Unit Ventilators, Heaters, and Makeup Air Equipment

Unit equipment represents a category of HVAC devices that combine heating, cooling, and ventilation functions in factory-assembled packages designed for specific space conditioning applications. These systems differ from central air handling systems by serving localized zones with self-contained controls and minimal field assembly.

Unit Equipment Categories

Unit Ventilators integrate outdoor air ventilation with heating and optional cooling in a single cabinet. Classroom applications dominate this market segment, where variable outdoor air dampers enable economizer operation while maintaining minimum ventilation rates. The face-and-bypass damper arrangement allows capacity modulation without airflow variation, critical for maintaining acoustic comfort in educational spaces.

Unit Heaters provide space heating through forced convection without mechanical cooling capability. Horizontal propeller fan units deliver high throw distances for industrial and warehouse applications. Vertical downflow units with centrifugal fans suit retail and commercial spaces requiring quieter operation and more uniform distribution patterns.

Makeup Air Units replace air exhausted from buildings by kitchen hoods, laboratory fume hoods, paint booths, and industrial processes. These systems handle 100% outdoor air and may incorporate tempering (minimal heating), full conditioning (heating and cooling), or direct-fired burners that add combustion products to the supply airstream for maximum efficiency.

Destratification Fans reduce thermal stratification in high-bay spaces by circulating warm ceiling air downward to occupied zones. Slow-speed operation minimizes drafts while capturing otherwise wasted heat, reducing heating energy consumption by 20-30% in facilities with ceiling heights exceeding 20 feet.

Unit Ventilator Operating Cycles

Unit ventilator control strategies follow three standardized operating cycles defined by ASHRAE, each optimizing energy performance while maintaining ventilation requirements.

Cycle I (Outdoor Air Heating) maintains 100% outdoor air across all operating modes. Outdoor air dampers remain fully open during occupied periods, while return air dampers stay closed. This cycle suits applications requiring continuous ventilation without recirculation, including certain healthcare and laboratory spaces. Energy consumption exceeds other cycles due to continuous conditioning of cold outdoor air during heating season.

Cycle II (Changeover) transitions between 100% outdoor air and 100% recirculated air based on outdoor temperature. When outdoor air temperature provides free cooling below space temperature setpoint, dampers admit 100% outdoor air. As outdoor conditions fall below the changeover temperature (typically 55-60°F), the system switches to 100% return air with minimum outdoor air for ventilation. This cycle delivers economizer benefits while minimizing heating energy.

Cycle III (Blended Air) modulates outdoor air and return air dampers proportionally to maintain mixed air temperature at coil entering conditions. The economizer sequence increases outdoor air fraction when temperature falls within the economizer range, providing free cooling. When outdoor air drops below economizer range, dampers modulate to minimum position while the heating coil tempers the mixed airstream. Cycle III offers the most sophisticated control and optimal energy performance across varying outdoor conditions.

Modern installations favor Cycle III with demand-controlled ventilation, using CO₂ sensors to modulate outdoor air dampers based on actual occupancy rather than design occupancy assumptions. This approach reduces heating and cooling energy by 15-30% in spaces with variable occupancy patterns such as classrooms, meeting rooms, and assembly spaces.

Unit Heater Configurations

Unit heaters provide economical space heating for applications tolerating moderate temperature stratification and elevated noise levels. Configuration selection depends on mounting location, throw requirements, and architectural constraints.

Horizontal Propeller Units mount on walls or suspend from structures, delivering horizontal airflow patterns with throw distances reaching 100-150 feet. Propeller fans generate high airflow rates (3000-20000 CFM) at low static pressure (0.1-0.2 in. wg), making these units ideal for warehouses, manufacturing facilities, and loading docks. Heating coils use hot water, steam, or direct gas firing. Sound levels range from 55-70 dBA at 15 feet, acceptable for industrial environments but excessive for occupied commercial spaces.

Vertical Downflow Units incorporate centrifugal fans delivering air vertically downward through ceiling-mounted diffusers. This configuration provides more uniform temperature distribution than horizontal units while operating at lower sound levels (45-55 dBA). Applications include retail stores, automotive service bays, and commercial entrances. Throw distances limit mounting heights to 20-30 feet depending on capacity and discharge velocity.

Propeller Cabinet Units combine propeller fan efficiency with cabinet construction that reduces noise transmission and improves aesthetics. Free-blow discharge or short duct connections distribute heated air. These hybrid units suit light commercial and agricultural applications where appearance matters but budget constraints preclude conventional air handling equipment.

Capacity modulation in unit heaters occurs through staged heating (multiple coil circuits) or modulating control valves for hydronic units. Gas-fired models employ modulating burners or high-low-off control. Constant fan operation during occupied periods maintains air circulation even when heating demand ceases, reducing stratification and improving comfort.

Makeup Air Unit Types

Makeup air units (MAUs) condition outdoor air to replace exhaust while preventing negative building pressure that compromises envelope performance, increases infiltration, and creates door operation problems.

Tempered Makeup Air Units provide minimal heating to prevent supply air temperatures below 50-55°F during cold weather. Heating capacity ranges from 20-40% of full conditioning capacity, sufficient to prevent frosting and maintain marginally acceptable discharge conditions. Applications include warehouses and industrial facilities where process heat contributions supplement space heating. First cost and operating cost remain minimal compared to fully conditioned units.

Indirect-Fired Makeup Air Units use conventional heat exchangers separating combustion products from supply airstream. Gas burners or oil burners heat a metal heat exchanger, transferring thermal energy to outdoor air passing through the opposite side. Thermal efficiency reaches 80-85% based on higher heating value, with 15-20% losses through flue gas exhaust. Indirect units suit any application requiring heated outdoor air without combustion product contamination.

Direct-Fired Makeup Air Units inject natural gas or propane burner flame directly into the outdoor airstream, adding combustion products (primarily CO₂ and water vapor) to supply air. This approach achieves 90-95% thermal efficiency by eliminating flue losses and capturing both sensible and latent heat from combustion. Direct-fired units require adequate building volume to dilute combustion products below OSHA permissible exposure limits. The stoichiometric combustion of natural gas produces approximately 1.1 pounds of water vapor per cubic foot of gas consumed, increasing space humidity alongside CO₂ concentration. Applications include warehouses, manufacturing plants, and athletic facilities with sufficient air volume for dilution.

Makeup air volume matches exhaust airflow to maintain slight negative pressure (0.02-0.05 in. wg) preventing uncontrolled infiltration while avoiding positive pressure that forces conditioned air through building envelope leaks. Kitchen exhaust hoods require 100% makeup air replacement, while laboratory and industrial exhausts may operate with partial replacement if building air balance tolerates the deficit.

Destratification Fan Applications

Thermal stratification in high-bay spaces creates temperature gradients where ceiling air temperatures exceed occupied zone temperatures by 15-30°F. This wasted heat accumulates at the ceiling due to natural convection while thermostats sense cold conditions at floor level, driving excessive heating system operation.

Destratification fans reverse this pattern by gently circulating air from ceiling to floor. Large-diameter (8-24 feet) slow-speed (50-100 RPM) fans create gentle air currents (200-400 FPM) that mix stratified layers without creating uncomfortable drafts. The mixing action recaptures ceiling heat, raising occupied zone temperatures 5-10°F with corresponding reductions in heating system runtime.

Energy savings calculations for destratification fans compare heating energy reduction against fan power consumption:

$$\text{Savings} = Q_{reduced} - P_{fan}$$

where heating reduction follows:

$$Q_{reduced} = \dot{m}{air} \times c_p \times \Delta T{recapture} \times \text{hours}$$

Typical installations achieve heating energy reductions of 20-30% with fan power consumption representing 1-3% of total facility energy. Return on investment periods range from 1-4 years depending on heating fuel costs, ceiling height, and climate severity.

Applications include manufacturing plants, warehouses, distribution centers, athletic facilities, and aircraft hangars. Installation requires structural analysis verifying ceiling or roof capacity to support fan weight (200-1000 pounds) plus dynamic loading from operation.

School Building Applications

Educational facilities represent the largest application segment for unit ventilators due to perimeter classroom configurations requiring individual room control and code-mandated ventilation rates.

Classroom unit ventilators mount under windows or in floor-ceiling chases, delivering 200-800 CFM depending on room size and occupancy. Outdoor air dampers modulate based on occupancy schedules and CO₂ sensors, reducing ventilation during unoccupied periods to minimum damper positions (10-15% of maximum airflow). This strategy prevents coil freezing while minimizing energy waste.

Heating coils use hot water from central boiler plants, with two-way control valves modulating flow based on discharge air temperature. Cooling coils connect to chilled water systems in fully conditioned schools or remain absent in heating-only installations common in moderate climates.

Sound level specifications limit unit operation to 35-40 dBA in classrooms, requiring careful fan selection and inlet/discharge treatment. Low-speed centrifugal fans with acoustical liner and flexible duct connections achieve these targets.

Modern school unit ventilators integrate with building automation systems for centralized scheduling, alarm monitoring, and energy reporting. Networked controls enable demand response participation where utility signals reduce heating/cooling during peak periods while maintaining minimum ventilation for occupied spaces.

Warehouse and Industrial Applications

Warehouse facilities favor unit heaters and destratification fans over central systems due to high-bay configurations, minimal cooling requirements, and cost sensitivity.

Unit heater layouts follow throw distance calculations ensuring adequate air circulation throughout the conditioned space. Spacing between units typically ranges from 40-80 feet depending on mounting height and unit capacity. Horizontal propeller units mount on walls or columns at 12-20 feet above floor level, delivering heated air across the space. Thermostat locations require careful placement avoiding direct airflow impingement or dead zones where air circulation proves inadequate.

Industrial facilities with process exhaust systems require makeup air units preventing negative building pressure. Paint booths, welding areas, and chemical processes exhaust contaminated air requiring outdoor air replacement. Makeup air units locate near exhaust points when possible, minimizing cross-contamination risks and reducing distribution ductwork.

Destratification fan installations in warehouses mount fans in a grid pattern with spacing equal to 8-10 times fan diameter. A 20-foot diameter fan requires 160-200 foot spacing between units. Reversible operation provides summer destratification, creating gentle air movement that enhances evaporative cooling without mechanical refrigeration.

Control strategies for warehouse equipment emphasize simple, reliable operation over sophisticated sequences. Time clock scheduling, single-stage thermostats, and manual override switches suit facilities with minimal operating staff and budget constraints. More sophisticated facilities implement occupancy-based scheduling, zone temperature control, and building automation integration.

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