1 Scope
NOTE This standard covers self-contained, fan-driven unit heaters that deliver heat directly into a space by forced convection, including propeller-fan suspended and cabinet types served by hot water, steam, gas combustion, or electric resistance. (1.1)
NOTE Unit heaters are used to maintain space temperature in unconditioned or semi-conditioned, unoccupied or intermittently occupied areas where ducted comfort conditioning is neither warranted nor economical. (1.2)
NOTE Typical applications include parking and repair garages, warehouses and distribution centers, loading docks and overhead-door bays, mechanical and electrical rooms, entry vestibules, aircraft hangars, fabrication shops, and similar spaces. The heater discharges directly into the occupied volume; no supply ductwork distributes the air except where a high-static blower type is specifically selected for a short duct connection. (1.3)
NOTE The standard addresses heating-capacity selection, heating-medium selection, fan and discharge configuration, mounting, controls integration, clearances to combustibles, and the venting and combustion-air provisions required for gas-fired units. (1.4)
NOTE It applies to both new construction and retrofit on commercial, industrial, and institutional projects. (1.5)
1.6 Equipment furnished under this standard shall be a factory-assembled, factory-tested unit heater listed and labeled for its heating medium and intended application.
1.6.1Each unit heater shall be a complete factory-assembled package comprising the heating element or heat exchanger, fan and motor, casing, and integral controls as scheduled.
1.6.2Each unit heater shall be listed and labeled by a nationally recognized testing laboratory for the heating medium and the service conditions in which it is installed.
1.6.3Field-assembled combinations of separately furnished heating coils, fans, and casings shall not be substituted for a listed unit heater.
1.7 Exclusions
NOTE Fin-tube radiation, convectors, panel radiators, and radiant floor or ceiling panels are terminal heating devices covered by sync/terminal-heating-units and are excluded here. (1.7.1)
NOTE Architectural cabinet units with integral cooling coils or full-perimeter finished enclosures are covered by sync/fan-coil-units and are excluded here. (1.7.2)
NOTE Duct-mounted heating coils and inline duct heaters without an integral fan belong to the air-handling and ductwork scope and are excluded here. (1.7.3)
NOTE Infrared radiant tube heaters and radiant ceiling panels, which heat by radiation rather than forced convection, are a separate product class not addressed in this standard. (1.7.4)
NOTE Hydronic supply and return piping upstream of the unit-heater connection is covered by sync/hydronic-piping and is excluded here. (1.7.5)
NOTE Natural-gas branch piping and the gas connection to the heater are covered by sync/natural-gas-piping and are excluded here. (1.7.6)
NOTE Central air-handling units serving multiple zones through ductwork are covered by sync/hvac-fans and are excluded here. (1.7.7)
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 referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
| Standard |
Title |
| ANSI Z83.8 / CSA 2.6 |
Gas Unit Heaters, Gas Packaged Heaters, Gas Utility Heaters, and Gas-Fired Duct Furnaces |
| UL 2021 |
Fixed and Location-Dedicated Electric Room Heaters |
| NFPA 54 |
National Fuel Gas Code |
| NFPA 70 |
National Electrical Code (NEC) |
| NFPA 88A |
Standard for Parking Structures |
| ASHRAE Handbook |
HVAC Systems and Equipment, Chapter 28 (Unit Ventilators, Unit Heaters, and Makeup Air Units) |
| ASHRAE 90.1 |
Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings |
| IMC |
International Mechanical Code |
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following action submittals for each unit heater type and tag before fabrication or ordering:
- Product data with rated heating capacity, fan airflow, motor data, and electrical characteristics for each tagged unit.
- Manufacturer's certified capacity data at the scheduled entering medium conditions (entering water temperature and flow, or steam pressure, or gas input, or electric kW).
- Dimensioned drawings showing casing size, discharge arrangement, mounting provisions, and connection locations.
- Wiring diagrams showing integral controls, motor protection, and field control terminations.
- Listing and labeling documentation identifying the heating medium and any hazardous-location rating.
- For gas-fired units, vent category, vent connection size and material, and combustion-air requirement.
☐ Product data with rated capacity and electrical data
☐ Certified capacity at scheduled entering conditions
☐ Dimensioned unit drawings with connection locations
☐ Control and power wiring diagrams
☐ Listing/labeling documentation
☐ Gas vent category, size, material, and combustion-air data
3.2 Closeout Submittals
3.2.1The Contractor shall submit the following closeout submittals before Substantial Completion:
- Operation and maintenance manuals for each unit heater type.
- As-installed wiring diagrams reflecting field control terminations.
- Startup and combustion-test reports for gas-fired units, signed by the startup technician.
- Air-balance report entries for each unit heater, coordinated with sync/testing-adjusting-and-balancing.
- Executed warranty documents.
☐ Operation and maintenance manuals
☐ As-installed wiring diagrams
☐ Gas-fired startup and combustion-test reports
☐ Air-balance report entries
☐ Executed warranty documents
3.3.1The Contractor shall submit the following informational submittals for review:
- Heat-loss calculation summary supporting the scheduled heating capacity for each space served.
- Combustion-air calculation for each gas-fired unit, demonstrating compliance with NFPA 54.
- Hanger and seismic-restraint details for suspended units.
☐ Heat-loss calculation summary
☐ Combustion-air calculation per NFPA 54
☐ Hanger and seismic-restraint details
4 Quality Assurance
NOTE Listing-and-labeling enforcement is the load-bearing quality control for unit heaters because a field substitution of an unlisted assembly defeats the combustion-safety and electrical-safety basis of the design. (4.1)
4.1.1Gas-fired unit heaters shall be listed and labeled in accordance with ANSI Z83.8 / CSA 2.6.
4.1.2Electric unit heaters intended for permanent installation shall be listed and labeled in accordance with UL 2021.
4.1.3The manufacturer shall have produced unit heaters of the specified type and capacity range for not less than five years.
4.1.4Capacity ratings shall be established by test in accordance with the applicable listing standard and shall not be derived solely by interpolation outside the tested range.
NOTE Manufacturer-agnostic procurement (4.2)
4.2.1Substitutions shall match the scheduled heating capacity, airflow, discharge arrangement, electrical characteristics, and physical envelope without requiring modification of structure, piping, venting, or electrical service.
5 Environmental and Service Conditions
NOTE Because unit heaters serve unconditioned spaces, the service environment, not the comfort design point, governs many selections: combustion air availability, freeze exposure, airborne contaminants, and door-open infiltration all drive the equipment choice. (5.1)
NOTE Infiltration is the most commonly underestimated load. (5.2)
NOTE Loading-dock and overhead-door bays lose large quantities of heat whenever a door is open, and a heater sized only to the steady-state envelope loss will not recover the space. The heating-capacity selection shall account for the door-open infiltration load for the design door-open condition, not the closed-envelope load alone. (5.3)
5.3.1The heating-capacity selection shall include the infiltration and door-open heat loss for spaces served by overhead or large vehicular doors.
5.3.2Where a space is exposed to outdoor air through frequently opened doors, the design entering-air temperature for capacity rating shall be the space design temperature, not a warmer assumed mixed-air temperature.
NOTE Freeze protection (5.4)
NOTE Hydronic and steam units installed in vestibules, loading docks, and other spaces that can fall below freezing when unoccupied are exposed to coil freeze damage, especially when served from a system without antifreeze. The design shall provide a freeze-protection strategy for any wet-coil unit in a freeze-exposed location. (5.5)
5.5.1Hydronic unit heaters in freeze-exposed locations shall be provided with a freeze-protection means: a glycol-rated coil on a glycol system, a low-limit aquastat that maintains minimum coil temperature, or a coil drain provision, as scheduled.
5.5.2A hydronic unit heater shall not be specified for a freeze-exposed location on a non-glycol system without one of the freeze-protection means above.
NOTE Airborne contaminants and hazardous locations (5.6)
5.6.1Cabinet (blower-type) unit heaters installed in spaces with suspended dust, sawdust, or metal filings shall be furnished with totally enclosed fan-cooled (TEFC) motors.
5.6.2Unit heaters installed in classified hazardous locations shall be listed for the applicable class and division, and shall not be field-modified to achieve that rating.
● Heated, weather-protected (mechanical room, interior bay)
○ Freeze-exposed (vestibule, loading dock)
○ Door-open infiltration (overhead-door bay, dock)
○ Contaminant-laden (shop, fabrication area)
○ Classified hazardous location
● None (general purpose)
○ Class I, Division 2
○ Class I, Division 1
○ Class II, Division 2
6 Heating Medium and Capacity
NOTE The heating medium is the first selection and it cascades into nearly every other decision: capacity rating basis, control method, venting, clearances, and electrical or piping service. (6.1)
NOTE The four media in common use are hot water (hydronic), steam, gas-fired combustion (natural gas or propane), and electric resistance. Hydronic and electric units are the most common in new commercial construction; gas-fired units are common where a central heating plant is absent; steam units are now rare in new work but frequent in older industrial renovations and shall be supported where an existing steam distribution system is the available medium. (6.2)
6.2.1The unit-heater heating medium shall be selected from the available building distribution system or fuel source and coordinated with the Engineer of Record.
● Hot water (hydronic)
○ Steam
○ Gas-fired, natural gas
○ Gas-fired, propane (LP)
○ Electric resistance
NOTE Heating capacity (6.3)
NOTE Heating capacity shall be established by a heat-loss calculation for the served space at the design conditions, including the infiltration provisions above. Capacity is expressed in MBH (thousands of Btu/h) for hydronic, steam, and gas units, and in kW for electric units. (6.4)
6.4.1The scheduled heating capacity shall be derived from a heat-loss calculation for the served space and shall not be assigned by floor area alone.
6.4.2The rated capacity shall be certified at the scheduled entering medium condition, not at a nominal catalog condition more favorable than the design.
NOTE Hydronic entering conditions (6.5)
NOTE For hot-water units the rated output depends on entering water temperature and flow; the design shall pin both so the certified rating matches the served load. (6.6)
6.6.1The rated output of a hydronic unit heater shall be certified at the scheduled entering water temperature and design water flow.
6.6.2The hydronic coil shall be selected for the system entering water temperature; a unit selected for a higher temperature than the system delivers shall not be substituted.
NOTE Steam conditions (6.7)
6.7.1A steam unit heater shall be selected and certified at the scheduled steam supply pressure.
6.7.2Where the steam system is high pressure, the steam pressure used for capacity rating shall match the pressure delivered at the unit, accounting for distribution losses.
● Low pressure (≤ 103 kPa / 15 psig)
○ Medium pressure (103-862 kPa)
○ High pressure (> 862 kPa)
7 Fan and Discharge Configuration
NOTE The fan type and discharge orientation determine where the heater can be applied and how the warm air is distributed; a propeller unit and a cabinet/blower unit are not interchangeable. (7.1)
NOTE Propeller-fan unit heaters move large airflow against low static pressure and are intended for open, unfinished spaces with direct discharge into the volume. Cabinet (centrifugal/blower) units develop higher static pressure, accept short duct connections or filter sections, and suit finished or semi-finished spaces. Calling out only "unit heater" without distinguishing the two routinely generates submittal rejections, because a propeller unit cannot serve a ducted or finished application. (7.2)
7.2.1The fan type shall be specified as propeller-fan or cabinet (centrifugal/blower); a unit heater shall not be scheduled without this distinction.
7.2.2A propeller-fan unit heater shall not be connected to supply ductwork.
7.2.3A cabinet (blower-type) unit shall be specified where a short duct connection, filter section, or finished casing is required.
● Propeller fan (open, unfinished spaces)
○ Cabinet / centrifugal blower (finished or ducted)
NOTE Discharge orientation (7.3)
NOTE Horizontal-discharge units throw warm air across the space and suit lower mounting heights and wider coverage; vertical-discharge (downblow) units project heat to the floor from high mounting and suit tall spaces. Multi-position units can be field-set to either. The discharge orientation shall be coordinated with mounting height and the space geometry shown on the drawings. (7.4)
7.4.1The discharge orientation shall be specified as horizontal, vertical (downward), or multi-position, coordinated with mounting height.
7.4.2Vertical-discharge units serving tall spaces shall be furnished with an air-deflection or anti-stratification means where scheduled to drive warm air to the occupied level.
● Horizontal discharge
○ Vertical (downward) discharge
○ Multi-position (field-adjustable)
8 Mounting and Clearances
NOTE Mounting configuration and clearance-to-combustibles requirements are safety and serviceability provisions, not aesthetics, and the gas-fired and high-pressure-steam cases carry code-mandated minimums. (8.1)
8.1.1The mounting configuration shall be specified as ceiling-suspended, wall-mounted, or floor-standing, coordinated with the structural support shown on the drawings.
8.1.2Suspended units shall be supported from building structure by hangers rated for the unit operating weight, not from ductwork, piping, or ceiling systems.
● Ceiling-suspended
○ Wall-mounted
○ Floor-standing
NOTE Mounting height for steam and high-temperature units (8.2)
NOTE High-pressure steam and high-temperature hot-water units discharging into occupied or parking areas can present a burn hazard at low mounting heights; NFPA 88A and ASHRAE establish a minimum discharge height above the floor in those areas. (8.3)
8.3.1High-pressure steam unit heaters discharging into occupied or parking areas shall be mounted with the discharge not less than 4 m (13 ft) above the finished floor.
8.3.2The mounting height of each suspended unit shall be coordinated with the discharge orientation so the throw reaches the occupied level without creating an objectionable draft.
NOTE Clearances to combustibles (8.4)
8.4.1Gas-fired and high-temperature units shall be installed with the clearances to combustible construction marked on the listing label and required by NFPA 54 and the IMC.
8.4.2Service access clearance shall be maintained at the burner, control, and fan-motor side of each unit for inspection and component replacement.
8.4.3Clearance reductions using listed protective assemblies shall be applied only as permitted by the listing and the adopted mechanical code.
9 Gas-Fired Units
NOTE Gas-fired unit heaters add combustion safety to the design: venting category, combustion-air supply, ignition type, and gas connection must all be coordinated, and each is a common source of field RFIs and AHJ rejections when omitted. (9.1)
NOTE Venting category and vent material (9.2)
NOTE The vent category determines the vent material. Category I natural-draft appliances vent into a conventional vent under negative static pressure; Category IV positive-pressure (condensing) appliances require a sealed, corrosion-resistant vent rated for positive pressure and condensate, such as stainless steel or a listed special vent. Specifying a standard single-wall or Type B vent on a Category IV appliance is a rejection, as is the reverse mismatch. (9.3)
9.3.1The vent category of each gas-fired unit shall be specified, and the vent material shall match that category.
9.3.2Category IV positive-pressure units shall be vented with a vent system listed for positive pressure and condensate (stainless steel or listed special vent); standard Type B vent shall not be used.
9.3.3Separated-combustion (sealed-combustion) units serving tightly insulated spaces shall be furnished with the manufacturer's listed intake and exhaust terminations.
● Category I (natural draft, Type B vent)
○ Category III (positive pressure, non-condensing)
○ Category IV (positive pressure, condensing)
○ Separated combustion (sealed, direct vent)
○ Unvented (only where code-permitted)
● Type B double-wall (Category I)
○ Single-wall metal (where permitted)
○ AL29-4C / stainless special vent (Category III/IV)
○ PVC/CPVC sealed (separated combustion, where listed)
NOTE Combustion air (9.4)
NOTE NFPA 54 requires a verified volume of combustion air for fuel-burning appliances in confined spaces. Omitting the combustion-air calculation for units in tight mechanical rooms is a frequent cause of field RFIs and plan-review rejections. The design shall demonstrate adequate combustion air by room volume or provide a dedicated combustion-air opening or duct. (9.5)
9.5.1Each gas-fired unit heater shall be provided with combustion air verified in accordance with NFPA 54.
9.5.2Where the room volume is insufficient under NFPA 54, a combustion-air opening or duct sized per the code shall be provided and shown on the drawings.
9.5.3A combustion-air calculation shall be submitted for each gas-fired unit demonstrating compliance.
NOTE Ignition (9.6)
NOTE Standing-pilot ignition wastes fuel and is increasingly disallowed; ASHRAE 90.1 and many jurisdictions require an intermittent or direct-spark ignition (DSI) system for commercial gas appliances. Specifying a standing pilot without a code check frequently fails plan review. (9.7)
9.7.1Gas-fired unit heaters shall be furnished with intermittent or direct-spark ignition (DSI); standing-pilot ignition shall not be used unless specifically permitted by the adopted code and the Engineer of Record.
● Direct spark ignition (DSI)
○ Intermittent (hot-surface) ignition
○ Standing pilot (only where code-permitted)
NOTE Fuel and gas connection (9.8)
9.8.1The fuel type shall be specified as natural gas or propane and the unit shall be furnished or factory-converted for that fuel.
9.8.2The gas connection shall terminate at a manual shutoff and union ahead of the unit for service isolation, coordinated with sync/natural-gas-piping.
● Natural gas
○ Propane (LP)
10 Electric Units
NOTE Electric unit heaters avoid combustion and venting entirely and suit spaces where no fuel or central plant is available; their constraints are electrical service capacity and element staging rather than combustion safety. (10.1)
10.1.1Electric unit heaters shall be furnished with the heating elements, contactors, and integral high-limit protection required for the scheduled kW and voltage.
10.1.2Electric elements shall be staged where scheduled to limit inrush and to permit modulating control of output.
10.1.3Electric units in classified hazardous locations shall be of the listed explosion-proof type for the applicable class and division.
● Single stage
○ Two stage
○ Multi-stage / step controller
11 Electrical and Motor
NOTE Motor voltage and phase must be coordinated with the building electrical distribution, and the NEC imposes disconnect and overload-protection requirements that are frequently missed on suspended units. (11.1)
11.1.1The motor voltage and phase shall be coordinated with the electrical distribution serving the space and scheduled on the drawings.
11.1.2A disconnecting means shall be provided within sight of and readily accessible to each unit-heater motor in accordance with NEC Article 430.
11.1.3Motor overload protection shall be provided as required by NEC Article 430.
11.1.4The fan motor shall be of the enclosure type suited to the service environment (TEFC where dust or contaminants are present).
○ 120 V, 1Φ
● 208 V, 1Φ
○ 208 V, 3Φ
○ 240 V, 1Φ
○ 277 V, 1Φ
○ 480 V, 3Φ
● Open drip-proof (ODP)
○ Totally enclosed fan-cooled (TEFC)
○ Explosion-proof (hazardous location)
12 Controls
NOTE Controls integration is where the unit heater is most often left in a coordination gap, because the thermostat wiring falls between the mechanical and electrical drawings and the BAS scope. (12.1)
NOTE The control method ranges from a unit-mounted thermostat, through a remote wall thermostat, to full DDC control by the building automation system with a modulating valve on hydronic units or staged elements on electric units. The control method and the responsibility for control wiring shall be defined so the wiring does not fall into a scope gap. (12.2)
12.2.1The control method shall be specified as unit-mounted thermostat, remote wall thermostat, or BAS/DDC control.
12.2.2Control wiring responsibility shall be assigned on the drawings so unit-heater thermostat wiring appears on either the mechanical or electrical documents, not neither.
12.2.3Hydronic units under BAS/DDC control shall be furnished or paired with a modulating control valve coordinated with sync/hydronic-piping.
12.2.4A fan time-delay or fan-thermostat interlock shall be provided on hydronic and steam units to prevent the fan from blowing cold air before the coil is warm and to purge residual heat after a call ends.
○ Unit-mounted thermostat
● Remote wall thermostat
○ BAS / DDC control
● Fan thermostat (coil-temperature sensing)
○ Fan time-delay relay
○ Continuous fan (no interlock)
13 Coil, Casing, and Construction
NOTE Construction requirements protect the heat-transfer surface and the casing from the harsh, often corrosive environments unit heaters serve. (13.1)
13.1.1Hydronic and steam coils shall be copper-tube with aluminum or copper fins, factory leak-tested at not less than the rated working pressure.
13.1.2The casing shall be steel with a factory-applied corrosion-resistant finish suitable for the service environment.
13.1.3Adjustable discharge louvers or a diffuser shall be furnished to direct the air pattern as required by the application.
13.1.4Steam coils shall be of the non-freeze (steam-distributing tube) type where the unit is exposed to entering air below freezing.
● Copper tube, aluminum fin
○ Copper tube, copper fin
○ Non-freeze steam-distributing coil
● Standard baked enamel
○ Epoxy / corrosion-resistant coating
○ Galvanized
14 Testing
NOTE Field testing confirms that the installed unit operates safely and delivers its rated output, and that gas-fired combustion is within the manufacturer's setup. (14.1)
14.1.1Each unit heater shall be started and functionally tested for heating output, fan operation, and control response.
14.1.2Gas-fired units shall be combustion-tested at startup and the results recorded on a signed startup report.
14.1.3Each unit heater airflow shall be verified and recorded as part of the project air balance under sync/testing-adjusting-and-balancing.
14.1.4Freeze-protection devices on wet-coil units shall be tested to confirm they activate at the scheduled setpoint.
15 Installation
NOTE Installation provisions ensure the unit is supported, connected, vented, and accessible per its listing and the mechanical code. (15.1)
15.1.1Each unit heater shall be installed level and supported in accordance with the manufacturer's instructions and the mounting details on the drawings.
15.1.2Suspended units in seismic regions shall be restrained in accordance with the project seismic-restraint requirements.
15.1.3Hydronic and steam connections shall include isolation valves and unions at the unit for service removal, coordinated with sync/hydronic-piping.
15.1.4Gas-fired units shall be vented and provided with combustion air as required by NFPA 54 and the IMC before being placed in operation.
15.1.5Piping connected to wet-coil units shall be insulated where required by sync/mechanical-insulation.
15.1.6Routing of piping, gas, vent, and electrical connections to each unit shall follow the arrangement shown on the drawings. unit connection routing 16 Delivery, Storage, and Handling
16.1Unit heaters shall be delivered in the manufacturer's packaging with the listing label intact.
16.2Units shall be stored indoors, protected from weather and construction dust, until installation.
16.3Units shall be handled by their designated lifting points and shall not be lifted by the coil connections, vent collar, or discharge louvers.
17 Warranty
17.1The manufacturer shall warrant each unit heater against defects in materials and workmanship for not less than one year from Substantial Completion.
17.2Gas-fired heat exchangers shall carry an extended warranty of not less than ten years where scheduled.
○ 5 years
● 10 years
○ 15 years
18 Spare Parts
18.1The Contractor shall furnish one complete set of spare ignition components for each gas-fired unit-heater type.
18.2The Contractor shall furnish spare fan-motor capacitors and one spare fan motor for each electric and propeller-fan unit-heater type where scheduled.
☐ Spare ignition components (per gas-fired type)
☐ Spare fan-motor capacitors
☐ One spare fan motor per type