VAV Boxes: Energy Efficiency, Air Leakage and ASHRAE 90.1 - NEBB

ATU Boxes: Energy Efficiency,
Air Leakage and ASHRAE 90.1
by John Gierzak, P.E.
Rev: 04/21/2012

Goals Today
• ATU Terminology
• ATU Types
• ATU Motors
• ATU w/ECM Selection Sweet Spot
• ATU Box Leakage – Casing/Valve
• ASHRAE 90.1 Requirements
• AHRI Response

ATU Terminology
• ATU = air terminal unit
• VAV Box = ATU
• ATU inlet valve = damper
• Primary air = air from AHU that enters through inlet valve
• Plenum air = air from ceiling space that enters ATU through
induction port (fan powered boxes)
• Inlet flow sensor + pressure transducer + controller =
knowing flow rate through primary

ATU Terminology
• ATU inlet valve = damper
• Primary air = air from AHU that enters through inlet valve
Primary Air
Inlet valve

ATU Box Types
Non Fan Powered
• Single-duct (controls cfm)
• Dual-duct (controls cfm and temperature)
Fan powered
• Series boxes (on all the time)
• Parallel boxes (fan on during heating demand)

ATU Box Types
Single-Duct
Dual-Duct
AHU air
Mixing Downstream
Cold deck
Hot deck
Series
Parallel
Plenum air
AHU air
Cool Primary Air
Warm Plenum Air
Fan ON all the time
Fan ON in heating
Energy Savings:
1) Reduce ATU motor electrical requirements
2) Reduce amount of AHU air leakage

ATU Motor Types - PSC
6-pole = 1075 rpm max
AC power in
550 min rpm (vertical)
400 min rpm (horizontal)
Sleeve bearing

ATU Fan Curve (PSC motor)

ATU Motor Types - ECM
Ball bearings
AC power in
Inverter and electronics
Brushless DC motor
200 min rpm
1400+ max rpm
ECMs are programmed

Programming ECMs
•Laboratory testing.
•ECM is “taught” relationship between rpm,
torque, airflow and external static pressure.
•ECM is programmed.
•ECM is specific to the ATU.
T A
A 3 A 4
1RPM
A2
RPM
CFM

ATU Fan Curve (ECM)
10 VDC
(100%)
100
2 VDC
(0%)
000

ECM Factory Set Airflow
000
100
%PWM
Flow Rate
(cfm)
0 650
10 825
20 1000
30 1175
40 1350
50 1525
60 1700
70 1875
80 2050
90 2225
100 2400

So how efficient are ECMs

Energy Usage - PSC vs. ECM
Measure Watts vs. CFM for Air Terminal Unit

Test VAV Energy Usage (PSC vs ECM)
Watts/cfm vs. Outlet Ps
Average Watts/CFM
Outlet Ps
PSC
ECM
0.1 0.44 0.09
0.2 0.44 0.17
0.3 0.45 0.20
0.4 0.43 0.22
0.5 0.44 0.25
0.6 0.43 0.27
Average: 0.44 0.20

0.25

0.25

ATUs with ECM – Bottom Line
•Series boxes best application (Fan ON all the time)
•Select ATU where
cfm design < 75% of ATU cfm max

ATU Leakage

ATU Leakage – Single and Dual-Duct
Casing Leakage
Valve Leakage

ATU Leakage – Series Box

ATU Leakage – Parallel Box
Inlet Valve
Leakage
Casing Leakage
Backdraft Damper
Leakage (shut)

ATUs – ASHRAE Research
Leakage of Parallel ATUs was
Significant
Up to 30%!
Forced Manufacturers to Design Better Box

ASHRAE 90.1 – Now
2010 Addendum cq
•Ductwork and plenums = Seal Class A
•Round and rectangular duct: Leakage Class 4
•Test 25% of all duct over 3 in.wg.
•Test 25% of outdoor duct

ASHRAE 90.1 - Future
•Single and Dual-duct VAV box leakage requirements
•Efficiency requirements for fractional hp motors

ASHRAE 90.1 - Future
•Single and Dual-duct VAV box leakage requirements

AHRI Certification – ATU Leakage
AHRI requires certified leakage ratings
•Casing leakage for single-duct ATUs at 0.5 in.wg.
•Damper leakage for single-duct ATUs at 1 in.wg.
•Casing leakage for dual-duct ATUs at 1 in.wg.
•Damper leakage for dual-duct ATUs at 6 in.wg.

Summary
• ATU w/ECM efficiency far exceeds standard ATU boxes
• Series ATU best suited for ECM option
• Select ATU w/ECM at up to 75% box capacity
• Pre-2010 Parallel box backdraft damper leakage is significant!
• ASHRAE 90.1
• Does not cover fractional hp motors
• Will have single and dual-duct casing and inlet valve leakage limits
• AHRI
• Added casing and valve leakage to the VAV Certification program

For a copy of PowerPoint:
www.oriflow.com/nebb.htm