Latest in High Performance Outdoor Air Systems (DOAS)

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Latest in High‐Performance Dedicated Outdoor Air Systems (DOAS)

Arthur D. Hallstrom, P.E. ASHRAE Fellow, BEMPahallstrom@ad‐hall.net

Updated Spring 2020

© 2020 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

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Copyright

Copyright 2017, 2018, 2019, 2020 by ASHRAE. All rights reserved. No part of this presentation may be reproduced without written permission from ASHRAE, nor may any part of this presentation be reproduced, stored in a retrieval system or transmitted in any form or by any means (electronic, photocopying, recording or other) without written permission from ASHRAE.

ASHRAE has compiled this presentation with care, but ASHRAE has not investigated and ASHRAE expressly disclaims any duty to investigate any product, service, process, procedure, design or the like, that may be described herein. The appearance of any technical data or editorial material in this presentation does not constitute endorsement, warranty or guaranty by ASHRAE of any product, service, process, procedure, design or the like. ASHRAE does not warrant that the information in this publication is free of errors. The user assumes the entire risk of the use of any information in this presentation.


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AIA/CES Registered Provider• ASHRAE is a Registered Provider with The American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non‐AIA members are available on request.

• This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.


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GBCI cannot guarantee that course sessions will be delivered to you as submitted to GBCI. However, any course found to be in violation of the standards of the program, or otherwise contrary to the mission of GBCI, shall be removed. Your course evaluations will help us uphold these standards.

Course ID: 920014509

Latest in High-Performance Dedicated Outdoor Air Systems (DOAS) By ASHRAE

Approved for:

3General CE hours

0LEED-specific hours


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Learning Objectives

• Describe DOAS terminology, systems and units with a knowledge of the latest innovations.

• Be able to explain why DOAS is an excellent system choice for high‐performance buildings

• Know how to properly apply and control a DOAS system

• Be able to describe differences between VAV DOAS and mixed‐air VAV systems


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Course Outline

1. System Overview – Terminology, Configurations

2. DOAS in High Performance Buildings

3. DOAS System – Core functions ‐ Ventilation, Humidity, Building Pressure Control

4. DOAS H/C Unit ‐ Core function ‐ Heating/Cooling

5. DOAS Unit (DOASu) Options, Selection, Control

6. VAV DOAS ‐Mixed‐Air VAV System Comparison

System Overview


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Terminology: Dedicated Outdoor Air System (DOAS)

Uses a DOAS unit (DOASu) to condition all of the outdoor air brought into a building and delivers it to each space, either directly or indirectly through heating/cooling (H/C) units

System Overview


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Terminology DOAS (System)

DOASu

Heating/Cooling (H/C) Units

Outdoor Air

Exhaust Air

SA Supply Air

Return Air

Chiller

System Overview

Unitary VAV DOAS VAV DOAS


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DOAS Terminology

DOASuOA

EA

Air‐to‐Air (ATA) Recovery Device

This is a space, also called

room or zone

H/C units are also called local or zone

units

System Overview

Filters


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Delivers prescribed amount of conditioned outdoor air

Controls building humidity

Controls building pressurization

DOAS System Primary FunctionsDOASu

OA

EA

H/C UnitsControls space temperature

RAEA

System Overview


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One or Two Cooling Systems

DOASu


OA

EA

SA SA

System Overview


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DOAS is Growing Very Popular. Why?DOAS advantages:1. Right amount of clean ventilation air to spaces.2. Controls space humidity independent of space

temperature 3. Controls building pressurization 4. Comparatively easy to control and maintain5. Good Indoor Air Quality 6. Low energy use7. Competitive first cost

System Overview


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DOAS Use Examples

• Required by Washington State Building 2015 Code. • Two options – DOAS or Enhanced VAV• Any other alternative needs to better with modeling

• U.S. Air Force/Navy/Army• Unified Facility Criteria (3‐410‐02 Nov 2017)• DOAS required

• ASHRAE HQ—Tullie Circle and new HQ building

System Overview

• AEDG Guides (Required system in the Zero Energy Guides)


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Key Observation

“For humid climates and high‐occupancy buildings, dedicated outdoor air systems improve humidity control and may offer first‐cost as well as operational savings. Preconditioning fresh air with a desiccant dehumidification system eliminates the need use mechanical air conditioning systems for that purpose.”

ENERGY STAR. 2016. Building Upgrade Manual. https://www.energystar.gov/sites/default/files/buildings/tools/EPA_BUM_Full.pdf

DOAS System


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Published in 2017

Developed with the support of ASHRAE TC 8.12, 8.10, 5.5,

and 1.12

System Overview


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VAV DOAS Direct to Space System

DOAS System


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DOASu and ductwork sends conditioned OA direct to each space.

H/C units provide space temp control

VAV DOAS Direct to SpaceDOASu

SA SA

RA

SA

OA

EA


EA

DOAS System Direct to Space


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VAV DOAS Direct to Space H/C Unit Options

Radiant Heating and/or Cooling Radiant Heating and/or Cooling

Other sensible unit options exist

Fan CoilFan Coil PTACPTAC



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DOAS Direct to Space Key Points

• Uses separate air diffusers. Good practice—use high‐induction (Coanda) VAV diffusers to avoid dumping problems.

• If DOASu has its own cooling system, it can operate with H/C units off. Separate systems. Offers some redundancy.

• H/C units are sensible fluid‐based units. • Generally a low noise system.



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VAV DOAS Indirect throughVAV H/C Units System

DOAS System Indirect to Space


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VAV DOAS—Indirect through H/C Units

DOASu

SA

OA

EA


SA

Conditioned outdoor air (OA) goes to VAV H/C unit, becomes supply air (SA)

RARA



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VAV DOAS through H/C Unit Options

Plenum‐Mounted Chilled Water DOAS H/C Units

Fluid-based units

Active Chilled Beams



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• Ventilation air is dispersed throughout zone by H/C unit fan

• Both DOAS and H/C fans must be on to get ventilation air

• Many new VAV DOAS H/C units are now available

VAV DOAS through H/C Units



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Advantages of DOAS ‐ H/C Systems

• Small ventilation ducts. • Small or no plenums• Both DOAS and H/C systems can be controlled in ‘’real time’’ to minimize fighting and minimize energy use

• Downsized simple H/C units can help lower system first cost



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Advantages of VAV DOAS• Air valves control space ventilation based on people, contaminants, humidity, ATA recovery operation

• Space ventilation OA is measured; much easier to commission, recommission, control with DDC. No multiply space equations.

• Space ventilation can be reset in real time if space conditions change

• ASHRAE Standard 62 changes• Occupancy changes• Bad IAQ (maybe more ventilation needed)• Building pressure management

• Spaces not in use can be set at minimum ventilation



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Building Model: Climate Zone 4, 50% variable occupancy, Total Recovery DOAS

Crowther and Ma 2016

72% Energy Savings

System Overview

Make up air/exhaust

CV DOAS

VAV DOAS

Annual Cost $12,00010,0008,0006,0004,0002,000

0

Supply Fan

Exhaust Fan

Supply Fan

Cooling Coil

Energy of Three System Options


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Why? H‐P buildings have lower sensible loads. Roughly

same latent loads.

Result: Latent (humidity) space control becomes a significant issue.

Solution: DOAS. It independently controls humidity, building pressure and air quality.

DOAS is ideal for High‐Performance Buildings

DOAS in High Performance Buildings


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Mixed‐air system at room level (8.3 tons)System Comparison Example

Cooling Loads Sensible Load Latent LoadBtu/h (W) Btu/h (W)

Conduction through roof 12,312 (3563)Conduction through exterior wall 502 (144)Conduction through windows 1210 (359)Solar radiation through windows 22,733 (6477)People 4500 (1350) 3600 (990)Lights 22,097 [6480)Equipment 8184 (2404) 1540 (450)Infiltration 2988 (876) 3969 (1159)Total space cooling load 74,626 (21,623) 9109 (2599)Ventilation 6540 (2047) 8820 (2709)Total coil cooling load 81,266 (23,670) 17,929 (5308)

SHR = 0.82



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DOAS (OA Dehumidification Only) + H/C SystemCooling Loads Sensible Load Latent Load

Btu/h (W) Btu/h (W)

Conduction through roof 12,312 (3563)

Conduction through exterior wall 502 (144)

Conduction through windows 1210 (359)

Solar radiation through windows 22,733 (6477)

People 4500 (1350) 3,600 (990)

Lights 22,097 (6480)

Equipment 8184 (2404) 1540 (450)

Infiltration 2988 (876) 3969 (1159) Btu/h tons

Total space cooling load 74,626 (21,623) 9109 (2599) 83735 7.0

Ventilation 6540 (2047) 8820 (2709) 15360 1.3

SHR = 0.89 0.43

System Comparison – Two Systems



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DOAS (Total dehumidification) + H/C SystemCooling Loads sensible load latent load

Btu/hr [W] Btu/hr [W]

conduction through roof 12,312 [3563]

conduction through exterior wall 502 [144]

conduction through windows 1210 [359]

solar radiation through windows 22,733 [6477]

people 4500 [1350] 3600[990]

lights 22,097 [6480]

equipment 8184 [2404] 1540 [450]

infiltration 2988 [876] 3969 [1159] Btu/h tons

total space cooling load 74,626 [21,623] 9109 [2599] 74626 6.2

ventilation 6540 [2047] 8820 [2709] 24469 2.0

SHR = 1 0.27

System Comparison‐ Two Systems



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DOAS (handles all latent) + H/C systemBtu/hr Btu/hr

conduction through roof 6000

conduction through exterior wall 300

conduction through windows 800

solar radiation through windows 7000

people 4500 3600

lights 8000

equipment 6500 1240

infiltration 2000 2657 Btu/h tons

total space cooling load 35,100 7497 35,100 2.9

ventilation 6540 8520 22,557 1.9

SHR = 1.00 0.33

Decrease 53% 8%

more insulation, light‐color roof, and better glass

better glass

LED lightinglaptops and flat monitors

tighter construction

High‐Performance Building

DOASu is now 40% of tons –almost half of tonnage – makes DOASU the most important unit on the job


Load is now just 4.8 tons

Load reduction is mostly sensible


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DOAS Ventilation Control

DOAS System – Ventilation© 2020 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

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ASHRAE 62.1‐2019 – OA Amounts

• Design Ventilation is allocated by people and space• Example

Design CFM (62.1‐2019) CFM per square foot at full load

Per Person Per Sq Ft

Classroom 10 .12 1.12

Lecture 7.5 .06 1.31


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ASHRAE 62.1‐2019 Part Load

• Demand Control Ventilation (DVC) (Reset) • Increasing encouraged by ASHRAE 62.1• Use in VAV DOAS spaces.

• Reduce the occupant related space ventilation air based on occupancy load.

• Recalibrate sensors with software or testing

• Key design question – what sets the minimum amount of ventilation air?


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DOAS Humidity Control

DOAS System – Humidity


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System Usage180

160

140

120

100

80

60

40

20

H

Hum

idity ratio, grains/lb of dry air

11030 40 50 60 70 80 10090Dry-bulb temperature, °F

80

70

50

4030

60

30

40

5055

60

65

70

75

80

Dew

point temperature, °F

DOASu

H/C Unit



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Best Practice—Use DOASu to Remove Internal Latent Load



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180

160

140

120

100

80

60

40

20

H

Hum



80

70

50

4030

60

30

40

50

55

60

65

70

75

80

Dew


SA

OA

SpaceRemove spacelatent loadRemove spacelatent load

DOASu Can Keep Interior Spaces Dry More comfortable

spaces H/C unit coils and

piping stay dry Reduced chance of

mold


Remove outsidelatent loadRemove outsidelatent load


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Measuring Dehumidification—Humidity Removal

How to measure?

Humidity ratios (gr/lb)before and after the coil



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Dehumidification Accomplished by the Cooling Coil

l/s × 4.32 × (entering g/kg – leaving air g/kg)1000

= kg/hr

cfm × 4.5 × (entering gr/lb – leaving air gr/lb)7000

= lb/hr



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VAV DOAS Dew‐Point Design

Calculate peak humidity load for each space and block humidity load for the building

For a given supply air dewpoint, peak is the space cfm. Block is the DOASu cfm.

Meet the block and peak dehumidification need with proper amount of DOASu ventilation air and leaving air dew point



42DOAS System – Humidity


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comfort.cbe.berkeley.eduDOAS System – Humidity


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Select DOASu Coil on Design Dew Point

Wet Bulb Design



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Outdoor Humidity Impacts DOAS

Source: Harriman et al. 1997.

These are averages, not peak.



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Tropical Climates

• Weather is great: temperature is “good” but humidity varies to the oppressive level

• Envelopes are “light,” natural ventilation often encouraged

• Consider air motion e.g. ceiling fans to “cool” people



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180

160

140

120

100

80

60

40

20

H

Hum



80

70

50

4030

60

30

40

50

55

60

65

70

75

80

Dew


SA

OA

Space

DOASu Can Keep Interior Spaces Dry



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Indoor Humidity Calculation• Assumption: Outside windows are closed, building is at indoor dewpoint design


Per Person Per Square

FootAverage square feet per person

62.1 CFM/Sq Ft

Design CFM from 62.1‐2019 Classrooms 10.00 0.12 10.00 1.12

Lecture Hall 7.50 0.06 6.00 1.31

Indoor Humidity removal dewpoint

grains at dewpoint

grains removed to get to 60 dewpoint target

cfm needed at 30 btuh/person

cfm needed at 60

btuh/person60 (max target) 77.6 0 infinity infinity 55 65.6 12 2.5 550 53.6 24 1.25 2.5045 45 32.6 0.92 1.8440 36.5 41.1 0.73 1.46


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DOAS—H/C Combined Operation

What is better? DOAS Delivering Cool Air or Room‐Neutral Air?

Cool is Generally Better



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Major change – ASHRAE 90.1‐2019 Established a 60F Indoor Dew Point Limit


5.9 Maximum indoor air dewpoint in mechanically cooled building. Provide dehumidification equipment that limits indoor humidity to 60F dewpoint during occupied and unoccupied hours when outdoor dewpoint is above 60F. [Design used] is design dewpoint and coincident dry‐bulb, space interior is at cooling design values and space solar loads are at zero.


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Bottom line: When Building is in Cooling Mode, Cool Air is Better than Neutral Air

• Makes sense: If majority of zones are in cooling mode, cool DOAS air will reduce H/C system cooling energy. Less system fighting. It might increase the DOAS energy use but overall, the building energy is lower.

• DOAS needs enough dry air to meet the 60F dew point requirement every hour.



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Improvements defined by Std 62.1‐2019

• For (any) SYSTEM DESIGN… no higher than a 60F(15C) dew point

• 60F dew point design limit applies for BOTH occupied and unoccupied system operation

• In effect… designs need to include equipment and controls that will provide DH independently from cooling.

• Exception: Overnight periods of less than 12 hours need not be limited by 60F dewpoint… provided that RH does not exceed 65% during those hours.

• However: This requirement applies to designs, so that owners have systems that CAN control humidity…. Whether or not he/she choses to DO SO.. is up to the owner.

Cool air can create first cost savings in H/C unit sizing



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Key Takeaway One

VAV DOASu needs to dehumidify at low loads and airflows.

• Indoor dew‐point control by DOASu is critical for H/C units without “connected” drain pans.

• Any cold surfaces below space dew point can condense—potentially creating indoor rain.



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Key Takeaway Two

Avoid short‐cycling DX DOASu cooling coil for better space humidity control

• DX coils take time to start dehumidifying and they re‐evaporate a lot of water on the coil when turned off

• Consider using variable capacity compressors, staged coil or bypass dampers



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Humidity Capacitance inside Structures

Buildings can store water vapor in carpets, drywall, and ceilings. Keeping dew point under control all the time:

• Enhances occupant comfort• Permits faster recovery from night or summer setback

• Building start‐up—dry out—is a critical operational step. • Once building is dry, keep it dry. Some energy modeling programs can now model humidity capacitance of a building. Makes equipment selections, sizing, operation more accurate. Wild card: Natural ventilation



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DOAS Building Pressure Control

DOAS System – Building Pressurization


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Incoming OA quantity is set by space ventilation and exhaust requirements.



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Control building pressure by changing exhaust fan airflow (e.g., direct space pressurization).

Control this



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Don Gatley’s, ASHRAE Fellow Voice of Experience79 moisture investigations in 25 years

All, except four, were caused by building suctionand/or excess humidity in ventilation and makeup air:•Too much exhaust (not enough dry makeup air)•Not enough drying of the makeup/ventilation air

Don’t overcomplicate humidity troubleshooting:• Make sure the building does not suck, and• Make sure the ventilation and makeup air is

dry all the time.


From Lew Harriman ASHRAE ALI Humidity Course II


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Building Pressure Recap:

• Get a good envelope• Maintain positive building pressure by varying DOASu exhaust air amount

• Exfiltration and remote exhaust defines the minimum OA needed

• Remember the ATA device needs exhaust air to work



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DOAS Can Isolate a Building• In the event of unacceptable outside air (fumes, chemicals) DOAS units can control building pressure by controlling OA/EA quantities.

• DOAS generally supplies air to many zones, so building has fewer OA/EA openings.

• OA and exhaust airflow is through DOASu. Under DOAS control.

• DOASu can be enhanced with custom air cleaning options for reducing outside and exhaust contaminates. (More on this later.)



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DOAS Night Setback or “Light Occupancy” Control

DOAS System – Night


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DOAS Night SetbackStop this part of unit and open night setback damper

RA



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Night Setback Control

• Cycle DOASu to control dewpoint, not temperature

• Calculate dew point from temperature and relative humidity. More stable, less expensive than chilled mirror



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DOAS Morning “Warm‐Up”

• Strategy depends on factors like climate and energy costs.

• Generally simpler to use H/C units. Space is dry, so recovery is generally quick.

• Cold Climate can use DOASu heat • Hot water, gas, electric, renewable electric heat, etc.



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VAV DOAS through H/C Units

EA

SA SA

CA

DOASuOA

EA

RA RA

Next topic, this DOAS H/C unit

It is a CW VAV single-zone mini air handler

DOAS H/C Unit


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DOAS H/C Unit

Photos Courtesy of Trane/IR.

DOAS H/C Unit


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DOAS H/C Unit

Photos Courtesy of Johnson Controls.

• Dedicated Outdoor Air Supply

• Primary Air Inlet Connection

• Non-Condensing Cooling Coil (Sensible Cooling)

• Hot Water (Option)

• Integral Drip Tray

DOAS H/C Unit


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Fan Operation • Uses variable‐speed electronically commutated motor

(ECM) fan. 90+ efficient• Continuous variable speed operation during occupied

modes• Intermittent operation during unoccupied modes• Fan speed modulates slowly as needed to maintain desired

zone temperature and ventilation deliveryAir Valve—sized to meet the design outdoor airflow requirement for the zone. Has flow measurement, inlet static.Acoustics—select for design background sound pressure levels in the occupied space

DOAS Chilled‐Water (CW) H/C Unit

DOAS H/C Unit


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DOAS H/C Unit Control

• Control o Wireless communicationo H/C unit controller uploads many points to

master controllers and enterprise servers (e.g., temperature/occupancy/CO2 sensors)

o Remote reset of setpoints allows for interval optimization or ongoing commissioning

o Condensate pan water indicator

DOAS H/C Unit


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DOAS CW H/C Unit Control (cont.)

• Use one thermostat per control zone

• Or if multiple H/C units serve one space link thermostats to a common control point. This practice avoids H/C units trying to heat and cool in same zone.

DOAS H/C Unit


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DOASu Evolution

DOAS Unit (DOASu)


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DOASu Evolution

DOASuTwo Fans,

ATA Recovery

Makeup AirOne Fan/ Filter

Air HandlerOne or Two Fans/Filter

ATA or ERV

Exhaust Fans

DOAS Unit (DOASu)


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AAERV System Classifications

Air Handling SystemAAERV component is incorporated within the AHU; no refrigeration, no controls

Packaged SystemAAERV component is incorporated along with a refrigeration system and controls

Integrated Systems

Photos Courtesy of Paul Pieper.

DOAS Unit (DOASu)


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DOASu Today• Ventilation Clean Air—Right Amount, Filtered• Dehumidified Air

• First‐stage air‐to‐air energy recovery—reduces energy use• Second‐Stage enhanced dehumidifier—provides low (44°F) dewpoints

• Building Pressure Control (with Integral Exhaust Fan)• VAV Fan Air Delivery• Interval Commissioning for Real Time Control

DOAS Unit (DOASu)


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Overview – DOASU are FlexibleDOASu options can be selected for the specific

climate, altitude, and application

Accurate equipment performance prediction selection and energy modeling programs are critical to success and first‐cost decisions

DOAS Unit (DOASu)


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Dedicated Outdoor Air Unit

• One DOASu can serve a building, floor, or space.• Oversizing the DOASu for lower internal pressure drops. Requires a cost/energy savings evaluation.

• Leakage and thermal insulation in exterior walls, interior walls, and around components is important detail to “specify.” DOASu is the coldest component in the system. Needs the best construction.

DOAS Unit (DOASu)


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Cold Coil

• Supply dew‐point design drives the coil‐compressor‐chilled selection.

• Design dew points are typically between 48°F–55°F. • Requires cooling coil that can seriously dehumidify outside air (e.g., 6/8 row, lower fpm)

• Coils need to dehumidify from peak dew‐point design at almost “any” OA flow condition to meet 90.1‐2019 standard

DOASu Design

DOAS Unit (DOASu) Coil


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35°F 40°F 45°F 50°F 60°FDew Point of Conditioned Outdoor Air (CA)

55°F

Packaged DX

CW system

Split or DOAS DX system

Dew point capability

DOASu Cooling Coil Sources

Ice Storage

Light Medium Heavy …



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1. Ground storage (lowest energy use— generally most expensive)

2. Water (towers, etc.) options. Permits ice or water storage

3. Air cooled (generally lowest cost, highest energy use)

DOASu Condensing Options



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DOAS Design Question

Use Chillers or Compressors?

Depends on:

• DOASu and H/C loads• When they occur• System efficiencies• Desire to transfer energy between the two systems

• Second stage enhanced dehumidification

• Redundancy requirements



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DOAS Design Question

Use Chillers or Compressors?

Depends on:

• What can operators maintain?

• What are electric utility costs now and in future?

• kW, kWh, time of day—impacts thermal storage decisions

• System cost

Customization = Flexibility



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DOASu First‐Stage ATA Recovery

Outside Air ConditionsOA with ATA RecoveryOA with ATA Recovery —Unequal Airflow

DOAS Unit (DOASu) ‐ ATA


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ATA Enthalpy Transfer Technologies

Paper‐Based and Polymeric Latent

PlatesPhotos Courtesy of Paul Pieper.

Enthalpy EnergyRecovery Wheels



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Useful Resource

ASHRAE Learning Institute:• Air‐to‐Air Energy Recovery Fundamentals—presented by Paul Pieper, P.Eng., Member ASHRAE



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Stage 1 DOASu Enthalpy Transfer



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Energy Recovery Wheel

• In US, two substrates available: silica and molecular sieve

• AHRI 1060 certification• 70%–85% total effectiveness• Wheel rotation speed indicates wheel type—color does not. Typically rotates around 20 rpm

• Filter air into both sides of wheel• Use bypass dampers for economizer, fan energy savings, frost protection

Bypass dampers



88DOAS Unit (DOASu) ‐ ATA


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ATA Fan Placement

Provides lowest amount of exhaust air transfer (EATR)

Photos Courtesy of Paul Pieper.



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“As‐Applied” Versus “Rated” Effectiveness

• Be careful not to confuse “as‐applied” enthalpy recovery ratio (ERR) (required by ASHRAE Standard 90.1) with “rated” effectiveness (AHRI 1060)

• Strive for balanced airflows• Bring back as much exhaust air to DOAS unit as possible



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ASHRAE/IES Standard 90.1‐2016

• Changed the language to use the term enthalpy recovery ratio (ERR), rather than the term effectiveness. This enthalpy recovery ratio is defined as

ERR = (h1 – h2)/(h1 – h3)where hx corresponds to the enthalpy of the respective airstreams.



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Mumma, Stan. 2014. Understanding & Designing Dedicated Outdoor Air Systems. Atlanta: ASHRAE.



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ERR—fraction of total available energy transferred to (or from) the outdoor airstream. ERR accounts for air imbalance between the outdoor and exhaust airstreams as well as the effects of purge air. Using this measure, ASHRAE/IES Standard 90.1‐2016 requires that the exhaust air energy recovery device has an ERR of 50% or greater.



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Manufacturers should provide performance program that calculates DOASu ERR for a specific set of application conditions based on AHRI certified data.

Note: This should a key part of a DOAS UnitPrediction Program



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Exhaust‐Air Energy RecoveryGood Idea—Recover Restroom Exhaust Energy



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ATA Recirculated Air Limit• From ASHRAE 62.1‐2019 Informative Note

• Class 1: Low contaminant, sensory‐irritation intensity, inoffensive odor.

• Unlimited recirculation. • Class 2: Moderate or mild conditions. E.g. bathroom exhaust.

• 10% limit. • Class 3: Significant or offensive air.

• 5% limit.• Class 4: Highly objectionable, potentially dangerous.

• 0% limits or as approved by AHJ.



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ATA Recovery Transfer

Flow and enthalpy measurements on all four quadrants can tell how ATA is performing.

Easy way to do this—measure pressure drop across ATA and RH/Temp in 4 quadrants.

If no transfer check for hardware failure.



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Wheel or Plate Freeze Protection

• In cold climates to prevent water freezing on ATA device use:

• A preheat coil before the energy recovery device

• A modulating bypass damper in the OA recovery section. Note: this may limit ATA recovery so DOASu heat may still be needed



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Second Stage DOASu Humidity Reduction

Type 3 Isotherm Desiccant Wheel

DOAS Unit (DOASu) – Type 3


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Type 3 Desiccant Wheel Rotates at 8 RPH


Can lower dewpoint by 8 or more points


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Isoterm Type 3 Wheel Can Save Cooling and Reheat Energy

1

1

2

23

34

4 5

53a

6

6

7

7

Source: Mumma et al. 2013.



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ASHRAE Tullie HQ Unit

Peak Dew‐Point Design



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Outdoor Air Cleaning• Particulate filters protects coils, wheels, people. MERV 8 is common, MERV 13 is better. Electric enhancement increases MERV without added pressure drop. Be careful.

DOAS Unit (DOASu) Air Cleaning


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Outdoor Air

Cleaning

• Goes beyond filters • Air cleaning can reduce

• Odors• VOC (Gas, Diesel, etc.)• Pathogens

• Viruses• Spores

• Fumes • Ozone



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Outdoor Air Ozone now has Limits

• ASHRAE 62.1‐2019 moving to a health standard• 6.1.4.3 Ozone air cleaning needed if outdoor air ozone is serious for 8 hours as defined by USEPA (>.1 ppm)

• 3 exceptions. • Also, if national outdoor air standards for contaminants are exceeded, assumptions and calculations related to IAQ impact need to be documented. Opens the door.


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Air Cleaning Application

• Location: Source – Path – Receiver• Is the source in the outdoor air or indoor air?

•Reduction or deactivation – generally not elimination (99.9%)



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Air Cleaning Groups

Carbon, Potassium‐Permanganate(Capture and Hold)

Photocatalytic Oxidation (PCO)UV, TiO2 (Catalytic Conversation)



108DOAS Unit (DOASu) Air Cleaning

UVc is different than PCO


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Photo‐catalysis create hydroxyl radicals



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ROS Life and oxidation potential



111DOAS Unit (DOASu) Air Cleaning

Airborne Particle Size

N95‐95% of .3um. Virus is .17, with moisture about 5 um. HEPA is 99.97% of .3um.


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Types of Building Related Air Cleaning



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DOASu Fans

oSelect based onoEfficiencyoUnloadingoNoiseoMaintenance

Picture is Direct Drive AF Fans with VFD

DOAS Unit (DOASu) ‐ Fans


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For larger cfm units• Offers

• Redundancy• Energy efficiency• Unit acoustics• Reliability• Serviceability• Small unit footprint

Fan Array or Wall

Photo Credit: Trane/IR .



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Fan Flow Measurement• Inverters are popular modulation option• Inlet cone flow measurement is low cost, effective



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Component Bypass Dampers

Component bypass dampers• Used with any large pressure drop component (ATA, coil, second stage component) to save fan energy

• ~0.3 in. versus ~1.0 in. wheel pressure drop

• Might also provide frost protection and flow control through a component

EA



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DOASu Cooling System Energy

Use

• Look at systems demand and timing of demand

• Consider separate DOAS (DOASu) and H/C chiller or compressor systems

• For lowest energy use, don’t burden a lightly loaded system with the needs of a more demanding system

DOAS Unit (DOASu) ‐ Cooling


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DOASu Energy

Use

• Know the equipment power use

• 100%, 75%, 50%, and 25%. Typical third party certification points.

• 0%. Off mode is not a AHRI rating point. Measure it!

DOAS Unit (DOASu)


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DOASu Control

• Factory packaged DDC controls with communicating controls for data logging, fault detection, and building level unit optimization

• Key DOASu control points:• Dewpoint and supply air temperature after each component

• Supply and exhaust fan volume and lift, energy use• Component on/off, setpoint, and operating status• Discharge supply OA dry bulb and dewpoint

DOAS Unit (DOASu)


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DOASu Control (cont.)

• DOASu can have upwards of 50 sensor and control points. Some have more than 100. DOASu factory‐mounted‐controls cost has come down. Called smart sensors. (Similar in concept to sensors and controls being added to cars.)

• Key point—DOASu can be 50% of building tonnage and operationally is often the HVAC largest energy user. Extra control points may have value.

DOAS Unit (DOASu)


121DOAS Unit (DOASu)© 2020 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

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• Moisture removal efficiency (MRE) ‐ unit dehumidification efficiency. Capacity (lb water/h divided by the power input to the unit (kW)

• Integrated seasonal moisture removal efficiency (ISMRE): seasonal efficiency of the four dehumidification moisture removal efficiency (MRE) ratings expressed in (lb of moisture/kWh)

• Integrated seasonal coefficient of performance (ISCOP): seasonal efficiency number of two COP values for the heating season of a DX‐DOAS unit water or air source heat pump, expressed in W/W.

AHRI Standard 920-2015

DOAS Unit (DOASu)© 2020 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

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AHRI Standard 920‐2020 Rating Standard

DOAS Unit (DOASu)

Supply Air Dew Point Temperature. After establishing Standard Airflow, Supply Air dew point temperature be 55.0 ºF + 2.31 × (14.7 – Pbar) or below for all Standard Rating Conditions A through D.

Condition OA Dry Bulb OA Wet BulbA 95 78B 80 73C 70 66D 63 59


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Mixed‐Air Systems—The Other Choice

AHU or Rooftop

Variable Air Volume

OA

Terminal ReheatDouble DuctMultizoneSingle Zone

Single Path—Must Choose Temperature or Humidity

VAV DOAS ‐Mixed‐Air VAV Comparison


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Mixed‐Air SystemReturn air and outdoor air is mixed before cooling/heating coil (e.g., HVAC). Typical rooftop or unit vent. Series airflow system

DOAS SystemOutdoor air goes directly to zone; parallel units handle space temperature control. Parallelairflow system

DOAS ‐Mixed‐Air VAV Comparison


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Mixed‐Air Systems

• OA mixes with return air• Requires full amount of supply air to be dehumidified

• Air stratification in “mixing box” can be a problem, especially in CW air handlers

• Can have air economizer; requires full relief fan system, a cost not required with DOAS

AHU or Rooftop

Variable Air Volume

OA

Constant Volume



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VAV Mixed‐Air Systems

• Humidity control is generally ignored. Implied.

• Some zones need higher OA amounts to properly ventilate other zones per Vpz factor

• Complex controls if highly efficient to meet latest codes

AHU or Rooftop

Variable Air Volume

OA



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2015 Washington State Code AllowsHigh‐Performance VAV System in Lieu of DOAS

• 16 criteria, which include:• Must have air‐side economizer and DDC controls• If zone is >2500 cfm must have

• System OA flow measurement with VAV terminal feedback reset• VAV terminal supply airflow measurement

• Primary supply air reset based on zone feedback• Spaces larger than 150 ft2 and >25 people/1000 ft2 must have VAV terminal with CO2 demand reset and occupancy sensor

• Computer, server rooms must have separate system• Hydronic: staged chillers or storage, >90% efficiency gas boilers• Fan‐powered motors—ECM with 66% minimum turndown• Fault management, monthly VAV terminal diagnostic checks• Designed and configured to ASHRAE Guideline 36



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DOAS or 100% OA Economizer?

• Question ‐ Does the application need a 100% economizer with its

• related cost• control complexity • potential loss of good humidity control?

• A significant fraction of the expected benefits of airside economizers are not borne out in practice because of improper installation, poor relief or operation, stratification/frozen coil, and equipment failure over time. (Stan Mumma)



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100% OA Economizers

• OA Economizers controlled by dry‐bulb temperature (particularly those using temperature reset) can introduce excess humidity, which can lead to added energy consumption, uncomfortable spaces, and IAQ problems (Mumma 2005a, 2006).

• Mixed air system solution: Use coil dew‐point control; monitor outdoor air dew point. Set a dew point limit on the outdoor air economizer (no extra outdoor air above a 55°F dew point. Reset coil SA to hold space dewpoint. Very hard to do in a VAV system

• Remember 100% relief is needed to make economizers work—adds first cost.



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Getting Code Acceptance

• State code mandates DOAS as the system of choice. • 90.1 Exception 1—individual fan‐cooling units have a supply capacity less than the minimum listed in Table 6.5.1‐12. (Allocate DOAU fan energy to the local H/C units to meet Table 6.5.1.)

• 90.1 Exception 10—For comfort cooling where the cooling efficiency meets or exceeds the efficiency improvement requirements in Table 6.5.1‐2. (May require modeling.)



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• DOAS is input as a separate ventilation system• Generally modeled as a part of each H/C thermal zone

• Ventilation (DOAS) equipment assigned to zones• DOAS system performance calculated first and passed on to each zone. H/C system by zone calculated next. Sum results.

• Program needs to model all types of DOAS units under consideration

• Program should model structure thermal and humidity capacitance

How to do DOAS Energy Modeling



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Conclusion

• Remember, DOAS can offer the following benefits:• Good IAQ with low energy use

• Assured ventilation performance• Excellent humidity and building pressure control

• In general, easy to understand controls • Fairly competitive life‐cycle‐based first cost

Congratulations to those folks already designing/building/using DOAS!


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Would like to recognize three experts for their early efforts in championing DOAS

ASHRAE Fellow Dr. Stan Mumma, Ph.D. P.E., ASHRAE Fellow Donald P. Gatley, P.E.

Bill Coad, P.E.

Acknowledgements:


135© 2020 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

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