Rooftop air handling units, commonly referred to as “rooftop” units, are air conditioning and ventilation systems installed directly on a building’s roof. For several decades, they have successfully dominated the air conditioning market for large-scale facilities in Western markets and are now gaining traction in Poland as well. This is due to their recognition among designers, architects, investors, and HVAC contractors. Thanks to their compact, monoblock design, they integrate all the most important functions into a single housing: ventilation, heating, cooling, heat recovery, and often also air humidification and dehumidification.

What is a rooftop unit and how does it work?

A rooftop unit is a complete ventilation and air conditioning system that is mounted on the building’s roof structure or directly on a roof opening. Outside air is drawn in through an intake, filtered, thermally conditioned (heated or cooled), and then supplied to the served spaces. Exhaust air, in turn, flows to a heat recovery exchanger, where it transfers or absorbs energy, and is then discharged outside. The heart of the system is typically a reversible heat pump, which enables both effective cooling in the summer and heating in the winter.

Translated with DeepL.com (free version)Kluczowym elementem poprawiającym efektywność energetyczną całego układu jest wysokosprawny wymiennik odzysku ciepła – najczęściej obrotowy (entalpiczny lub sensybilny) lub krzyżowy. Dzięki niemu możliwe jest odzyskanie od 70% do ponad 80% energii zawartej w powietrzu wywiewanym, co znacząco obniża koszty eksploatacji instalacji.

Construction of Roof-Mounted Air Handling Units

Modern roof-mounted air handling units are complex systems comprising numerous mechanical, refrigeration, and electrical components.

A typical monoblock enclosure consists of:

• a frame structure based on aluminum profiles, resistant to corrosion and mechanical deformation,

• insulated enclosure panels made of galvanized sheet metal, additionally coated with a polymer (polyester) coating, ensuring high resistance to weather conditions,

• a support frame equipped with transport handles to facilitate crane installation,

• an interlocked air intake and exhaust duct, ensuring the separation of supply and exhaust air streams without mutual interference.

The following are installed inside the housing as standard:

• supply and exhaust air filters (classes M5/F7, PM10 65% / PM1 55%)

• high-efficiency rotary heat recovery exchanger

• recirculation damper (economizer) allowing for the adjustment of fresh air supply,

• supply and exhaust fans with EC motors (brushless DC motors with electronic commutation)

• built-in reversible heat pump (refrigeration system with scroll compressor)

• integrated automatic control system (PLC controller with operator panel).

Depending on the specific requirements of a given facility, roof-mounted units can be equipped with optional top-mounted heaters: water-heated, electric, or gas-fired with a built-in burner. Thanks to the unit’s modular design, installing an additional heat source does not require changing the overall dimensions of the unit.

Larger roof-mounted units (with higher capacity) can be designed as two-section units—consisting of a separate ventilation block and a rotating heat exchanger and filtration block. This solution facilitates transporting the unit to the roof and installing it in areas with limited access.

Operating Modes of Roof-Mounted Units

Modern roof-mounted units equipped with advanced automation systems can operate in several modes, adjusting their performance to the current room load and outdoor conditions.Tryby pracy central dachowych

Nominal Operating Mode

Activated when the room is at full capacity—typically during the day. The unit maintains a nominal, constant supply and exhaust airflow, monitored by flow sensors. The economy damper automatically adjusts the fresh air supply from a minimum value (set by the user) up to 100%, depending on the room temperature and CO₂ concentration. The system automatically switches between heating, cooling, and ventilation modes according to current demand.

Thermostatic operating mode

Activated during reduced or no load—most often at night. The unit enters standby mode, but the measurement system remains active, monitoring the temperature via a room sensor. If the temperature drops below the setpoint, the system automatically starts the unit at nominal airflow using recirculated air. It is also possible to switch to full recirculation mode to quickly equalize the temperature, e.g., after detecting frost on the heat exchanger.

Freecooling Mode

Activated in the summer when the outdoor temperature is lower than the indoor temperature and the room is warmer than the setpoint. In this mode, the compressor-based cooling system and the rotary heat recovery exchanger remain off—all cooling energy comes from the fresh, cool outdoor air supplied to the building. This minimizes electricity consumption while lowering the temperature inside the building without engaging the compressor system.

Areas of application

Roof-mounted units are designed for all large-scale and industrial facilities where simultaneous ventilation, heating, and cooling are required without the need to install an extensive network of internal ducts. The main areas of application include:

• production halls and industrial facilities – where strictly defined air parameters (temperature, humidity, cleanliness) must be maintained for technological processes and employee comfort

• warehouses and logistics centers – large-volume facilities where effectively conditioning the space using traditional methods is costly and complicated

• big-box stores and shopping malls – where customers’ thermal comfort directly translates into sales results

• sports arenas and recreational facilities – requiring the supply of large volumes of fresh air while maintaining temperature control

• exhibition and trade show pavilions</strong

• schools, universities, and conference centers with large auditoriums or multipurpose halls.

Advantages of rooftop units – an edge over conventional solutions

Choosing a rooftop unit as an HVAC solution offers a number of tangible benefits compared to traditional systems based on separate air handling units, chillers, heat pumps, and heaters.

Compactness and space savings

All components of the HVAC system are enclosed in a single housing mounted on the roof. There is no need to allocate additional technical space inside the building for a mechanical room. This is particularly important for facilities where every square meter of usable space has tangible value.

Simplified and quick installation

Installing the rooftop unit involves three main steps: constructing the roof base with a roof penetration, positioning the unit on the prepared base using a crane, and connecting the electrical power and utilities (water, gas—if applicable). This eliminates the need for complex installations between individual system components, which shortens the project timeline and reduces the risk of installation errors.

Optimization of investment costs

By integrating all components into a single unit, the total length of piping, electrical wiring, and ventilation ducts is reduced. This lowers both material and installation costs. Compact shipping dimensions simplify logistics and reduce delivery costs.

Operational flexibility and modularity

They can operate with various heat sources (electricity, gas, hot water). The economizer damper ensures flexible control of fresh air intake, and automation systems enable communication between multiple units operating in a single space (Master-Slave systems), allowing for precise management of conditions in large-area facilities.

Low maintenance costs

Service access to the unit is from outside the building—from the roof, through a wide access door. There is no need to enter the building, which is particularly important in production facilities operating on continuous shifts.

Automation and management systems

Modern rooftop units are equipped with advanced controllers featuring intuitive touch panels and the ability to integrate with building management systems (BMS) via Modbus RTU, BACnet, or TCP/IP protocols. This enables remote monitoring and control of the unit, archiving of operational data, remote fault diagnostics, and the execution of schedules.

W przypadku obiektów obsługiwanych przez wiele central dachowych pracujących równolegle stosuje się hierarchiczne systemy automatyki klasy Master-Slave. Sterownik nadrzędny (Master) zbiera dane z wszystkich sterowników podrzędnych (Slave), analizuje pomiary temperatur w pomieszczeniu ze wszystkich central. Na podstawie danych sets the desired supply air temperature and transmits it to the local controllers. In the event of a communication failure, the slave controllers execute the programmed emergency strategy—autonomous operation or shutdown.

Integration with CO₂ sensors enables demand-controlled ventilation (DCV), which automatically increases the proportion of fresh air based on the current carbon dioxide concentration in the room. This allows for additional energy savings when the building’s occupancy varies.

Installation and Design Guidelines

Proper installation of a rooftop unit requires careful design of the roof base and roof penetrations. Key design guidelines for installing rooftop units include:

• The roof base should be made of a steel structure, leveled with particular precision—the deviation from level should not exceed the values specified by the unit manufacturer.

• The base must provide linear support for the unit frame along its perimeter—the dimensions of the frame and the base must be coordinated.

• Roof openings (for air supply and exhaust) must precisely align with the air inlet and outlet on the unit; as a standard, the openings are enlarged by 50 mm on each side relative to the unit’s connection flange dimensions.

• The clearance beneath the unit (between the bottom edge of the unit and the roof surface) should be at least 60 cm – this ensures unobstructed connection of ventilation ducts and service access.

• Ensure that the base components do not interfere with the roof structure, downspouts, or other obstacles.

• Thermal and acoustic insulation of roof penetrations must be installed in accordance with the design requirements for the specific building.

The unit is installed on the roof using a crane or hoist, with lifting slings attached to the transport handles integrated into the unit’s frame. Once the unit is positioned on the base, it is bolted to the base, and then the electrical power supply is connected to the built-in distribution panel.

Selection and Design Criteria

Selecting the appropriate roof-mounted unit requires consideration of a number of technical and economic factors:

Required air capacity

Airflow is selected based on the standard fresh air supply rate per person or per unit area (in accordance with EN 16798-1), the technological requirements of the production process, or air change rates. For commercial and sports facilities, the air change rate is typically 4–8 h⁻¹, while for production halls it can reach over a dozen.

Heat Load

The cooling and heating capacity must cover the building’s calculated heat gains and losses, taking into account gains from solar radiation, mechanical and electrical equipment, lighting, and occupants. Peak demand under extreme weather conditions is also a critical factor.

Acoustic requirements

The sound pressure level of roof-mounted units is typically 57–63 dB(A). For facilities requiring special acoustic conditions (e.g., offices, schools), it is necessary to analyze noise propagation through ventilation ducts and building partitions, and possibly install acoustic dampers.

Refrigerant and legal requirements

When selecting rooftop units, the requirements of the F-Gas Regulation (EU No. 517/2014) regarding refrigerants must be taken into account. Refrigerants with low GWP (Global Warming Potential), such as R32 (GWP = 675), are increasingly being used to replace the older R410A (GWP = 2088). Regulatory trends in the EU are moving toward further restricting the use of high-GWP refrigerants.

Summary

Roof-mounted air handling units are a mature, versatile, and energy-efficient solution for large-scale facilities. Their main advantages include compact design, ease of installation, and the integration of all HVAC functions into a single unit. Another advantage is the advanced automation system. All these features make them the optimal choice for investors seeking an efficient and cost-effective HVAC system.

Growing requirements for building energy efficiency, increasingly stringent CO₂ emission standards, and the rapid development of heat pump technology mean that rooftop units will play an even more important role in the design of HVAC systems in the coming years.