ATU Boxes: Energy Efficiency, Air Leakage and ASHRAE 90.1

Rev: 04/21/2012

by John Gierzak, P.E.

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

Inlet valve

Primary Air

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)

Single-Duct

AHU air

Energy Savings: 1) Reduce ATU motor electrical requirements 2) Reduce amount of AHU air leakage

Dual-Duct

ATU Box Types

Cold deck

Hot deck

Mixing Downstream

Cool Primary Air Warm Plenum Air

Series Plenum air

AHU air

Parallel

Fan ON all the time Fan ON in heating

ATU Motor Types - PSC

AC power in

550 min rpm (vertical)

400 min rpm (horizontal) Sleeve bearing

6-pole = 1075 rpm max

ATU Fan Curve (PSC motor)

ATU Motor Types - ECM

AC power in

Inverter and electronics

Brushless DC motor

Ball bearings

200 min rpm

ECMs are programmed

1400+ max rpm

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.

43 AA21 CFMRPMARPMAT

ATU Fan Curve (ECM)

2 VDC

(0%)

10 VDC

(100%)

000

100

ECM Factory Set Airflow

000 %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 100

So how efficient are ECMs?

Energy Usage - PSC vs. ECM

Measure Watts vs. CFM for Air Terminal Unit

Outlet Ps

Average Watts/CFM 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

Test VAV Energy Usage (PSC vs ECM)

Watts/cfm vs. Outlet Ps

0.25

0.25

ATUs with ECM – Bottom Line

•Series boxes best application (Fan ON all the time) •Select ATU where

cfmdesign < 75% of ATU cfmmax

ATU Leakage

ATU Leakage – Single and Dual-Duct

Casing Leakage

Valve Leakage

ATU Leakage – Series Box

ATU Leakage – Parallel Box

Backdraft Damper

Leakage (shut)

Casing Leakage

Inlet Valve

Leakage

Leakage of Parallel ATUs was

Significant

Up to 30%!

ATUs – ASHRAE Research

Forced Manufacturers to Design Better Box

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 – Now

•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

ASHRAE 90.1 - Future

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