Unit Heaters (Hydronic)

Hydronic unit heaters deliver concentrated forced-convection heating through steam or hot water coils with high-capacity fans, providing output densities of 30,000-300,000 Btu/h per unit suitable for large open spaces including warehouses, manufacturing facilities, aircraft hangars, and loading docks where ceiling heights of 12-40 feet preclude effective use of perimeter terminals. The combination of finned-tube heat exchangers and propeller or centrifugal fans creates downward air throw patterns that project heated air 20-60 feet, counteracting stratification and delivering heat to occupied zones despite high mounting positions.

Unit Heater Configurations

Horizontal Propeller Fan Unit Heaters

Construction:

Cylindrical or rectangular cabinet
Hot water or steam coil mounted behind propeller fan
Direct-drive or belt-drive fan motor
Propeller diameter: 12-36 in typical
Horizontal discharge (parallel to mounting surface)

Airflow characteristics:

High volume, low static pressure
Free delivery (no ductwork)
Throw distance: 20-50 ft depending on size and fan speed
Spread pattern: Cone-shaped, 30-60° divergence

Mounting:

Suspended from ceiling or roof structure
Swivel bracket allows directional adjustment
Typical mounting heights: 12-30 ft
Downward angle: 0-45° adjustable

Capacities:

Small: 30,000-75,000 Btu/h (12-18 in fan)
Medium: 75,000-150,000 Btu/h (18-24 in fan)
Large: 150,000-300,000 Btu/h (24-36 in fan)

Vertical Downblow Unit Heaters

Construction:

Vertical cabinet with top-mounted heat coil
Centrifugal blower discharging downward
Compact footprint (12-24 in square)
Height: 24-48 in typical

Airflow characteristics:

Medium volume, medium static pressure
Concentrated downward discharge
Throw distance: 15-40 ft
Narrow spread pattern (12-24 in diameter at floor)

Mounting:

Suspended from ceiling
Fixed downward discharge (typically not adjustable)
Mounting heights: 10-25 ft typical
Applications: Spot heating, loading docks, entrance vestibules

Capacities:

Small: 20,000-50,000 Btu/h
Medium: 50,000-100,000 Btu/h
Large: 100,000-200,000 Btu/h

Cabinet Unit Heaters (Centrifugal Fan)

Construction:

Enclosed sheet metal cabinet
Centrifugal blower and coil in single enclosure
Optional duct collar for discharge
Floor-mounted or wall-mounted

Airflow characteristics:

Low to medium volume, high static pressure
Capable of short duct runs (up to 25 ft equivalent length)
Throw distance: 10-30 ft
Directional grilles for throw control

Applications:

Smaller spaces (shops, garages, utility rooms)
Where ducting is beneficial
Lower mounting heights (8-15 ft)
Noise-sensitive applications (quieter than propeller)

Capacities:

Residential/light commercial: 10,000-60,000 Btu/h
Commercial: 60,000-150,000 Btu/h

Coil Design and Heat Transfer

Steam Coil Construction

Coil configuration:

Copper or steel tubes with aluminum or steel fins
Fin density: 6-10 fins per inch
Rows deep: 1-2 rows typical
Full condensing design (vapor in, liquid out)

Steam supply:

Low-pressure steam: 2-15 psig (219-250°F)
High-pressure steam: 15-50 psig (250-298°F) - industrial applications
Steam pressure reducing valve if required

Condensate removal:

Steam trap at coil outlet
Pitch coil toward trap (1/4 in per foot minimum)
Trap sizing: Adequate for condensate load plus safety factor
Condensate return: Gravity or pumped

Heat transfer:

Steam provides constant temperature across coil:

$$Q = UA(T_{steam} - T_{air,avg})$$

Where:

$U$ = 60-90 Btu/h·ft²·°F (steam to air, forced convection)
$A$ = Coil surface area
$T_{steam}$ = Saturation temperature at steam pressure
$T_{air,avg}$ = (Entering air + leaving air) / 2

Advantages of steam:

High heat transfer (latent heat of condensation)
Uniform coil temperature
Freeze protection (rapid drainage when off)
Lower fluid volume (reduced weight)

Hot Water Coil Construction

Coil configuration:

Copper tubes with aluminum fins
Fin density: 8-12 fins per inch
Rows deep: 2-4 rows (more rows compensate for lower water temperature vs. steam)
Multi-pass circuiting for velocity and heat transfer

Water temperature:

High temperature: 180-200°F supply
Medium temperature: 160-180°F supply
Low temperature: 140-160°F supply (condensing boiler compatible)
Design temperature drop: 20-40°F typical

Heat transfer:

Water temperature varies through coil:

$$Q = 500 \times GPM \times \Delta T_{water}$$

Also:

$$Q = UA \times LMTD$$

Where LMTD accounts for temperature change of both air and water streams.

Advantages of hot water:

Modulating capacity control (via valve position)
No steam trap maintenance
Better suited to outdoor reset control
Quieter operation (no steam flash or condensate issues)

Freeze protection:

Must drain coil when off in freezing conditions
Glycol solution alternative (reduces capacity ~10%)
Face and bypass dampers for temperature control in mild weather

Coil Performance and Sizing

Coil face velocity:

$$V_{face} = \frac{CFM}{A_{face} \times 60}$$

Typical: 400-800 FPM for unit heaters.

Higher velocity increases heat transfer coefficient but increases pressure drop and noise.

Coil capacity:

Approximately proportional to airflow and temperature difference:

$$Q = 1.08 \times CFM \times (T_{leaving air} - T_{entering air})$$

Typical performance:

Water Temp	Entering Air	Airflow	Leaving Air	Capacity
180°F	50°F	2,000 CFM	95°F	97,000 Btu/h
180°F	60°F	2,000 CFM	100°F	86,400 Btu/h
160°F	50°F	2,000 CFM	82°F	69,000 Btu/h

Leaving air temperature limited to avoid discomfort (typically 95-110°F maximum).

Fan Performance and Air Distribution

Propeller Fan Characteristics

Fan curve:

High flow at zero static pressure
Flow drops rapidly with static pressure increase
Not suitable for ducted applications
Efficiency: 40-60%

Throw distance:

Empirical relationship:

$$D_{throw} = C \times \sqrt{CFM}$$

Where $C$ = 0.8-1.2 depending on unit design.

Example: 2,000 CFM unit

Throw = 1.0 × √2000 = 45 ft

Spread pattern:

Heated air jet expands with distance:

At discharge: Diameter = fan diameter
At 20 ft: Diameter ≈ 2× fan diameter
At 40 ft: Diameter ≈ 3-4× fan diameter

Directional control:

Adjust unit mounting angle (0-45° downward typical)
Horizontal: Maximum throw, minimal downward penetration
45° down: Reduced throw, better occupied zone heating

Centrifugal Fan Characteristics

Fan curve:

Lower flow than propeller at zero static
Flow more stable with static pressure
Can overcome duct resistance
Efficiency: 50-70%

Air distribution:

More concentrated discharge
Smaller spread angle
Better penetration of stratified layers
Suitable for long, narrow spaces

Sizing and Selection Methodology

Heat Load Calculation

Space heating load:

$$Q = Q_{envelope} + Q_{infiltration} + Q_{ventilation} + Q_{process}$$

High-bay considerations:

Increased infiltration at loading docks, overhead doors
Stratification losses (heat rise to ceiling)
Process loads (equipment, product cooling)
Pickup load for morning warm-up

Effective heat loss:

Account for stratification:

$$Q_{effective} = Q_{calculated} \times F_{stratification}$$

Where $F_{stratification}$ = 1.1-1.3 for ceiling heights 20-40 ft.

Unit Selection Procedure

Step 1: Determine space heat loss

Include stratification factor and pickup load (20-40% for high thermal mass).

Step 2: Determine heating medium

Steam or hot water based on availability, control requirements, freeze protection needs.

Step 3: Calculate number and location of units

$$N_{units} = \frac{Q_{total}}{Q_{unit}} \times F_{safety}$$

Where $F_{safety}$ = 1.1-1.15.

Distribute units to provide uniform coverage, considering:

Throw distance (units spaced ≤ 2× throw distance)
Major heat loss areas (near doors, exterior walls)
Obstruction avoidance

Step 4: Verify throw and coverage

Check each unit’s throw reaches intended area without excessive leaving air temperature.

Step 5: Select fan speed and control

Single-speed: Simple, lowest cost, on-off control only
Two-speed: Better part-load efficiency, reduced noise during mild weather
Variable-speed (VFD): Best efficiency, modulating capacity, quiet operation

Sizing Example

Warehouse:

Dimensions: 100 ft × 200 ft × 24 ft high
Heat loss: 480,000 Btu/h (calculated)
Stratification factor: 1.15
Pickup load: 25%
Heating: 180°F hot water available

Calculation:

Effective load = 480,000 × 1.15 × 1.25 = 690,000 Btu/h
Select unit size: 100,000 Btu/h capacity @ 180°F water, 50°F air
Number of units = 690,000 / 100,000 = 6.9 → 7 units
Verify coverage:
- Unit throw ≈ 35 ft
- Space long dimension: 200 ft
- Maximum spacing: 2 × 35 = 70 ft
- Minimum units along length: 200 / 70 = 2.86 → 3 rows
- Units across width: 7 / 3 ≈ 2-3 per row
Layout: 3 rows of 2-3 units = 7 units total ✓

Installation and Control

Mounting and Suspension

Structural support:

Unit weight: 50-300 lb depending on size
Water-filled coil adds weight (0.5-2.0 gal water per unit)
Vibration isolation: Required for centrifugal units, recommended for propeller

Suspension hardware:

Steel rod or cable suspension (1/4 in to 1/2 in diameter)
Swivel bracket for propeller units (directional adjustment)
Fixed bracket for vertical units
Safety cable or chain (redundant support)

Clearances:

Discharge clearance: 4-8 ft minimum (prevent direct impingement on people/objects)
Service clearance: 3-4 ft minimum for coil access
Ceiling/roof: 1-3 ft (thermal clearance, avoid overheating)

Piping Connections

Steam systems:

Supply: Reduced pressure and temperature if high-pressure source
Steam trap: At coil condensate outlet, sized for load plus safety factor
Drip leg: Ahead of coil supply (remove condensate from steam main)
Vacuum breaker: If condensate returns to vacuum system

Hot water systems:

Supply and return: 3/4 in to 2 in depending on capacity
Control valve: Two-way modulating or on-off
Air vents: At coil high point (automatic vents)
Isolation valves: Service and balancing
Freeze protection: Drain valves, glycol, or heat trace if freezing risk

Piping pitch:

Steam coils: Pitch toward trap (1/4 in per foot)
Hot water: Pitch for air removal (toward vents)

Control Strategies

On-off control:

Thermostat energizes fan and opens valve
Simple, low cost
Temperature swing: ±3-5°F typical
Suitable for unoccupied or tolerant applications

Two-stage control:

Stage 1: Low fan speed, valve open
Stage 2: High fan speed, valve wide open
Reduced swing, better comfort
Lower noise during mild weather

Modulating control:

Proportional valve position based on temperature
Fan runs continuously or cycles with valve
Best comfort (±1-2°F)
Higher cost, complexity

Freeze protection:

Low-limit sensor in discharge air (55-65°F)
Shuts valve if air temp drops (prevents coil freeze)
Critical for hot water systems in cold climates

Applications and Best Practices

Warehouse and Distribution Centers

Design approach:

Perimeter units beneath skylights/roof (address major heat loss)
Interior units for supplemental heating
Door units for infiltration load
Mounting height: 20-30 ft typical

Control:

Zone by use pattern (high-activity areas vs. storage)
Setback during unoccupied periods (55-60°F)
Morning warm-up staging (prevent overcurrent)

Manufacturing Facilities

Considerations:

Process heat gains reduce heating load
Local exhaust may create negative pressure (increase infiltration)
Equipment clearances (cranes, conveyors)
Noise tolerance (production areas less sensitive)

Unit selection:

Higher capacity units (reduce quantity, installation cost)
Two-speed or VFD for part-load efficiency
Industrial-grade construction (heavy-duty motors, corrosion-resistant finish)

Loading Docks and Vestibules

Challenge:

High infiltration during door open cycles
Spot heating requirement
Avoid direct blow on personnel

Solution:

Vertical downblow units above door
Horizontal units angled to create air curtain effect
High leaving air temperature acceptable (short exposure)
Rapid response (two-speed or modulating)

Aircraft Hangars and High-Bay Spaces

Unique requirements:

Ceiling heights 40-80 ft
Massive volume, high stratification
Equipment clearance critical

Design:

Combine unit heaters with radiant heating (infrared)
Unit heaters for occupied zone, radiant for floor/equipment
High-capacity units (200,000-300,000 Btu/h)
Confirm throw distance adequate (may require 60+ ft)

Maintenance and Troubleshooting

Routine Maintenance

Monthly (during operation):

Inspect for unusual noise or vibration
Verify fan operation
Check for steam/water leaks

Seasonal:

Clean coil fins (vacuum or compressed air)
Lubricate fan bearings if required
Inspect belt tension and condition (belt-drive units)
Test controls and safeties

Annual:

Steam trap inspection and testing
Coil pressure test (hot water systems)
Motor inspection and testing
Vibration isolation check

Common Problems

Inadequate heating:

Insufficient airflow (dirty coil, failed fan, belt slippage)
Low steam pressure or water temperature
Control valve not opening (check actuator, control signal)
Air trapped in hot water coil (bleed air)

Short throw/poor coverage:

Fan running backwards (check motor rotation)
Low fan speed (check voltage, motor condition)
Coil air bypass (leaking cabinet seams)

Noise:

Loose fan mounting or blades
Failed bearings
Vibration transmission to structure (check isolation)
Steam trap malfunction (condensate backup, water hammer)

Freeze damage:

Coil freeze due to low airflow or water temperature
Prevention: Drain coil when off, use glycol, maintain airflow and water flow

Hydronic unit heaters provide high-capacity forced-convection heating for large open spaces through suspended propeller or centrifugal fan units with steam or hot water coils. Proper selection integrates space heat load with throw distance requirements and mounting constraints, while effective control strategies optimize comfort, efficiency, and equipment protection in warehouse, industrial, and high-bay applications.