1 Scope
NOTE This specification covers factory-assembled dedicated outdoor air systems (DOAS) that draw 100% outdoor air, recover energy from the building exhaust, deep-cool and dehumidify the ventilation air to a controlled leaving dewpoint, reheat and temper the supply air as required, and deliver it to occupied spaces or to the inlets of a parallel sensible-conditioning system. (1.1)
NOTE Equipment covered includes the unit casing, the outdoor air and exhaust air fans, the air-to-air energy recovery section, the cooling and dehumidification section and its cooling source, the reheat section, the heating section, filtration, dampers, drain pans, and the factory controls that sequence dehumidification and reheat. (1.2)
NOTE Both indoor and outdoor (rooftop or grade-mounted) arrangements are addressed. (1.3)
NOTE A DOAS exists to decouple the building's latent (moisture) load, handled by the ventilation air at a controlled low dewpoint, from the sensible (temperature) load handled by the parallel zone system, because the two loads do not track each other and a single mixed-air coil cannot serve both without overcooling and wasteful reheat. (1.4)
NOTE The DOAS takes responsibility for the entire latent load of the building by delivering air at a low, controlled dewpoint, and the parallel sensible system handles only the dry sensible load of each space without ever having to dehumidify, which is why the dewpoint of the supply air, not its dry-bulb temperature alone, is the controlling design parameter. (1.5)
NOTE Air-to-air energy recovery is mandatory in most climates and at most ventilation rates under ASHRAE 90.1, and the recovery device's effectiveness, frost behavior, and cross-leakage are central engineering decisions, not accessories. (1.6)
NOTE Achieving a low leaving dewpoint requires cooling the air well below the desired supply temperature and then reheating it, and how that reheat is supplied (recovered "free" reheat or purchased energy) is the second central design decision. (1.7)
1.8Equipment shall comply with AHRI 920 for performance rating of DX and heat-pump DOAS units, AHRI 1060 for air-to-air energy recovery ventilation equipment, AHRI 430 and AHRI 410 where central-station fans and coils are used, ASHRAE 62.1 for ventilation rates and air-stream classification, ASHRAE 90.1 for energy recovery and fan power, ASHRAE 55 for occupant comfort, the applicable AMCA fan and damper/louver standards, NFPA 90A for fire and smoke safety, and the applicable UL listing for electrical and refrigerant safety.
NOTE The boundary of work under this standard is the DOAS unit and its factory components, from the outdoor air intake and exhaust air connections to the supply air discharge connection. (1.9)
NOTE Supply and exhaust ductwork is covered by
Hvac Ductwork; the diffusers, grilles, and registers that distribute DOAS air are covered by
Hvac Air Distribution Devices; the parallel sensible terminal equipment is covered by its own standards; field refrigerant piping for split DX arrangements is covered by
Refrigerant Piping; chilled water and hot water piping to coils is covered by the hydronic piping standard; and controls programming, graphics, and trending are covered by
Building Automation System.
(1.10) 2 Referenced Standards
2.1Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
2.2Where conflicts exist between referenced standards, the adopted energy and mechanical codes, the equipment manufacturer's instructions, or the contract documents, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
2.3 Referenced standards list
| Standard |
Title |
| ANSI/AHRI 920 (I-P) |
Performance Rating of DX-Dedicated Outdoor Air System Units |
| ANSI/AHRI 1060 (I-P) |
Performance Rating of Air-to-Air Exchangers for Energy Recovery Ventilation Equipment |
| ANSI/AHRI 430 (I-P) |
Performance Rating of Central Station Air-handling Unit Supply Fans |
| ANSI/AHRI 410 |
Performance Rating of Forced-Circulation Air-Cooling and Air-Heating Coils |
| ANSI/ASHRAE 51 / ANSI/AMCA 210 |
Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating |
| ANSI/AMCA 300 |
Reverberation Room Methods of Sound Testing of Fans |
| ANSI/AMCA 500-L |
Laboratory Methods of Testing Louvers for Rating |
| ANSI/AMCA 500-D |
Laboratory Methods of Testing Dampers for Rating |
| ANSI/ASHRAE 52.2 |
Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size |
| ANSI/ASHRAE 62.1 |
Ventilation and Acceptable Indoor Air Quality |
| ANSI/ASHRAE 55 |
Thermal Environmental Conditions for Human Occupancy |
| ANSI/ASHRAE/IES 90.1 |
Energy Standard for Buildings Except Low-Rise Residential Buildings |
| ANSI/ASHRAE 170 |
Ventilation of Health Care Facilities (where DOAS serves health-care spaces) |
| ASHRAE Guideline 36 |
High-Performance Sequences of Operation for HVAC Systems |
| NFPA 90A |
Standard for the Installation of Air-Conditioning and Ventilating Systems |
| UL 1995 |
Heating and Cooling Equipment (where accepted by AHJ) |
| UL 60335-2-40 |
Safety of Household and Similar Electrical Appliances — Particular Requirements for Heat Pumps, Air-Conditioners, and Dehumidifiers |
| SMACNA HVAC Duct Construction Standards |
SMACNA HVAC Duct Construction Standards — Metal and Flexible |
| ASHRAE Handbooks |
HVAC Systems and Equipment, Fundamentals, Applications |
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following for the Engineer's review and acceptance before equipment is ordered.
3.1.2Fabrication and procurement shall not proceed until action submittals have been reviewed and returned.
3.1.3Submittals shall be internally coordinated with the parallel sensible system and with the building automation system before any item is submitted.
3.1.4 Submit the following action submittals:
- Manufacturer's product data for each DOAS unit, including model designation, an airflow schematic showing the sequence of energy recovery, cooling, reheat, heating, and filter sections, and the component schedule
- AHRI 920 certified performance ratings for DX and heat-pump units, including the integrated seasonal moisture removal efficiency (ISMRE2, or ISMRE270 where the unit reheats to a neutral leaving temperature with supplemental heat) and, for heat-pump units, the integrated seasonal coefficient of performance (ISCOP2)
- AHRI 1060 certified performance ratings for the energy recovery component, including sensible and latent effectiveness at 100% and 75% of rated airflow for both summer and winter conditions, the exhaust air transfer ratio (EATR), and the outdoor air correction factor (OACF)
- A psychrometric analysis for each unit plotting the air state points (entering outdoor air, after energy recovery, after the cooling/dehumidification coil, and supply air) at the cooling/dehumidification design condition, and the corresponding heating-season state points, with the leaving dewpoint and supply air temperature at each
- Capacity data at the cooling design condition and at the specified part-load (turndown) condition, demonstrating that the unit holds the leaving dewpoint setpoint at reduced ventilation flow and reduced load, where part-load dehumidification is required
- Fan performance curves at the design operating point per AHRI 430 / AMCA 210, with power and efficiency, and octave-band sound power levels per AMCA 300 for both supply discharge and casing radiation
- Coil selection data per AHRI 410 for cooling, reheat, and heating coils, with entering and leaving conditions, face velocity, rows, fin spacing, and air- and water-side pressure drop
- Filter section data with MERV rating certified per ASHRAE 52.2, initial and final resistance, and frame and seal construction
- Casing construction details, panel R-value, interior liner material, and, for outdoor units, the weatherproofing and finish system
- Energy recovery frost-control description (the method, the outdoor temperature at which it activates, and its effect on ventilation delivery during frost control)
- A controls points list and a written sequence of operation describing dehumidification control, reheat control, supply air temperature/dewpoint reset, energy recovery bypass and frost control, and the interface to the parallel sensible system and the Building Automation System
- Electrical data: voltage, phase, minimum circuit ampacity, maximum overcurrent protection, and the refrigerant type and charge for DX and heat-pump units
☐ Product data and unit airflow schematic
☐ AHRI 920 performance ratings (ISMRE2 / ISMRE270 / ISCOP2)
☐ AHRI 1060 energy recovery ratings (effectiveness, EATR, OACF)
☐ Psychrometric state-point analysis (design and heating)
☐ Capacity at design and part-load dehumidification
☐ Fan curves and AMCA 300 sound power levels
☐ Coil selection data (AHRI 410)
☐ Filter MERV certification (ASHRAE 52.2)
☐ Casing construction and weatherproofing details
☐ Energy recovery frost-control description
☐ Controls points list and sequence of operation
☐ Electrical data and refrigerant charge
3.2 Closeout Submittals
3.2.1The Contractor shall provide the following at or before substantial completion:
- Operation and maintenance manuals for each unit and all major components, organized with a table of contents
- As-built unit configuration and as-built controls points list reflecting any field modifications
- Factory test reports for each unit
- Startup and commissioning records, including the functional test of dehumidification, reheat, energy recovery frost control, and the interlock to the parallel sensible system
- Refrigerant charging records (DX and heat-pump units), documenting refrigerant type and total charge
- Energy recovery component maintenance data, including wheel drive belt and seal replacement intervals for rotary devices, and core cleaning procedure for fixed-plate devices
- Filter media installed at substantial completion with MERV certification and installation date tags
- Warranty documentation from the manufacturer and from any compressor and energy recovery sub-suppliers
☐ Operation and maintenance manuals
☐ As-built unit configuration and controls points list
☐ Factory test reports for each unit
☐ Startup and commissioning records
☐ Refrigerant charging records (DX and heat-pump units)
☐ Energy recovery component maintenance data
☐ Filter media with MERV certification and date tags
☐ Warranty documentation (manufacturer and sub-suppliers)
4 Quality Assurance
4.1 Manufacturer Qualifications
4.1.1DOAS units shall be manufactured by a company with a minimum of ten years of continuous experience designing and fabricating packaged ventilation and air-conditioning equipment.
4.1.2The manufacturer shall maintain a quality management system.
4.1.3The manufacturer shall be able to provide replacement parts and factory service support for a minimum of fifteen years after the date of manufacture.
4.2 Single-Source Responsibility
4.2.1For each DOAS unit, the casing, fans, energy recovery section, cooling and dehumidification section, reheat and heating sections, filters, and dampers shall be provided as a coordinated factory assembly by or through the DOAS manufacturer.
NOTE The interaction between the energy recovery device, the dehumidification coil, and the reheat source is what determines whether the unit holds its leaving dewpoint, and resolving that interaction at the factory rather than assembling independent components in the field is the reason for the single-source requirement. (4.2.2)
4.3 AHRI Certification
☐ DOAS unit performance per AHRI 920 (ISMRE2 / ISCOP2)
☐ Energy recovery component per AHRI 1060
☐ Supply fan performance per AHRI 430
☐ Coils per AHRI 410
4.3.1DX and heat-pump DOAS units shall be rated under the AHRI Certification Program for DX-Dedicated Outdoor Air System Units per AHRI 920, and published ISMRE2, ISMRE270 (where applicable), and ISCOP2 ratings shall bear the AHRI certification mark.
4.3.2Air-to-air energy recovery components shall be rated under the AHRI Certification Program for energy recovery ventilation equipment per AHRI 1060, and the published effectiveness, EATR, and OACF shall bear the AHRI certification mark.
4.3.3Where central-station fans and coils are used, ratings shall conform to AHRI 430 and AHRI 410.
4.3.4AHRI-certified ratings, not generic catalog values, shall be the basis of selection.
4.4 Factory Run Test
4.4.1Each unit shall receive a factory functional run test before shipment, operating the refrigeration or heat-pump circuit (where integral), the fans, the energy recovery drive, and the dehumidification and reheat sequence, and confirming that the unit produces the specified leaving dewpoint at the test condition.
4.4.2A factory test report shall be submitted at closeout.
4.5 NRTL Listing
4.5.1The complete DOAS assembly, including all electrical and (where integral) refrigerant components, shall be listed and labeled by a Nationally Recognized Testing Laboratory to UL 60335-2-40, or to UL 1995 where the Authority Having Jurisdiction continues to accept that listing for equipment manufactured before the transition date.
5 Environmental and Service Conditions
5.1The unit shall be selected and rated for the design conditions at the installation site.
5.3 Design Outdoor Air Condition
Per ASHRAE 0.4% dehumidification design dewpoint for project location (see mechanical schedules)
5.3.1Because the DOAS conditions 100% outdoor air, it shall be selected at the project's dehumidification design condition — the design dewpoint or design wet-bulb condition from ASHRAE Fundamentals for the project location — and not merely at the cooling dry-bulb design.
NOTE The dehumidification design condition, which typically pairs a moderate dry-bulb with a high coincident moisture content, is the condition that sizes the cooling coil and the reheat, and it is frequently more demanding for a DOAS than the peak dry-bulb hour. (5.3.2)
5.4 Supply Air Leaving Targets
NOTE The controlling leaving condition is the supply air dewpoint, because the DOAS is selected to absorb the building latent load. (5.4.1)
4058
40454850525558
Default: 52 °F
○ Neutral supply — reheat ventilation air to near space-neutral temperature (68-72°F); space load carried entirely by parallel sensible system (standard)
○ Cold (DOAS-to-space) supply — deliver cool, dry air directly to the space to offset part of the sensible load; reheat only to the dewpoint-control minimum
○ Reset supply temperature — neutral in cooling season, warm (tempered) in heating season
5.4.2The leaving dewpoint setpoint shall be low enough that the DOAS supply air, after mixing with space air and absorbing the space moisture gains, holds the space relative humidity within the design range under ASHRAE 55.
NOTE A leaving dewpoint of about 50°F to 55°F is typical for comfort applications; lower dewpoints are specified for spaces with strict humidity limits. (5.4.3)
NOTE A neutral-supply DOAS delivers ventilation air at a temperature close to the space setpoint so it neither heats nor cools the space, and it is the most common and most forgiving arrangement because the parallel system and the DOAS never fight each other. (5.4.4)
NOTE A cold-supply (DOAS-to-space) arrangement intentionally delivers cool dry air to offset part of the space sensible load, which reduces the parallel system's capacity but requires careful diffuser selection to avoid drafts and requires that the parallel system not overcool and drive the space humidity back up; this strategy decouples humidity control from the parallel system only if the DOAS dewpoint is held independent of its dry-bulb, which is exactly what the reheat section provides. (5.4.5)
5.5 Unit Installation Location
Outdoor — rooftop on roof curb
Outdoor — grade-mounted on housekeeping pad
Indoor — mechanical room or penthouse
6 Unit Configuration and Arrangement
6.1The energy recovery type, the cooling source, the dehumidification and reheat method, and the supply strategy are interdependent and shall be selected together and reviewed against the psychrometric analysis as a set.
6.2 Airflow Capacity
50060000
Default: 4000 cfm
6.2.2The ventilation rate shall be established by the ASHRAE 62.1 ventilation rate procedure for the served spaces.
6.3 External Static Pressure
0.54
0.50.7511.251.522.534
Default: 1.5 in. wg
6.4 Configuration Summary
Enthalpy (total energy) rotary wheel
Sensible-only rotary wheel with purge sector
Fixed-plate enthalpy (membrane) core
Fixed-plate sensible (aluminum) core
Heat-pipe (sensible)
Runaround coil loop (sensible, separated air streams)
None (energy recovery not required by code for this unit)
Integrated DX (packaged compressorized cooling)
Air-source heat pump (DX cooling and heating)
Water-source heat pump (DX, condenser water loop)
Chilled water coil (central plant)
Hot-gas (refrigerant) reheat — recovered condenser heat (DX/heat-pump units)
Modulating hot-gas reheat — variable leaving-temperature control
Wraparound heat pipe — passive precool/reheat around the cooling coil
Wraparound runaround coil — pumped passive reheat
Dehumidification (passive desiccant) wheel — sensible reheat from process air
Hot water or electric reheat coil — purchased reheat energy
None — supply delivered cold (cold-supply strategy)
Heat pump (integral, air-source or water-source)
Indirect gas-fired furnace
Electric resistance heating coil
Hot water heating coil (central plant)
None — supplemental heating provided downstream
○ Neutral supply (standard)
○ Cold supply (DOAS-to-space)
○ Reset (neutral cooling, tempered heating)
MERV 8 — minimum for ventilation air
MERV 11 — enhanced commercial
MERV 13 — fine particulate (PM2.5), standard for occupied ventilation
MERV 14 — superior filtration
○ Direct-drive plenum (plug) fan with ECM or VFD-driven motor (standard)
○ Direct-drive backward-curved plenum fan, VFD-driven
○ Belt-driven centrifugal fan (legacy / large units)
7 Energy Recovery
NOTE Air-to-air energy recovery is the feature that makes a DOAS practical. (7.1)
7.2Recovering most of the conditioning load from the building exhaust, which is already near the space condition, cuts the cooling and heating capacity the unit must purchase by half or more.
7.3ASHRAE 90.1 requires exhaust air energy recovery for most ventilation systems based on the percentage of outdoor air and the design supply airflow, with a minimum enthalpy recovery ratio of at least 50% at the design condition for the climate zone.
7.4Recovery shall be provided wherever required by the adopted energy code; the only DOAS units that omit it are those that fall below the code threshold or that qualify for a specific exception.
7.5 Recovery Device Type — Sensible vs. Total
○ Total energy (sensible + latent) — recover moisture; reduces coil latent load (standard for comfort DOAS)
○ Sensible only — temperature recovery, no moisture transfer (contaminated exhaust or no-cross-transfer applications)
NOTE Total-energy (enthalpy) devices — rotary enthalpy wheels and fixed-plate membrane cores — transfer both heat and moisture between the air streams; in summer they pre-dry the incoming air using the drier exhaust, and in winter they return moisture to the incoming air, reducing dry indoor conditions and the need for humidification. (7.5.1)
7.5.2Sensible-only devices — heat pipes, sensible plates, sensible wheels, and runaround loops — transfer heat only, and a sensible device shall be selected where moisture transfer is undesirable or prohibited, where the exhaust is contaminated and any cross-transfer to the supply is unacceptable, or where the design intent is to maximize the moisture removed by the cooling coil rather than recover it.
7.6 Cross-Contamination and EATR
NOTE AHRI 1060 quantifies cross-transfer from the exhaust stream into the supply stream as the exhaust air transfer ratio (EATR) and the outdoor air correction factor (OACF). (7.6.1)
Class 1 — general comfort exhaust (any recovery type acceptable)
Class 2 — moderate-contaminant exhaust (EATR ≤ 10%; purge required on wheels)
Class 3/4 — high-contaminant exhaust (separated-stream sensible recovery only)
○ Purge sector required (Class 2 exhaust or low-EATR requirement)
○ Purge sector not required (Class 1 exhaust)
○ Not applicable (non-rotary recovery device)
7.6.2Where the exhaust is ASHRAE 62.1 Class 1 air (general comfort exhaust), a small EATR is acceptable.
7.6.3Where the exhaust contains Class 2 air, ASHRAE 62.1 limits the EATR to no more than 10% and requires that any transferred air be accounted for in the ventilation calculation.
7.6.4Where the exhaust is Class 3 or Class 4, energy recovery that allows cross-transfer is prohibited and a sensible device with fully separated streams (heat pipe or runaround loop) shall be used.
7.6.5Rotary wheels intended for Class 1/Class 2 service shall include a purge sector, which uses a small pressure-driven flow to sweep carryover air back to the exhaust before the wheel rotates into the supply stream, reducing EATR.
7.7 Frost Control
NOTE In cold climates the warm, moist exhaust gives up its moisture as it is cooled by the incoming air, and that moisture can freeze on the recovery surface, blocking the exhaust path and destroying recovery. (7.7.1)
Modulating wheel speed (rotary wheels)
Timed defrost cycle (rotary wheels)
Preheat of entering outdoor air upstream of recovery (plates, heat pipes)
Exhaust-side or supply-side bypass damper
Not required (climate does not produce recovery frost)
7.7.2Every recovery device in a freezing climate shall have a defined frost-control strategy.
7.7.3Wheels shall be frost-controlled by modulating wheel speed or by a timed defrost; fixed plates and heat pipes shall be frost-controlled by preheating the incoming air, by an exhaust-side bypass, or by recirculation.
7.7.4The frost-control method, its activation temperature, and its effect on ventilation flow during frost control shall be submitted.
7.8 Recovery Bypass
○ Provide recovery bypass damper (economizer-capable or code-required)
○ No bypass (recovery active whenever the unit runs)
7.8.1A bypass damper around the recovery device shall be provided where the recovery would otherwise be active during economizer hours, or where frost control or the energy code economizer provisions require the recovery to be bypassed.
8 Cooling and Dehumidification
NOTE The cooling section of a DOAS is selected to drive the air to a low leaving dewpoint, which usually means cooling the air below the temperature the space actually wants, then reheating it to the desired supply temperature while the moisture stays removed. (8.1)
NOTE To reach a 50°F to 55°F leaving dewpoint at a humid design condition, the coil leaving dry-bulb must be at or below that dewpoint (the air leaves the coil at or near saturation), which is colder than a comfortable supply temperature. (8.2)
NOTE AHRI 920 rates this behavior directly through its integrated seasonal moisture removal efficiency (ISMRE2, in pounds of moisture removed per kilowatt-hour), and AHRI 920 testing is run to a 55°F leaving dewpoint reference. (8.3)
8.4 Cooling Source
○ Integrated DX — single or multiple compressor stages
○ Integrated DX with variable-capacity (variable-speed or digital) compressor
○ Heat pump — reversible refrigerant circuit (cooling and heating)
○ Chilled water coil — central plant
4055
404548505255
Default: 52 °F
8.4.1The cooling source shall be selected to match the project's energy infrastructure and the unit type.
8.4.2A chilled water DOAS shall obtain its reheat from a separate source, since there is no condenser heat to recover.
NOTE Integrated DX is the most common cooling source for standalone packaged DOAS because it makes recovered reheat available from the condenser, a heat pump adds heating from the same circuit, and a chilled water coil is used where a central chilled water plant exists and the DOAS is one of many loads on it. (8.4.3)
NOTE Variable-capacity compressors are strongly preferred for DOAS because the latent load varies enormously across the year, and a variable-capacity circuit can hold the leaving dewpoint at part-load ventilation rates and at the many mild, humid hours when a fixed-stage unit would short-cycle and deliver wet air. (8.4.4)
8.5 Part-Load Dehumidification
Single stage — full capacity only (small units, mild climates)
Two-stage — approximately 50% capacity at low stage
Variable capacity — continuous modulation to approximately 25% capacity
8.5.1The DOAS shall hold the leaving dewpoint setpoint across the range of operating conditions, not only at the design hour.
8.5.2The unit shall provide dehumidification down to the specified minimum turndown, by compressor staging or variable capacity, and the part-load performance shall be documented in the submittal.
NOTE Many of the wettest hours of the year are mild — a rainy 70°F day at near-saturation carries a large latent load with almost no sensible load — and a unit that can only dehumidify at peak sensible conditions will deliver humid air during exactly those hours. (8.5.3)
8.6 Reheat (Dehumidification Reheat)
NOTE Reheat is the mechanism that makes low-dewpoint dehumidification possible without delivering uncomfortably cold air, and on a neutral-supply unit it is a continuous part of normal operation. (8.6.1)
NOTE The decision is where the reheat energy comes from. (8.6.2)
○ Hot-gas reheat — single-circuit condenser heat to a reheat coil (standard DX)
○ Modulating hot-gas reheat — staged or valved for variable leaving-temperature control
○ Wraparound heat pipe — passive, sealed refrigerant loop around the cooling coil
○ Wraparound runaround coil — pumped glycol loop around the cooling coil
○ Dehumidification (desiccant) wheel — passive sensible reheat from the process air
○ Hot water reheat coil — purchased energy (where recovered reheat unavailable)
○ Electric reheat coil — purchased energy (where recovered reheat unavailable)
5575
55606568707275
Default: 70 °F
8.6.3Recovered reheat shall be used wherever the unit type allows it.
8.6.4Purchased reheat (a hot water or electric coil) shall be limited to applications where no recovered source is available or where the energy code permits it.
NOTE Recovered reheat — hot-gas (refrigerant) reheat that takes heat rejected by the unit's own condenser, a wraparound heat pipe or runaround coil that precools the entering air and reuses that captured heat to rewarm the leaving air, or a passive dehumidification wheel — spends no purchased energy on reheat, whereas purchased reheat spends energy heating air that was just cooled, which the energy code restricts. (8.6.5)
NOTE A wraparound device (heat pipe or runaround coil) improves dehumidification while it reheats: by precooling the entering air before the cooling coil it lets the coil reach a lower dewpoint for the same compressor capacity, and it then uses that captured heat to reheat the leaving air — typically lifting the supply from roughly 48°F to roughly 55°F — with no purchased energy and no controls. (8.6.6)
8.6.7A passive wraparound is largely self-regulating and cannot independently set the supply temperature; where precise leaving-temperature control is needed, modulating hot-gas reheat or a controlled wraparound is required.
9 Heating
9.1In the heating season the DOAS shall temper the cold ventilation air so that it does not chill the spaces or the inlets of the parallel sensible system.
9.2With energy recovery active, much of the heating is recovered from the exhaust and the heating source supplies the remainder; for a neutral-supply unit the heating section warms the air to near the space-neutral temperature, and for a tempered-reset unit it may warm it further to offset part of the space heating load.
Heat pump — integral reversible circuit
Heat pump with auxiliary electric or gas backup for low ambient
Indirect gas-fired furnace (stainless heat exchanger)
Electric resistance coil
Hot water coil — central plant
None — tempering by energy recovery only
6095
60656870758595
Default: 70 °F
9.3Air-source heat-pump DOAS units in cold climates shall include auxiliary heat sized to carry the unit through the heating design condition and any heat-pump defrost cycles.
9.4Where a hot water coil is the heating source in a freezing climate, freeze protection shall be provided as specified under Casing, Fans, Coils, Filters.
10 Casing, Fans, Coils, Filters
10.1 Casing
○ Double-wall, injected foam insulation (standard)
○ Double-wall, injected foam with thermal break at panel edges (cold climate / high-humidity)
R-6 to R-8 (standard commercial)
R-10 to R-13 (cold climate / outdoor DOAS)
☐ Sloped/pitched weather top panels
☐ Drainable intake/exhaust louvers (AMCA 500-L) with bird screen
☐ Drainable base rail and roof curb seal
☐ Gasketed access doors with hold-open and lockable latches
☐ Two-coat corrosion-inhibiting, UV-resistant exterior finish
10.1.1The casing shall be double-wall construction throughout.
10.1.2Panel insulation R-value shall be appropriate for the climate, with cold-climate and outdoor units warranting higher-R panels to prevent exterior condensation and limit conduction loss.
NOTE Double-wall construction is required because a DOAS handles cold, dehumidified air on the supply side and cold outdoor air on the entering side, and condensation will form on any cold single-wall panel. (10.1.3)
10.1.4Outdoor and rooftop units shall be provided with a factory weatherproof casing including pitched or sloped top panels, weather hoods and drainable louvers (rated per AMCA 500-L) over the outdoor air intake and exhaust, bird screens, a drainable base rail, gasketed access doors with hold-open hardware, and corrosion-resistant exterior hardware.
10.1.5The exterior finish on outdoor and rooftop units shall be a two-coat corrosion-inhibiting and UV-resistant system.
10.2 Fans
○ ECM (electronically commutated) motor — integral variable speed (standard for smaller units)
○ Premium-efficiency motor with variable frequency drive (larger units)
10.2.1Supply and exhaust fans shall be direct-drive plenum (plug) fans driven by electronically commutated (ECM) motors or by variable-frequency-driven premium-efficiency motors, so that airflow can be held constant as filters and the recovery device load and so that fan power complies with ASHRAE 90.1.
10.2.2Fan and airflow performance shall be rated per AHRI 430 / AMCA 210.
10.2.3Fans shall be selected at the design airflow and external static pressure with capacity to maintain design airflow at the loaded-filter and frosted-recovery condition.
10.3 Coils
○ Aluminum fins (standard inland environment)
○ Coated aluminum fins (coastal/corrosive — within ~3 mi of saltwater or industrial air)
350550
350400450500550
Default: 450 fpm
10.3.1Cooling, reheat, and heating coils shall be rated and certified per AHRI 410, with tube and fin materials suited to the airstream.
10.3.2Coastal and corrosive-environment units shall use corrosion-resistant fin treatment, because the cooling coil runs wet continuously and the entire airstream is outdoor air.
10.3.3Cooling coil face velocity shall be selected to limit moisture carryover off a continuously wet coil, with a face velocity at or below 500 ft³/min per ft² (cfm/ft²) of coil face recommended for DOAS, lower than a comparable mixed-air unit because the DOAS coil is always operating wet at design.
10.4 Drain Pan and Condensate
○ Type 304 stainless steel (standard)
○ Type 316 stainless steel (coastal/chemical environment)
10.4.1A sloped, fully draining stainless steel drain pan shall be provided under the cooling coil and under any other condensing section.
10.4.2The drain pan shall fully drain and shall be Type 304 stainless steel minimum (Type 316 in coastal or chemical environments).
10.4.3A condensate trap with seal depth matched to the section's static pressure shall be provided so the trap is neither blown nor sucked dry.
NOTE Because a DOAS coil runs wet continuously, standing water is both a Legionella risk and a corrosion risk. (10.4.4)
10.5 Hot Water Coil Freeze Protection
Serpentine-element low-limit thermostat (full-face coverage)
Glycol solution in the coil loop
Not applicable (no hydronic coil, or non-freezing climate)
10.5.1Where a hot water heating or reheat coil is used in a freezing climate, freeze protection shall be provided, and protection shall be a serpentine-element low-limit (freeze-stat) covering the coil face, glycol in the coil loop, or a face-and-bypass arrangement, as appropriate.
NOTE Freeze protection is required because the coil sees full design outdoor air with no warm return air to buffer it. (10.5.2)
10.6 Filtration
MERV 7 — synthetic media panel pre-filter (standard)
MERV 8 — pleated pre-filter
None (single-stage filtration)
10.6.1Final filtration shall be MERV 13 for occupied-space ventilation air unless the application requires higher efficiency, with a pre-filter to extend final-filter life.
10.6.2Filter frames shall seal positively to prevent bypass, which would void the rated efficiency.
11 Controls and BAS Integration
11.1The DOAS is a sequence machine: it must coordinate energy recovery, deep cooling, dehumidification, and reheat to hold a leaving dewpoint, and it must coordinate with the parallel sensible system so the two never undo each other's work.
BACnet MS/TP (RS-485)
BACnet IP (Ethernet)
Modbus RTU (RS-485)
Modbus TCP/IP (Ethernet)
Hardwired I/O only (no network)
11.2The unit shall be furnished with factory controls and a labeled terminal interface, and shall integrate with the Building Automation System. 11.3Sequences should follow the intent of ASHRAE Guideline 36 where applicable.
11.4 Control of Dehumidification and Reheat
○ Leaving air dewpoint sensor (direct dewpoint control)
○ Leaving air relative humidity and temperature (computed humidity ratio)
11.4.1The primary control loop shall sequence the cooling coil to maintain the leaving dewpoint (or leaving humidity ratio) setpoint.
11.4.2The reheat shall be sequenced independently to maintain the supply dry-bulb setpoint.
NOTE Holding these two loops independent is what lets the DOAS control humidity without coupling it to space temperature — the defining behavior of a decoupled-latent system. (11.4.3)
11.5 Supply Reset
Fixed leaving dewpoint and supply temperature setpoints
Dewpoint reset on space humidity; fixed supply temperature
Full reset of dewpoint and supply temperature by BAS
11.5.1Supply air temperature and dewpoint may be reset by the BAS based on outdoor conditions, space humidity, or the load on the parallel sensible system, to save reheat energy when the full dehumidification capacity is not needed.
11.6 Interlock with Parallel Sensible System
11.6.1The DOAS and the parallel sensible system shall be interlocked through the BAS so that the parallel system does not satisfy space temperature in a way that drives space humidity up, and so that the DOAS ventilation runs whenever the spaces are occupied.
11.6.2Occupancy scheduling, demand-controlled ventilation reset (where CO2 sensing is used), and the night/unoccupied mode shall be coordinated across both systems.
11.7 Minimum Control Points
☐ Outdoor air temperature and humidity sensors
☐ Supply air temperature sensor
☐ Supply air dewpoint or humidity sensor
☐ Cooling coil leaving air temperature sensor
☐ Compressor / cooling capacity control output
☐ Reheat control output (hot-gas valve or coil)
☐ Heating control output
☐ Energy recovery drive / speed and bypass control
☐ Energy recovery frost-control status
☐ Supply and exhaust fan speed control and status
☐ Filter differential pressure switch or transmitter
☐ Outdoor and exhaust air damper actuators and end switches
☐ Smoke detector(s) per NFPA 90A
☐ Condensate overflow / drain-pan high-level switch
☐ Unit alarm summary to BAS
12 Testing
12.1 Factory Testing
○ Standard production test plus functional run test (certified report submitted)
○ Witnessed factory test — provide 10 business days notice
12.1.1Each unit shall receive the manufacturer's standard production tests plus a functional run test confirming that the refrigeration or heat-pump circuit, fans, energy recovery drive, and the dehumidification-and-reheat sequence operate and that the unit produces the rated leaving dewpoint at the factory test condition.
12.1.2A drain-pan tightness test and an electrical continuity and insulation test shall be included.
12.1.3A factory test report shall be submitted at closeout.
12.2 Field Testing and TAB
☐ Ventilation and exhaust airflow set and documented (TAB)
☐ Leaving dewpoint setpoint held at test condition
☐ Reheat maintains supply temperature setpoint
☐ Energy recovery effectiveness / airflow ratio verified
☐ Frost-control sequence verified
☐ Parallel-system interlock and occupancy modes verified
12.2.2The TAB agent shall set and document the design outdoor (ventilation) airflow and the exhaust airflow through the recovery device, and shall confirm that the supply-to-exhaust airflow ratio is within the range the energy recovery device was rated for, since EATR and effectiveness are airflow-ratio dependent.
12.2.3Functional testing shall verify that the unit holds the leaving dewpoint setpoint, that reheat maintains the supply temperature setpoint, that energy recovery frost control operates, and that the interlock to the parallel sensible system behaves as designed.
13 Installation
13.1 Rigging and Mounting
13.1.1The unit shall be rigged and set per the manufacturer's instructions using furnished rigging points; cables shall not bear on casing panels.
13.1.2Rooftop units shall be set on a level, gasketed roof curb that matches the unit supply, exhaust, and intake openings; grade-mounted units shall be set on a level housekeeping pad.
13.1.3After setting, the unit shall be verified level so the drain pan slopes to drain.
13.1.4Vibration isolation shall be provided as required by the unit type and location.
13.2 Clearances
13.2.1Service clearances shall be maintained per the manufacturer's instructions for energy recovery wheel or core removal, coil pull, filter access, and compressor and control service, and for the airflow clearance at outdoor intakes and exhausts.
13.3 Outdoor Air Intake Location
13.3.1The outdoor air intake shall be located to draw clean air and shall comply with the ASHRAE 62.1 separation distances from exhaust outlets, plumbing vents, cooling towers, loading docks, and other contaminant sources.
13.3.2The intake and the unit's own exhaust discharge shall be separated so the exhaust is not short-circuited back into the intake.
NOTE The DOAS conditions 100% outdoor air, so a poorly located intake contaminates the entire ventilation supply. (13.3.3)
13.4 Condensate
13.4.1The condensate drain shall be trapped with a seal depth matched to the unit static pressure, run to an approved point of disposal with continuous slope, and provided with a union for trap cleaning.
13.4.2Where the unit is in a freezing location, the condensate drain shall be protected from freezing.
13.5 Duct, Intake, and Exhaust Connections
13.5.1Supply and exhaust ductwork shall connect through flexible connectors and shall be independently supported so duct weight is not carried by the casing.
14 Startup and Commissioning
Manufacturer startup with TAB verification
Manufacturer startup, TAB, and functional performance testing by commissioning agent (standard)
Manufacturer startup, TAB, FPT, and deferred seasonal (wet-condition) dehumidification test
14.1Startup shall be performed by a factory-trained representative of the manufacturer.
14.2The startup sequence shall verify refrigerant charge (DX and heat-pump units), fan rotation and airflow, energy recovery drive operation and purge (where provided), the dehumidification-and-reheat sequence holding the leaving dewpoint, the heating sequence, the frost-control sequence, and the reporting of all BAS points.
14.4Commissioning shall include a wet-condition test of dehumidification where the season permits, or a documented plan for deferred seasonal testing where it does not.
15 Warranty
15.1 Equipment Warranty
1 year parts and labor
2 years parts and labor
5 years parts, 1 year labor
15.1.1The manufacturer shall warrant the complete unit against defects in materials and workmanship from the date of substantial completion.
15.2 Compressor Warranty
○ Standard — 1 year same as equipment
○ Extended — 5 years on compressor
15.2.1The compressor(s) in DX and heat-pump units shall carry an extended warranty.
NOTE An extended compressor warranty is required because compressor replacement is the most consequential failure on a packaged DOAS. (15.2.2)
15.3 Energy Recovery Component Warranty
○ 1 year (minimum)
○ 5 years on energy recovery wheel/core and drive
15.3.1The energy recovery device — the wheel and its drive and seals, or the fixed-plate core, or the heat-pipe/runaround assembly — shall carry a warranty against defects and against loss of rated effectiveness from a defect.
15.4 Installation Warranty
15.4.1The Contractor shall warrant the installation workmanship, including the curb and intake/exhaust connections, condensate, flexible connections, and controls terminations, for one year from substantial completion.
16 Spare Parts
☐ One complete set of spare filters (each type and MERV)
☐ One energy recovery wheel drive belt (rotary units)
☐ One set of energy recovery wheel seals (rotary units)
☐ One set of access door gaskets
☐ Set of spare fuses for all fuse types installed
16.1Spare filters shall be stored in sealed manufacturer's packaging labeled with the unit designation, MERV rating, and the installation date of the set in service.
16.2The O&M manual shall include the filter change schedule, the energy recovery maintenance schedule (wheel seal and belt inspection, or fixed-plate core cleaning), and the condensate trap cleaning procedure.