When Do Variable Flow Fume Hoods Save
Energy? Implications for lab design and behavior
modification
Michael Gevelber, Robert Choate, Kevin Sheehan, Brian Lo
Boston University, Mechanical Engineering
Outline
•Why all the fuss about fume
hoods & energy?
•What is the conventional
wisdom to save energy?
•When won’t a VAV fume
hood save energy?
•Analysis of several BU lab buildings
•Pay back analysis
•Monitoring & behavior modification
How much energy/$ does a fume hood use?
LBL Fume Hood Energy Model: Provides basis to use local
climate, energy rates, efficiency to calc $/cfm & load costs
http://fumehoodcalculator.lbl.gov/index.php
• For $3.38/cfm, a 1000cfm hood costs $3,380/year
• Analysis by Mills and Sartor, LBL 2005 paper
National Impact of Fume Hoods
• Mills and Sartor 2006 paper
• Significant energy use in US: ~750,000 hoods
$4.2 billion/yr, 5100 MW of electric demand 26 TWh of
electricity & 193 X 1012 BTU/yr
• Solutions
– New designs
– What to do about existing hoods?
• Variable  close the sash
• Constant volume  should they be changed
to variable?
Question: When will these solutions for existing
hoods really pay off? Should we use variable for
new hoods?
What Does University Websites Say to Do?
Johns Hopkins
• A fume hood consumes 3.5 times the amount
of energy consumed by the average house
• One simple action can make the difference
between a wasteful lab and a responsible
lab: CLOSE THE SASH!
Stanford University
• Fume hoods are big energy hogs because
they use so much conditioned air. When the
fume hoods are not in use but left open, a
tremendous amount of energy is wasted by the
conditioned air flowing through the hoods and
out of the building.
Cal Tech
• Shut your sash! - A variable volume fume
hood is 60% more energy effective when the
sash is down when not in use
• One fume hood uses as much energy as 3
typical American homes
University of Notre Dame
• Keeping just one variable air volume hood
closed when not in use rather than leaving it
open all the time saves $1,000 a year and is
equivalent to taking 3 cars off the road.
Pennsylvania State University
• With several hundred fume hoods at Penn
State, we could save $250,000 to $500,000 in
energy costs if the sashes are closed when
the fume hoods are not being used.
Is it as simple as it seems?
No! Need to view the hood as part of the room’s HVAC system.
Variable Volume Fume Hoods
• System maintains constant face velocity, varies
flow rate
• High capital cost but lower operating costs
– IF the sash is closed
• Easy, inexpensive modification with
large return
– Reduce face velocity to 100 ft/min
– Education and monitoring program
Photo Credit: oregonstate.edu/vent/bypass
Constant Volume Fume Hoods
• “Constant” air flow regardless of sash height
• Lower capital cost, higher operating cost
• More challenging and expensive retrofit: need
a variable speed [ventilation fans/motors]
– Rebalance air system
– Reduce flow rate when closed
using a low position sensor
– Minimize face velocity to 100 ft/min
Photo Credit:
www.research.northwestern.edu/ors/labsafe/hoods
BU Fume Hood Background
• 385 fume hoods on Charles River campus
– $1.5 million (~$4K per hood) in fume hood energy
use costs
– 34% variable
66% constant
– Several new lab buildings and rehab, done with
phoenix valves, and all variable fume hoods.
What should we do to reduce waste?
- launched study to investigate cost of
converting constant to variable
Hood/Room Exhaust Savings Analysis
Q hood
Hood
Exhaust
Q
gen
• Total exhaust includes:
Q hood+ Q gen
• Min total exhaust based
on room ACH
• Lab safety requirements:
6<ACH<12
If
,
No Savings
Hood/Room Exhaust Savings
Savings per Hood
900
Savings Depend on:
• ACH spec: 6 – 10
500
• Hood min/ max flow
• User behavior
• Need to redo analysis for
different size hoods
Savings per hood CFM
• No. of Hoods in room
700
– UnOcc ACH reduction (4)
ACH 6
800
ACH 8
600
400
ACH 10
63%-88% of
“expected savings”
300
30%-70% of “expected
savings”
200
100
0
1
2
3
4
5
6
Number of hoods
7
8
Assumptions:1000 sq ft room
1000 cfm hoods
What Are Payback Implications?
• What is relative cost savings if switch/design
continuous to variable hoods?
# Hoods/Room
“Expected” Yearly Savings
Actual Savings
∆Cost of VAV
“Expected” Payback
Actual Payback
2 Hoods
$4,000
$1,600
$20,000
5 years
12.5 years
3 Hoods
$6,000
$3,600
$30,000
5 years
8.3 years
• Assumes: 1000cfm Hood, $4/cfm, $10,000 per hood
Closed 50% of time
Spreadsheet calculator for actual
fume hood savings
Can extend to actual savings by using $/cfm
A Look at Fume Hoods
in 3 Lab Buildings at BU
Photonics
8 St. Mary’s St
• 290,000 ft2
• 32 Fume Hoods
Life Sciences
24 Cummington
• 148,000 ft2
• 79 VAV Hoods
• 10 CV Hoods
Science Center
590 Comm
• 284,000 ft2
• 169 Fume Hoods
Hood Density Per Room
No Savings
Photonics
24 Cummington
20
25
No Savings
18
Number of Rooms
Number of Rooms
16
14
12
10
8
6
4
2
20
15
10
5
0
0
1
2
3
4
5
6
7
8
10
1
2
3
4
Fume hoods Per Room
Fume Hoods Per Room
Implications
590 Commonwealth
No Savings
In many cases, if just 1 fume hood
in room, should be constant
volume & used as part of room
exhaust.
40
Number of Rooms
35
30
25
20
15
–what ACH is required for safety?
10
5
0
1
2
4
6
Fume Hoods Per Room
8
10
–how implement safe unoccupied
mode for room  down to 4 ACH
Fume Hoods Size (cfm) Distribution
Photonics
30
14
25
12
Numer of Fume Hoods
Number of Fume Hoods
24 Cummington
20
15
10
5
8
6
4
2
0
0
400
600
800
1000
1200
1400
1600
1800
Fume Hoods Size (cfm)
60
50
40
30
20
10
0
600
800
1000
Fume Hood Size (cfm)
More
400
600
800
1000
1200
• Red indicates one hood
per room
70
400
2000
Fume Hoods Size
590 Commonwealth
Number of Fume Hoods
10
2000
More
Fume Hood Position Monitoring System For
Behavior Modification
• Use Building Automation System to track which hoods are
left open
– Report on monthly basis to lab managers, staff, &
professors
– Need a champion in building, track only hoods that
matter
• Simple iterative algorithm
• Lots of people will take your $ to provide a fancy system,
but the above is easy to implement
Trial Monitoring Results
1 Day Dynamic Results
Average over 70 days
120
828.1
100
80
828.4
848.3
724.2
60
828.2
40
20
0
12 AM
848.1
828.5
724.1
AVG Percent Open
Fume Hood Open Percent
120
100
80
60
50%
40
20
0
4 AM
8 AM
12 PM
4 PM
8 PM
•828.1, 828.4 never close their hoods
•724.2 seems to be open at 70% 24/7
828.1
828.2
828.4
828.5
848.1
848.3
724.1
724.2
Fume Hood
•724.1, 848.5, 848.1 doing a decent Job
•848.3 Modulates, but why not close more?
Summary
• Hoods are energy hogs, BUT
we need to look how they function as part of
room and building HVAC systems to figure out
potential savings
• Savings for VAV Hoods likely to be less for rooms
with only 2-3 hoods in them, maybe none for 1.
• For those rooms where there’s an impact,
consider Fume Hood monitoring, large variation
of behavior & changes term by term
Comparative Analysis of Two Size Hoods
Q_hex_max = 800cfm,
Q_hex_min = 200cfm
Q_hex_max = 1200cfm,
Q_hex_min = 200cfm
Average Over 70 day
120
Avg Percent Open
100
80
60
40
20
0
828.1
828.2
828.4
828.5
848.1
Fume Hood
848.3
724.1
724.2
1 day Dynamic Results
120
Fume Hood Open Percent
100
80
60
40
20
0
12 AM
4 AM
8 AM
12 PM
Time
4 PM
8 PM