1 Scope
1.1 General
NOTE This standard governs the furnishing, performance, fabrication, and installation of factory-packaged humidifiers and their dispersion assemblies serving ducted air systems and conditioned spaces. (1.1.1)
NOTE Humidifiers add water vapor to an airstream to maintain a minimum space relative humidity, protecting occupants, materials, processes, and the building envelope from the effects of excessively dry air. Dry winter air drives static discharge, respiratory discomfort, dimensional shrinkage of hygroscopic materials, and elevated airborne infection transmission; controlled humidification mitigates each of these. (1.1.2)
NOTE The work of this standard includes the humidifier, its dispersion assembly, integral and field controls, the high-limit safety, and coordination of water supply, drain, electrical service, and fuel or steam utilities. (1.1.3)
1.1.4The humidifier shall be a factory-assembled, listed package selected from a manufacturer's standard production model line.
1.1.5Custom-engineered humidifier assemblies shall not be substituted for a standard cataloged model without the Engineer's written approval.
1.1.6Selection of humidifier technology shall match the project's facility type, infection-control requirements, available utilities, and water quality.
1.1.7Coordination of the supply water source and treatment shall be governed by Hvac Water Treatment where treated water is required. 2 Referenced Standards
2.1 Referenced Standards List
NOTE The following standards are referenced in this document; the edition in force shall be that listed unless the Contract Documents require otherwise. (2.1.1)
| Standard |
Title |
| ANSI/ASHRAE/ASHE 170-2021 |
Ventilation of Health Care Facilities |
| ANSI/ASHRAE 62.1-2022 |
Ventilation and Acceptable Indoor Air Quality |
| ANSI/ASHRAE/IES 90.1-2022 |
Energy Standard for Buildings Except Low-Rise Residential Buildings |
| ANSI/ASHRAE 55-2023 |
Thermal Environmental Conditions for Human Occupancy |
| UL 998 |
Standard for Safety for Humidifiers |
| NFPA 90A-2024 |
Standard for the Installation of Air-Conditioning and Ventilating Systems |
| ASHRAE Handbook — HVAC Systems and Equipment |
Chapter 22, Humidifiers (primary reference for sizing, absorption-distance determination, and selection methodology) |
3 Definitions
3.1 Key Terms
NOTE The following terms are used throughout this standard: (3.1.1)
- Isothermal humidification adds water vapor as steam, releasing latent heat with the vapor so the dry-bulb temperature of the airstream is essentially unchanged.
- Adiabatic humidification atomizes or evaporates liquid water into the airstream; the latent heat of vaporization is drawn from the air, lowering its dry-bulb temperature as humidity rises.
- Absorption distance is the length of straight, unobstructed duct downstream of a dispersion assembly required for injected moisture to be fully absorbed without condensing on duct walls or downstream components.
- Non-wetting delivery means the humidifier is sized and located so that no droplet or condensate wets any downstream surface, coil, filter, damper, or duct wall.
- Conductivity is the electrical conductance of the supply water, expressed in microsiemens per centimeter (µS/cm); electrode humidifiers rely on it to generate heat.
- Cylinder service life is the cumulative operating time after which an electrode steam cylinder's electrodes are consumed or scaled and the cylinder must be replaced.
4 Submittals
4.1 Submission Requirements
NOTE The Contractor shall submit the following action submittals for review before fabrication or ordering: (4.1.1)
4.2 Action Submittals
4.2.1The Contractor shall submit the following action submittals:
- Product data for each humidifier type, including rated capacity, electrical or fuel input, water-quality requirements, and accessories
- Manufacturer's capacity-selection calculation showing peak winter latent load and selected unit size
- Manufacturer's absorption-distance calculation for the dispersion assembly at the design entering-air conditions
- Dispersion-assembly layout drawings showing manifold or lance arrangement and downstream clearance
- Wiring diagrams differentiating factory-installed and field-installed wiring
- Control sequence of operation and points list for facility-management-system integration
- Water, drain, and (where applicable) fuel or steam connection requirements
☐ Product data for each humidifier type
☐ Capacity-selection calculation
☐ Absorption-distance calculation
☐ Dispersion-assembly layout drawings
☐ Wiring diagrams (factory vs. field)
☐ Sequence of operation and points list
☐ Water, drain, fuel/steam connection data
4.3.1The Contractor shall submit the following informational submittals:
- UL 998 listing documentation confirming the listing scope covers the intended installation
- Manufacturer's water-quality specification stating acceptable conductivity, hardness, and treatment range
- For adiabatic units in healthcare, the infection-control compliance narrative required by ASHRAE 170
- For gas-fired units, AGA/CSA certification and flue category designation
- Field quality-control test plan
☐ UL 998 listing documentation
☐ Water-quality specification
☐ Healthcare infection-control narrative
☐ Gas certification and flue category
☐ Field quality-control test plan
4.4 Closeout Submittals
4.4.1The Contractor shall submit the following closeout submittals:
- Operation and maintenance manuals for each humidifier and dispersion assembly
- Startup and commissioning report including measured capacity and high-limit verification
- Cylinder, media, or nozzle replacement schedule with expected service intervals
- Spare-parts list with manufacturer part numbers
- Final as-built control sequence and points list
☐ Operation and maintenance manuals
☐ Startup and commissioning report
☐ Replacement-part service schedule
☐ Spare-parts list
☐ As-built sequence and points list
5 Quality Assurance
5.1 Listings and Qualifications
NOTE Quality-assurance requirements establish the listings, qualifications, and verifications that govern the equipment and its installation. (5.1.1)
5.1.2Each humidifier shall be listed to UL 998 for its rated voltage and mounting configuration.
5.1.3The Contractor shall confirm that the UL 998 listing scope includes duct or plenum mounting where the dispersion assembly is installed inside an air handler or duct.
5.1.4Gas-fired humidifiers shall carry AGA or CSA certification for the fuel and venting category specified.
5.1.5The installer shall be trained or certified by the humidifier manufacturer for the equipment furnished.
5.1.6A single manufacturer shall furnish the humidifier, dispersion assembly, and integral controls as a coordinated package.
5.1.7The manufacturer shall provide the absorption-distance calculation as a stamped or signed engineering submittal for the project's design entering-air conditions.
6 Environmental and Service Conditions
6.1 Selection Criteria
NOTE The humidifier shall be selected for the air, water, and ambient conditions at its point of installation. (6.1.1)
NOTE Space humidity setpoints govern equipment sizing and control tolerance, and differ markedly by facility type. (6.1.2)
6.1.3Patient-care areas shall be maintained within the relative-humidity band required by ASHRAE 170, generally 30% to 60% RH.
6.1.4Operating rooms shall be maintained within the relative-humidity band required by ASHRAE 170, generally 20% to 60% RH, with surgical suites and protective-environment rooms held to a tighter band of 45% to 55% RH where the project program requires.
6.1.5Comfort-driven spaces shall be maintained within the humidity limits of ASHRAE 55, with the dew point not exceeding 62.2°F.
6.1.6The supply-air dew point shall not be allowed to exceed the dew point at which condensation forms on the coldest downstream surface.
6.1.7The peak winter design latent load shall account for envelope infiltration, the moisture deficit of ventilation air, and any process latent demand.
99.6% heating dry-bulb
99% heating dry-bulb
Per drawings
7 Humidifier Type Selection
7.1 Technology Selection
NOTE The choice between isothermal and adiabatic technology is the controlling decision and is driven by facility type, infection control, available utilities, and water quality. Isothermal steam humidifiers do not depress supply-air temperature and are the default for winter heating applications and infection-sensitive environments. Adiabatic humidifiers consume far less primary energy but cool the airstream as they humidify, which must be offset by preheat in heating applications. (7.1.1)
7.1.2The humidifier type shall be selected from a single technology family appropriate to the served space and confirmed against the supply-water quality.
Electrode steam (isothermal)
Resistive-immersion electric steam (isothermal)
Gas-fired steam (isothermal)
Steam-to-steam jacketed (isothermal)
Direct steam injection from central main (isothermal)
High-pressure atomizing (adiabatic)
Ultrasonic (adiabatic)
Wetted-media evaporative (adiabatic)
7.1.3Adiabatic humidifiers shall not be applied to a heating airstream without preheat compensation sized for the evaporative temperature depression.
7.1.4The evaporative cooling effect of high-pressure atomizing and wetted-media units shall be included in the heating-load calculation for the air system.
7.2 Electrode Steam Humidifiers
NOTE An electrode steam humidifier passes current between electrodes immersed in the supply water, using the water's own conductivity as the heating element, and is self-regulating to incoming water conductivity. (7.2.1)
NOTE The electrode cylinder is a consumable; as minerals concentrate, the cylinder reaches end of life and is replaced as a unit. (7.2.2)
7.2.3Electrode humidifiers shall be supplied with potable municipal tap water or softened water within the manufacturer's stated conductivity range, typically 125 to 1,250 µS/cm.
7.2.4Electrode humidifiers shall not be specified where the available supply water is deionized, reverse-osmosis, or otherwise below the manufacturer's minimum conductivity, because the unit will produce no steam.
7.2.5Electrode cylinders shall be the replaceable type with clear end-of-life indication reported to the controls.
Potable tap water
Softened water
Replaceable disposable cylinder
Cleanable cylinder
7.3 Resistive-Immersion Electric Steam Humidifiers
NOTE A resistive-immersion humidifier boils water with submerged heating elements at a fixed rated capacity, independent of water conductivity. (7.3.1)
NOTE Because heat output does not depend on conductivity, resistive-immersion units are the correct electric technology for deionized or reverse-osmosis water. (7.3.2)
7.3.3Resistive-immersion humidifiers shall be specified where the supply water is deionized or reverse-osmosis and an electric steam source is required.
7.3.4Resistive-immersion humidifiers serving cleanroom or laboratory spaces shall use deionized or reverse-osmosis water to avoid mineral carryover.
7.4 Gas-Fired Steam Humidifiers
NOTE A gas-fired humidifier boils water in a stainless combustion chamber fired by an integral gas valve train, shifting the humidification energy off the electrical service. (7.4.1)
7.4.2Gas-fired humidifiers shall be furnished with the flue category and venting required by NFPA 90A and the manufacturer's certification.
7.4.3Condensate from a condensing flue shall be drained and neutralized as required by local code before discharge to the sanitary system.
7.5 Steam-to-Steam and Direct Steam Injection Humidifiers
NOTE A steam-to-steam humidifier uses facility steam to boil a clean secondary water charge through a heat exchanger, dispensing chemical-free steam into the airstream. (7.5.1)
NOTE Direct steam injection meters facility steam straight into the duct through a dispersion valve, which is acceptable only where the steam is clean enough for the occupied airstream. (7.5.2)
7.5.3Facility steam shall not be injected directly into an occupied airstream where boiler treatment chemicals such as amines or oxygen scavengers are present.
7.5.4Where facility steam contains treatment chemicals, a steam-to-steam heat exchanger shall be provided so that only clean steam enters the airstream.
7.5.5Direct steam injection shall be permitted only after the owner's facilities team confirms in writing that the steam is treated with FDA-accepted or culinary-grade chemistry.
7.5.6A direct-steam dispersion assembly shall include a steam separator, pressure-regulating valve, and trap upstream of the control valve to deliver dry steam and prevent water hammer.
7.5.7The steam control valve shall be sized by the manufacturer to both the minimum controllable flow and the full-load capacity to prevent hunting.
7.6 Adiabatic Humidifiers
NOTE Adiabatic humidifiers introduce liquid water as fine droplets or by evaporation from wetted media, consuming little primary energy but requiring stringent water quality and infection-control management. The principal types are: (7.6.1)
- High-pressure atomizing units pump water to 800 to 1,500 psig through fine nozzles, producing droplets that demand an extended absorption distance.
- Ultrasonic units use piezoelectric transducers to fracture water into a fine mist and are well suited to direct-space humidification in cleanrooms, laboratories, and museums.
- Wetted-media evaporative units pass air across water-wetted cellulose or synthetic media, adding humidity with no heating energy but only in warm, dry climates or pre-cooling applications.
7.6.2High-pressure atomizing and ultrasonic humidifiers shall be supplied only with reverse-osmosis or deionized water to prevent mineral aerosol and white-dust deposition.
7.6.3Adiabatic humidifiers shall not be supplied with untreated tap water.
7.6.4Adiabatic humidifiers serving healthcare facilities shall comply with the ASHRAE 170 infection-control path, including water treatment, biofilm management, and scheduled drain and flush cycles, each called out explicitly in the submittal.
7.6.5A blanket statement that adiabatic humidification is permitted shall not be accepted in lieu of the specific design measures required by ASHRAE 170.
Reverse-osmosis water
Deionized water
8 Capacity and Rating
8.1 Sizing and Selection
NOTE The humidifier shall be selected from the manufacturer's standard model line at a rated capacity equal to or greater than the peak winter latent load. (8.1.1)
NOTE Selecting from the standard model line avoids custom pricing and lead time and preserves spare-part availability. (8.1.2)
8.1.3The rated capacity shall be expressed in pounds per hour of moisture output at the design supply-water condition.
8.1.4The humidifier shall be sized so that its rated capacity equals or exceeds the calculated peak winter latent load with no negative spare margin.
8.1.5The selected unit shall not be oversized to the degree that minimum controllable output exceeds the smallest expected load, which would cause short-cycling.
0 (match design load)
5
10
15
9 Water Quality and Supply
9.1 Supply Requirements
NOTE The supply-water source shall match the humidifier technology, since the wrong water type either disables the unit or contaminates the space. Mismatching water quality to technology is among the most common and most consequential humidifier specification errors. (9.1.1)
9.1.2The supply-water source shall be confirmed against the manufacturer's water-quality specification before the unit is ordered.
Potable tap water
Softened water
Reverse-osmosis water
Deionized water
9.1.3Where reverse-osmosis or deionized water is required, its production and distribution shall be coordinated with the Plumbing Engineer.
9.1.5The supply-water connection shall include a shutoff valve, strainer, and any fill valve or float assembly required by the manufacturer.
10 Dispersion Assembly and Absorption Distance
10.1 Assembly and Distance Requirements
NOTE The dispersion assembly distributes steam or atomized water across the duct cross-section; its type and length set the required absorption distance. (10.1.1)
NOTE Non-wetting delivery is a hard requirement of ASHRAE 62.1: no droplet or condensate may wet any downstream surface. (10.1.2)
10.1.3Available straight duct downstream of the assembly shall equal or exceed the manufacturer-stated absorption distance for the design entering-air temperature, velocity, and humidity.
10.1.4No coil, filter, damper, elbow, sensor, or duct transition shall be located within the absorption distance downstream of the dispersion assembly.
10.1.5Where the available straight duct run is constrained, a short-absorption jacketed or multi-tube dispersion manifold shall be specified to reduce the required absorption distance.
10.1.6The absorption distance shall be verified by the manufacturer's calculation submitted for review, not assumed from a rule of thumb.
Single lance tube
Multi-tube manifold
Short-absorption jacketed manifold
Atomizing nozzle array
Air-handler internal (factory installed)
Duct mounted (field installed)
In-space direct-room unit
10.1.7The location of the dispersion assembly within the air handler or duct shall be coordinated with the air-handler manufacturer and shown on the drawings. dispersion assembly location 10.1.8Where the dispersion assembly is factory-mounted in an air handler, its location and clearances shall be coordinated under Air Handling Units. 11 Controls and Safeties
11.1 Control and Safety Requirements
NOTE The humidifier shall be controlled to maintain the space humidity setpoint within the required tolerance while preventing duct wetting and condensation. (11.1.1)
NOTE Modulating control gives the tightest humidity tolerance and is preferred for operating rooms, museums, data centers, and cleanrooms. Staged control is acceptable where a wider tolerance is permissible and lower cost is favored. (11.1.2)
11.1.3The humidifier shall be controlled by a fully modulating signal where the space tolerance is ± 5% RH or tighter.
Modulating (4-20 mA)
Modulating (0-10 V)
Staged (multi-step)
11.1.4The primary humidity sensor location shall be selected to give stable control and shall be shown on the drawings. humidity sensor location Room/space transmitter
Return-air duct
Supply-air duct
11.1.5The primary humidity sensor shall not be located near a supply diffuser, an exterior wall, or any moisture source.
11.1.6A duct-mounted high-limit humidistat or transmitter shall be installed downstream of the dispersion assembly to prevent over-humidification and duct wetting.
11.1.7The high-limit shall override the primary control and stop humidification before the supply-air dew point is reached.
11.1.8The high-limit safety shall not be omitted, since its absence permits duct wetting, condensation, and mold growth.
11.2 Facility Management System Integration
NOTE Integration with the facility management system enables fault detection, trending, alarming, and maintenance scheduling that standalone humidistats cannot provide. (11.2.1)
11.2.2The humidifier shall include a communication module for facility-management-system integration where the project uses a building automation system.
BACnet/MSTP
BACnet/IP
Hardwired analog and alarm contacts only
11.2.3The points list shall report capacity output percentage, alarm status, conductivity where applicable, and cylinder or media life remaining.
11.2.4Low-water, cylinder end-of-life, and drain-fault conditions shall each be reported as discrete alarms to the facility management system.
12 Drain, Blowdown, and Condensate
12.1 Drain and Condensate Management
NOTE Steam humidifiers manage mineral concentration through timed drain and blowdown cycles, and the resulting hot effluent must be handled safely. Electrode and resistive-immersion units require periodic drain-and-refill cycles to keep mineral concentration and conductivity within range. (12.1.1)
12.1.2The drain line shall be sized for full-flow discharge and shall not be trapped, since back-pressure prevents the unit from draining.
12.1.3Hot drain effluent, which may approach 200°F, shall be cooled by a dilution tee or neutralized before entering plastic drain piping.
12.1.4A condensate neutralizer shall be provided where required by local code for condensing gas-fired units or acidic effluent.
Dilution/tempering tee
Condensate neutralizer
Direct to floor drain (metallic piping)
13 Electrical and Fuel Service
13.1 Service Coordination
NOTE The electrical or fuel load of large humidifiers is substantial and must be coordinated early to avoid late panel and transformer upsizing. Large electrode and resistive units can draw 30 kW to over 100 kW, a load frequently missed in early electrical calculations. (13.1.1)
13.1.2The humidifier electrical load shall be coordinated with the Electrical Engineer and reflected in the panel and transformer sizing.
120 V, 1Φ
208 V, 1Φ
240 V, 1Φ
208 V, 3Φ
240 V, 3Φ
480 V, 3Φ
13.1.3Branch-circuit and overcurrent protection shall be sized to the manufacturer's nameplate rating for the selected unit.
13.1.4Gas-fired units shall be coordinated with the fuel piping and venting and reflected in the gas-load calculation for the building.
14 Energy Recovery Compliance
14.1 Compliance Verification
NOTE ASHRAE 90.1 ties humidification to energy-recovery requirements that must be confirmed at design to avoid plan-review findings. (14.1.1)
14.1.2Where active humidification is provided in Climate Zones 3 through 8, energy recovery shall be provided on the ventilation system as required by ASHRAE 90.1.
14.1.3The heat-exchanger enthalpy recovery ratio at heating design conditions shall meet the minimum threshold of ASHRAE 90.1 for the applicable climate zone.
14.1.4Compliance with the energy-recovery requirement shall be confirmed before the humidification design is finalized for permit submission.
15 Testing
15.1 Field Testing Requirements
NOTE Field testing verifies that the installed humidifier delivers rated capacity, controls to setpoint, and trips its high-limit safety without wetting the duct. (15.1.1)
15.1.2The humidifier shall be started and commissioned by a manufacturer-trained technician.
15.1.3Measured humidification capacity at design conditions shall be verified against the rated capacity during commissioning.
15.1.4Non-wetting delivery shall be confirmed by inspecting the downstream duct for condensation during sustained full-load operation.
15.1.5The duct high-limit safety shall be tested to confirm it interrupts humidification at its setpoint.
15.1.6Control stability shall be verified by confirming the space humidity holds within the specified tolerance without short-cycling.
☐ Capacity verification at design conditions
☐ Non-wetting/duct condensation inspection
☐ High-limit trip test
☐ Control-stability verification
☐ Alarm and FMS point verification
16 Installation
16.1 Installation Procedures
NOTE Installation requirements ensure the unit, its dispersion assembly, and its service connections are arranged for correct operation and future maintenance. (16.1.1)
16.1.2The humidifier shall be installed level and plumb on a base or hanger rated for its operating weight.
16.1.3The dispersion assembly shall be installed at the location and orientation shown, maintaining the full downstream absorption distance. dispersion assembly location 16.1.4Duct penetrations for the dispersion assembly shall be constructed and sealed in accordance with NFPA 90A.
16.1.5Steam, water, drain, and fuel connections shall be made with the materials, slopes, and traps specified by the manufacturer.
16.1.6Clearance for cylinder, media, or nozzle removal and for routine service shall be maintained around the unit.
16.1.7Dispersion-assembly and connecting-pipe insulation shall be installed in accordance with Mechanical Insulation. 17 Delivery, Storage, and Handling
17.1 Shipping and Storage
NOTE Humidifiers shall be delivered in the manufacturer's original packaging with cylinders, media, or nozzles protected. (17.1.1)
17.1.2Equipment shall be stored indoors, protected from freezing, moisture, and construction dust until installation.
17.1.3Consumable cylinders, media, and nozzles shall be kept dry and clean until commissioning.
18 Warranty
18.1 Warranty Terms
NOTE The manufacturer shall warrant the humidifier against defects in materials and workmanship for the period specified. (18.1.1)
18.1.2The warranty period shall begin at Substantial Completion or at documented startup, whichever the Contract Documents designate.
18.1.3Consumable cylinders, media, and nozzles are excluded from the workmanship warranty and shall be covered under the replacement-part service schedule.
19 Spare Parts
19.1 Parts and Replacement
NOTE Spare parts shall be furnished so that consumables can be replaced without unplanned downtime. (19.1.1)
NOTE Electrode cylinders have a finite service life, typically 1,500 to 4,000 operating hours depending on water quality, and must be replaced on schedule. (19.1.2)
19.1.3The Contractor shall furnish one complete set of replacement consumables matched to the installed unit.
☐ One replacement electrode/immersion cylinder
☐ One set of replacement atomizing nozzles
☐ One set of replacement evaporative media
☐ One set of fill/drain valve service parts
☐ One air/water inlet strainer screen
19.1.4Accessible clearance for cylinder, media, or nozzle removal shall be provided and confirmed during installation.
19.1.5The replacement-part service schedule shall state the expected service interval for each consumable based on the design water quality.