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Why VAV?

Ryan R. Hoger, LEED AP

Single Zone Systems

ZC

ZONE 1

TMT TM TM

ZONE 2 ZONE 3 ZONE 4

ROOF

Heating/CoolingUnit

Heating/CoolingUnit

Heating/CoolingUnit

Heating/CoolingUnit

TM

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Multiple Zone Systems

Why VAV?

• Comfort– Individual comfort controls

• Load Diversity– Different parts of the building require different loads during

the day• Energy Savings

– Improved part load performance• Design Flexibility

– Allow building to change throughout it’s life• Partial Operation

– Ability to cool or heat portions of the building

• Disadvantages?

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VAV SystemSystemComponents:1. VAV RTU or AHU2. VAV Terminals

with Heating Coil3. Supply Diffuser4. Control Wiring5. Return Grille6. Zone Thermostat7. Supply Fan VFD8. Duct Static

Pressure Sensor9. Supply Duct

Zone 8 Zone 9 Zone 10

RTU or AHU

Variable Supply Constant Temperature Airflow

Return

g

Multiple Compression

Stages

T

Full-Face Active Cooling Coil

VAV unit operation is based on leaving air temperatureVFD Fan

ModulationSupply Air Temperature Sensor from Evaporator

Return Air Temperature

Reset

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Zone Layout

INTERIOR ZONE

PERIMETER ZONE

Zone Layout

TTT

TTT

TTT

N

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Zone Layout

VAV Rules• VAV is a cooling only system by definition

– 55 deg supply air needed year round to take care of interior cooling loads

– Do not expect AHU to provide heat during the day– Use AHU heat for morning warm-up sequence only– If high OA requirements, use AHU heat to bring discharge temp back up

to 55– Maybe be ok to reset cooling setpoint from 55 to 60 if zone average is

satisfied -never reset higher• Heat is handled at the zone level, as needed

– REPEAT: Do not expect AHU to provide heat during the day– Baseboard, radiant, or overhead (inside VAV box)– If heating from overhead, stay under 90 deg discharge or 25% extra OA

required per ASHRAE 62.1

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VAV Rules (con’t)• Modulating cooling needed

– Unlike constant volume, cooling capacity control for VAV is based on supply air temp

– Need chilled water or variable speed compressors• Or several compression stages w/ HGB (6 stages or more is best, 4 at absolute

minimum)

• Variable capacity fans needed– As dampers shut down, static in duct builds, fan capacity needs to be

reduced– VFD most common method (older methods: IGV, DD, BPD, ride curve)– Control to static pressure 2/3 down the duct

• Variable exhaust needed– As fan speed changes, OA intake changes, and affects building

pressure, so building exhaust must be actively controlled– Control to building static pressure in hallway

Energy Savings• Diversity Credit

– Smaller equipment– Part load efficiency vs. on/off or even 2-stage cooling– Reduced compressor cycling

• Variable Frequency Drives (VFD)– Reduces fan horsepower– Affinity Laws

• Running at 75% CFM uses 42% of energy• Running at 50% CFM uses 12.5% of energy

• DDC w/ Supply Air Reset– Increases discharge air temp from 55 deg to a max of 60 on low load days to

reduce use of reheat in perimeter zones• DDC w/ Static Pressure Reset

– Reduces duct static pressure whenever possible– Looks at zone damper positions and if all zones are less than 80-90% open,

static is reset lower• DDC w/ CO2-based Demand Controlled Ventilation (DCV)

– Modulates OA damper below normal code minimum when less people are in the space

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12 Types of VAV Boxes:

•Single Duct

•Single Duct w/Reheat

•Retro-fit Dampers

•Supply-Return tracking

•Dual Duct

•Parallel Fan Powered

•Series Fan Powered

•Low Profile Fan Powered

When to Use Single Duct Terminals

• Interior spaces– Where only cooling is required. Heating not

necessary due to climate, internal loads, and heating from perimeter areas.

• Perimeter with reheat– Cooling along outer walls with some small

reheat on the coldest days.• Interior with reheat to maintain ventilation

minimums• Supply return tracking

– Airflow measurement and control into and out of a laboratory space.

LMHS –Standard

Single Duct

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When to Use Retro-Fit Single Duct Terminals

• Retrofitting old terminal units.– Mechanical constant volume dual duct dampers

for high pressure systems replaced with round retrofits for measuring and controlling airflow.

• Adding a control zone.– Round retrofits added to a round duct or slip-in

retrofits to a square duct to measure and control airflow in a new occupied zone monitored with room sensor.

• Replacing VVT-type zone dampers– Round or slip-in retrofits to replace existing high

pressure, high velocity zone controlled units with low pressure VAV units.

• As a return damper– Added to existing ductwork to measure and

control leaving airflow as required from controls.• As a bypass or ‘dump box’

– Installed at the end of a duct system to maintain relatively constant pressure at the air handler

Square Slip In Retro-Fit

Round Retro-Fit

When to Use Supply - Return Tracking

• Volumetric pressure control - Standard unit on supply, round unit on exhaust

• Stainless Steel construction available on exhaust unit

• Excellent low flow control is MANDATORY!• With several DDC systems, pressure control

logic is a standard configuration• Direct control of pressure requires door

interlock!

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By-Pass Terminals

• Low pressure systems only• Requires balancing damper upstream

of unit• Bypass damper should be set so A/H

unit sees little change in pressure/airflow.Bypass

By-Pass Terminals

Bypass

PrimaryInletTo VAV’s for

Occupied Spaces

Excess Returned to Air Handler

or Dumped into Plenum

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When to Use Dual Duct Terminals

• Perimeter Heating & Cooling– Allows air of required temperature as climate changes load.

• Positive Ventilation Control (measured ventilation to the zone)

– Allows volume of mixed air for ventilation requirements at temperature for comfort requirements.

• Laboratories / Constant Volume Applications– Allows constant volume of mixed air at needed comfort

temperature.• Supply / Return Tracking

– Can supply air of needed temperature at varying flow rates as ventilation changes.

• Underfloor Air Conditioning– Allows heating and cooling when both are available.

• Upstream vs. Downstream Flow Sensing Issues– Discharge sensor measures TOTAL heating and cooling

airflow for controls not set up to measure two different varying airflows.

LMHD – Non Mixing Dual Duct

LMHDT – Premium Mixing Dual Duct

Non-Mixing Dual Duct

l No Mixingl Two Single Duct Units

Joinedl S&D Dischargel Inlet Sensing Only

Standard Dual Duct

Std Single Ducts

Joining Strip

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Premium Dual Duct

l Attenuated Dual Ductl Inlet or Total Discharge

Sensingl Unequal Inlet Size

Combinationsl S&D Discharge Nozzlel Mixing Baffle Standard

(20:1 Mixing Ratio)l Accurate discharge flow

sensing Premium Dual Duct

20:1 Mixing Ratio

50%(95oF)+50%(55oF) = 75oF

95oF-55oF= 40oF

40oF ÷ 20 = 2oF

+/- 1oF IN DISCHARGE

74 75 74 76

74 75 75 76

75 74 74 75

76 74 75 76

95oF

55oF

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When to use fan powered terminals

• Perimeter Heating and Cooling– Unit can provide supplemental heat to plenum air

• Constant Volume Applications (Series)– Unit fan provides constant airflow with varying

temperatures.

• Variable Volume Applications (Parallel)– Unit provides variable primary airflow as required by

occupied zone temperature

• Interior Zones– Units can provide supplemental heat and provide

constant CFM and/or minimum airflow as required.

• Energy Concerns– Unit provides supplemental heat from warm plenum

air and may not require additional energy of heating cold primary air

QFC – Standard Fan Powered Terminal Unit

KQFS – Premium Fan Powered Terminal Unit

Series vs. Parallel Fan Boxes

SERIES FAN POWERED BOXES• Fan is on discharge of unit - all air

goes through the fan (primary airflow cannot exceed fan airflow)

• Fan runs at all times during occupancy• Fan must be started before primary air

is supplied (DDC logic?)• Note: there is no such thing as an “anti-

backward rotation device”.

• Radiated sound is critical sound application factor

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Series Operation

Heating Only

Series Operation

Min Cooling Make-Up Heating

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Series Operation

Cooling Only

Series Operation

1000 CFM FAN

1200 CFM PRIMARY

200 CFMLOST

Situation to Avoid:Short-circuiting can result in sub-cooling the plenum and loss

of control

IncorrectApplication

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Series OperationConstruction Options

Standard Premium

Construction Highlights

Perf Baffle –Blocks Primary Noise

Blower Panel –Isolates Motor-blower

From Casing

Baffle-Blocks Noise From Escaping to Plenum Opening

Integral Attenuator –Additional Insulation Absorbs Sound

Attenuator Panel –Gives Additional Rigidity to Unit

Larger Cabinet –Allows low airflow

velocities and additional insulation

Standard

Premium

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Series vs. Parallel Fan Boxes

PARALLEL FAN POWERED BOXES• Fan is in parallel with VAV damper• Fan airflow independent of primary

airflow• Fan only runs in heating mode• Fan is essentially first stage of heat• Backdraft damper is standard• Discharge sound is critical

application factor

Standard Parallel Operation

Cooling OnlyBack Draft

Damper Closed

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Standard Parallel Operation

Heating OnlyBack Draft

Damper Open

Parallel Construction Highlights

Discharge Opening –Straight path for motor blower

Blower Panel –Isolates Motor Blower From Unit Casing

Integral Attenuator –Additional Insulation

Absorbs Noise

Blower Orientation-Wheel turned sideway away from plenum opening

Attenuator Panel –Gives Additional Rigidity to Unit

Larger Cabinet –Allows low airflow

velocities and additional insulation

QFVKQFP

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Fan Box - Series vs. Parallel

Series Parallel

Fan Box - Series vs. Parallel Rules of Thumb

• Constant Volume = Series• Variable Volume = Parallel• Series Boxes

– Constant Fan Operation– Max Primary Airflow ≤ Fan Airflow

• Parallel Boxes– Fan is essentially 1st Stage of Heat– Fan Operates in Heating Mode Only– Fan Airflow ~ 40-60% Cooling Airflow (typically)

Note: There is no cure for a noisy fan box except to relocate or replace with quieter designClearly written specifications will reduce field issues

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• The use of ECM motors can reduce the energy consumption of Fan Powered Terminals

• An ECM motor is a brushless DC motor, with an integral computer controlled inverter.

• At reduced rpm and load, an ECM motor can use as little as 60% of the energy of a standard Permanent Split / Capacitor Start (PSC) motor

• An ECM motor can be programmed to be pressure independent, resulting in true constant volume performance, easier balancing, and even DDC controlled performance.

• ECM motor has greater turn-down.

ECM MotorsPSC vs. ECM Motors

PSC vs. ECM

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ECM Motor Controllers

ACU•CFM is adjustable

•0-10 dc Volt Signal from DDC Controls modulates Motor Speed.

Example

3.8 Volts = 38 Setting

VCU•CFM is constant

•CFM is set manually at the board with Fan Adjust Knob

•Board alternates display

•Fan Airflow Setting

•RPM

Air Flow Setting RPM Display

Some Cautions, however:• An ECM motor can add $250 to the purchase price of a Fan Powered Terminal.• Power consumption CAN be greater than an equivalent PSC motor, and the connected load requirement is usually higher because of this. • There are issues with the power factor of these types of devices which may cloud true energy savings.

ECM MotorsECM Motors

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ECM vs. PSC for Fan Powered Box

2500 cfm Series Fan Box, ECM/PSC motor

0

25

50

75

100

0.00 0.20 0.40 0.60 0.80

Efficiency, Watts/cfm

% F

low ECM

PSC

Which Fan Box to choose?

• Standard• When acoustics is not an issue or• Fan Air flows are < 1200 CFM

• Quiet• When premium construction is

desired• When double wall is specified• For wider selection of inlet/fan

combinations• Low Profile

• Space is restricted and• Airflows < 1200 cfm

What Product to Use

•Clearly written specifications will reduce field problems

•There is no field cure for a noisy fanbox except to replace it with a quieter design.

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• ½” thick 1.5lb dual density (Standard) • 1” thick 1.5lb dual density (drops 1 NC*)• Duct board with steel flanges (adds 7-10 NC*)• Double-Wall with 5 internal insulation options

– Solid metal (adds 12 NC*) – Perforated metal (adds 8 NC*)

• ½” or 1” thick foil faced fiberglass (Adds 7-10 NC*) • No liner (adds 12 NC*)• Cellular (adds 3-5 NC*)

– 3/8” standard– 3/4”now available on select single duct models

Up to 15 Lining Options

* Data for Premium Series Fan Box is shown above -the effect is not the same with all models.

Note: Not all programs show the effect of insulation options!

Sensor Options

• K4 LineaCross• Four quadrant center averaging• Good amplification for greater

turndown• Minimal increase in pressure drop• Barbed tubing connections

• Linear Probe• Two parallel averaging tubes• Installed diagonally across duct• No measurable pressure drop• No measurable additional sound

Both Give Equivalent Pressure Signals

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Multi-Point Inlet Sensors

INLET SENSORFROM BEHIND

INLET SENSORFROM FRONT

Sensor BasicsStatic Pressure – The outward force of air within a duct, measured in inches of water column.Velocity Pressure – The forward moving force of air within a duct, measured in inches of water column.

Inlet Sensor Pressure Signal

2.0”

2.0”1.97”1.2”1.0”Example 1: ∆P = 2.0 – 2.0 = 0Example 2: ∆P = 2.0 – 1.97” = 0.03”Example 3: ∆P = 2.0 – 1.2” = 0.8”Example 4: ∆P = 2.0” – 1.0” = 1.0”

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Minimum/Maximum Airflows

l Airflows for ‘Standard’ controls below minimums shown cannot be guaranteed to be maintainable “uncontrollable range”

l Maximums based on 1.0” W.G. differential pressure

l Minimums based on 0.03” W.G. differential pressure

LMHS UnitsMax. Primary

inlet size Airflow - CFM Standard* Electric Heat**4 230 40 555 360 62 856 515 89 1107 700 121 1408 920 159 1909 1160 201 240

10 1430 248 30012 2060 357 42514 2800 486 58016 3660 634 750

24 x 16 7000 1212 1800

Min Airflow, CFM

• 1 to 4 row for single duct• 1 to 2 Row for fan powered

– Fan powered terminals need less rows because they heat a mixture of warm plenum air and primary air

• Left or Right Hand Piping Connections• Access Panels to clean water coil supply

side

Hot Water Coil Features

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Electric Reheat Features

CAUTION: 480 3-Phase 3-Wirel Special for Single Ductl Must have Dual Point

Source for Fan Box

• 120/208/240/277 single phase • 208/3 phase (3-wire), 480/3 phase (4-wire) • 1-3 Stages (most types)• Side access - Leave clearance per NEC• Operation at minimum airflow is unlikely to provide comfort• Consider proportional electric heat with discharge temperature

control for maximum comfort

Patented LineaHeat Control:• Solid State Relay (SSR) vs. SCR

– Allows use of less expensive versions of DDC controllers (as much as $100 less)• Up to 4 VAV manufacturers offer SSR control now• Infinite adjustment (0 – 100%)• Low cost• Silent operation• Optional discharge temperature control• 7 different control inputs

Proportional Electric Reheat

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LineaHeat Control input types:• LX1 – 1 step heater (PWM) - {24Vac Pulse from Controls}• LX2 – 2-stage heat• LX3 – 0-10 Volt / 0-20mA signal – {Common Control Function}• LX4 – 2-10 Volt / 4-20mA signal• LX5 – Thermostat (incremental) – {Mirrors a NC Water Valve}• LX6 – Binary (1, 2, or 1 & 2 stages)• LX7 – Three Point Floating – {Mirrors a 24Vac 3 point Water

Valve}

Proportional Electric Reheat

• Empty Enclosure (Field Mounting)• Pneumatic • Analog• DDC• Transformers

Control Options

l Many Control Options

l Library of current DDC wiring diagrams for factory mounting

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VAV Terminals

POORLY ADJUSTED / SELECTED DIFFUSER

THERMOSTAT

Air Distribution, Poor Pattern Example

VAV Terminals

Air Distribution, Poor Pattern Example

THERMOSTAT

POORLY ADJUSTED / SELECTED DIFFUSER

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VAV Terminals

PROPERLY ADJUSTED DIFFUSER

THERMOSTAT

Good Pattern Example

VAV Terminals

Perimeter Considerations:

• Maximum delta-t for effective mixing when heating, per ASHRAE handbook = ?.

• = 15°F (90°F discharge), continuous operation.

• Throw toward and away from glass.• 150 FPM should reach 4-5 feet from the

floor.• ASHRAE 62.1 requires that ventilation be

increased by 25% when heating, if the above rules are not followed.

• Typical perimeters require only 8°Fdelta-t @ 1 cfm/sq.Ft.

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VAV Terminals

ASHRAE Journal, 2004

Ryan R. Hoger, LEED AP708.670.6383ryan.hoger@tecmungo.com