Unit Heaters

Unit heaters provide space heating through forced convection without mechanical cooling capability. The equipment combines a heating element (steam coil, hot water coil, or gas burner), fan, and basic controls in a compact cabinet suitable for ceiling suspension, wall mounting, or floor installation. Industrial facilities, warehouses, loading docks, and high-bay spaces represent primary applications where simple heating without distribution ductwork meets functional requirements.

Operating Principles

Unit heaters circulate room air across a heating element and discharge the warmed air in a directed stream. The air jet entrains surrounding room air through momentum transfer, eventually mixing with the entire space volume. Proper unit selection and placement create circulation patterns that distribute heat uniformly while avoiding cold spots and uncomfortable drafts.

The heating capacity follows the sensible heating equation:

$$Q_h = \dot{m} c_p \Delta T = \text{CFM} \times 1.08 \times (T_{discharge} - T_{room})$$

For a unit delivering 5,000 CFM with a 70°F temperature rise:

$$Q_h = 5000 \times 1.08 \times 70 = 378,000 \text{ Btu/hr}$$

This capacity remains constant at rated conditions but decreases as room temperature rises or entering air temperature to the coil increases. Modulating controls adjust output by cycling the burner or modulating water flow rather than varying airflow.

Horizontal Propeller Fan Units

Horizontal units use propeller fans to generate high airflow at low static pressure. The configuration produces high discharge velocities (1500-3000 FPM) and long throw distances, making these units ideal for spaces requiring heat distribution across large floor areas.

Physical Construction

Cabinet dimensions range from 18-36 inches square for residential/light commercial units to 48-72 inches for industrial models. The propeller fan mounts directly on the motor shaft without belt drives, reducing maintenance and improving reliability. Fan diameters range from 12-48 inches with blade counts of 3-6 depending on acoustic and efficiency requirements.

The heating coil mounts in the airstream behind the fan. Hot water and steam coils use finned tube construction with 8-14 fins per inch. Gas-fired units incorporate tubular heat exchangers with 80-85% thermal efficiency for atmospheric burners and 90-95% for sealed combustion designs.

Throw and Coverage Patterns

Throw distance represents the horizontal distance air travels before velocity decays to 50 FPM, the threshold where occupants no longer sense air motion. Manufacturer data provides throw distances at various temperature rises and mounting heights.

Empirical throw equations for horizontal propeller units:

$$L_{throw} = K \times \sqrt{\frac{\text{CFM}}{\Delta T}}$$

where $K$ ranges from 15-25 depending on fan design and discharge configuration. For the 5,000 CFM unit with 70°F rise:

$$L_{throw} = 20 \times \sqrt{\frac{5000}{70}} = 169 \text{ feet}$$

This throw distance assumes unobstructed airflow and nominal mounting height. Rack storage, machinery, and structural members reduce effective throw by 20-40%.

Coverage area depends on throw pattern and mounting configuration:

• Wall-mounted horizontal discharge: Covers rectangular area with length = throw distance and width = 0.3-0.5 × throw
• Ceiling-mounted horizontal discharge: Covers circular area with radius = 0.7 × throw
• Corner-mounted 90° discharge: Covers quarter-circle sector with radius = throw

Multiple units require 15-20% throw overlap at floor level to prevent cold zones between coverage areas.

Mounting Considerations

Horizontal propeller units mount on walls or suspend from ceilings using adjustable brackets. Wall mounting suits loading docks and perimeter heating, directing warm air across open bay doors. Ceiling suspension places units above obstruction height in warehouses and manufacturing spaces.

Discharge orientation matters for comfort and efficiency. Horizontal discharge parallel to exterior walls prevents cold air infiltration through doors and curtain walls. Downward discharge at 15-30° angles enhances air mixing in spaces with ceiling heights exceeding 25 feet.

Mounting height affects throw and drop characteristics. The heated air jet follows a parabolic trajectory, initially remaining horizontal then dropping as velocity decays. Mount units high enough to prevent occupant discomfort from high-velocity air but low enough to ensure warm air reaches the occupied zone before rising to the ceiling.

Vertical Centrifugal Fan Units

Vertical unit heaters employ centrifugal fans to deliver air downward, creating a floor-level circulation pattern. The design suits retail spaces, workshops, and facilities requiring quieter operation than propeller units provide.

Cabinet Configuration

Vertical units measure 24-48 inches square by 36-72 inches tall depending on capacity. The centrifugal fan (forward-curved or backward-inclined) mounts at the top or bottom with airflow entering horizontally and discharging vertically downward.

Top discharge units draw return air from the bottom and sides, creating an umbrella circulation pattern. The warm air spreads radially from the unit location, covering a circular floor area. Bottom discharge units pull air from the top and discharge downward with higher velocity, penetrating the occupied zone more effectively in high-ceiling applications.

Performance Characteristics

Centrifugal fans generate higher static pressure than propeller fans, enabling external static resistance from inlet grilles, heat exchangers, and discharge cones without severe performance degradation. Typical operating points range from 0.3-0.8 inches water gauge at rated CFM.

The higher static capability permits:

• Finer coil fin spacing for improved heat transfer
• MERV 8-11 filtration to improve indoor air quality
• Acoustic liner to reduce radiated noise
• Discharge ductwork for localized heat delivery (short runs only, typically under 10 feet)

Airflow ranges from 800-8000 CFM with heating capacities of 30-400 MBH depending on coil configuration and entering fluid temperatures.

Acoustic Performance

Centrifugal fans operate at lower tip speeds than propeller fans for equivalent airflow, reducing aerodynamic noise generation. Forward-curved fans produce 50-65 dB(A) at 3 feet, suitable for retail and light commercial applications with ambient noise levels of NC-40 to NC-45.

Backward-inclined fans reduce noise another 3-5 dB but increase first cost by 15-25%. The investment makes sense for office areas, fitness facilities, and hospitality venues where NC-35 to NC-40 sound levels matter for comfort.

Cabinet and Suspended Unit Heaters

Cabinet unit heaters enclose the fan and coil in an architectural cabinet with finished surfaces suitable for occupied space installation. Louvers control discharge direction while integrated controls and thermostats eliminate external wiring. These units suit retail back rooms, storage areas, and secondary spaces requiring finished appearance.

Suspended unit heaters hang from structural members using chains, threaded rods, or brackets. The open construction minimizes weight and cost while facilitating maintenance access. Industrial facilities, warehouses, and agricultural buildings use suspended units where aesthetics do not constrain equipment selection.

Steam vs Hot Water vs Gas-Fired

Steam Unit Heaters

Steam coils provide rapid heat-up and high capacity in compact coil sizes. Operating pressures range from 2-15 psig with corresponding saturation temperatures of 219-250°F. The high temperature differential enables smaller coils but creates challenges:

• Rapid capacity response: Steam condenses instantly when valves open, delivering full capacity within seconds
• Freeze protection: Condensate must drain continuously to prevent water accumulation and freezing
• Control difficulty: On-off steam valves cause temperature swings; modulating valves require vacuum breakers and proper trap sizing
• Safety: High temperature surfaces demand guards to prevent contact burns

Condensate drainage uses thermostatic or float traps sized for the coil load. Trap failures cause water logging, reducing capacity by 50-80% until corrected.

Hot Water Unit Heaters

Hot water systems provide modulating control and simplified piping compared to steam. Supply water temperatures range from 140-200°F depending on boiler capabilities and distribution losses.

The lower fluid temperatures compared to steam require larger coil face areas for equivalent capacity. A steam coil at 5 psig (227°F) delivers approximately twice the capacity per square foot of face area as a 180°F hot water coil under similar airflow conditions.

Control valves modulate water flow to vary output. Two-way valves maintain constant supply temperature with variable flow, simplifying boiler control. Three-way valves provide constant flow with blended water temperature, preventing low-flow conditions in primary-secondary systems.

Freeze protection requires either:

• Glycol antifreeze (15-50% by volume)
• Continuous circulation during freezing weather
• Automatic coil drainage with low-temperature alarms

Gas-Fired Unit Heaters

Gas-fired units eliminate hydronic piping and central boiler systems. Natural gas or propane burners provide heat directly to the airstream, achieving 80-95% thermal efficiency depending on combustion technology.

Atmospheric burners use natural draft for combustion air and vent products through Type B vents or chimneys. Thermal efficiency reaches 80-85% with Category I venting requirements.

Sealed combustion units draw combustion air from outdoors through dedicated intake pipes and exhaust through PVC or stainless steel vents. Thermal efficiency exceeds 90% through condensing operation, though installation costs increase due to venting and intake piping requirements.

Capacity modulation uses:

• On-off control: Simple and low cost, causes temperature swings of 3-6°F
• High-low-off firing: Two-stage burners reduce cycling and improve comfort
• Modulating burners: Variable firing rates from 20-100% provide optimal comfort and efficiency

Gas-fired units require gas piping sized for peak demand across all units operating simultaneously. Electrical requirements remain minimal (fan motors only), simplifying installation in buildings without adequate electrical service for electric heat.

Installation Requirements

Clearances

Manufacturers specify minimum clearances from combustible materials. Typical requirements:

These clearances ensure service access, prevent combustion air starvation (gas units), and protect building components from thermal damage.

Piping Connections

Hot water and steam connections require:

• Isolation valves for service without system shutdown
• Strainers to protect coil tubes from debris (40-60 mesh)
• Unions for unit removal
• Flexible connectors to isolate vibration

Condensate piping for steam units must slope continuously downward at 1/4 inch per foot minimum. Trap discharge lines require venting to prevent condensate backup.

Electrical Connections

Motor starters include overload protection sized at 115-125% of motor nameplate current. Disconnect switches mount within sight of the unit for service safety. Control circuits operate at 24VAC from transformers sized for contactor, valve actuator, and thermostat loads.

Variable-speed drives reduce energy consumption by 30-50% in applications with significant load variation. The VFD cost premium ($400-1200 depending on motor horsepower) pays back in 2-4 years through reduced fan energy in moderate-climate zones.

Unit heaters deliver cost-effective heating for industrial and commercial applications tolerating moderate noise levels and non-uniform temperature distribution. The wide range of configurations, fuel sources, and mounting options enables solutions for virtually any space heating requirement from small workshops to large distribution centers.

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