than 50% of cooling maximum

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
0.15
Cool Season All Occupied Hours
Low Minimum
30% Minimum
Low Minimum
30% Minimum
0.00
0.00
0.05
0.05
Density
Density
0.10
0.10
0.15
Warm Season All Occupied Hours
0
20
40
60
Flow Fraction [%]
80
100
0
20
40
60
Flow Fraction [%]
80
100
Total Electricity Usage
Energy Savings
Occupant Comfort Survey

Questions
•
When the logic changed from conventional
(High minimum) to Dual Maximum (Low
minimum):



Approach
•
3 surveys:


•
•
•

Did occupants become less comfortable?
Was their sense of air movement stronger due to
“dumping”?
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
% dissatisfied of people
30.0%
LOW min flow rate
249
25.0%
681
20.0%
15.0%
463
1408
10.0%
766
1793
5.0%
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/FT2 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
85
Low flow: average Tair = 73.3ºF
HIGH minimum flow rate
LOW minimum flow rate
75
70
Dates
10/03
10/04
51
74
1.5
45
74
1.1
64
73
1.9
25
74
1.1
37
74
1.4
67
75
1.3
27
75
1.2
48
73
2.1
10/20
09/30
30
73
1.6
10/19
09/29
23
72
0.89
10/18
09/28
65
71
2
10/17
09/27
34
71
1.5
10/14
47
70
1.6
10/13
41
71
1.8
10/12
35
71
1.4
10/11
36
71
1.5
10/10
45
72
1.6
10/07
35
71
1.5
10/06
23
71
1.8
10/05
27
72
2
09/26
60
N
14
Mean 72
sd
1.4
09/23
65
09/22
zone temperature [F]
80
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
[email protected]