**1. Introduction**

The use of environmental control systems has significantly increased in the building sector in order to reduce the energy consumption of heating, ventilation, and air-conditioning (HVAC) systems [1]. Air handling units (AHUs) are one of the most complex building service systems [2], and can include heating, cooling, humidifier, mixing element, and heat recovery units, in order to provide the required indoor air quality and thermal comfort in conditioned spaces [3].

In a typical AHU, chilled water in the cooling coils cools the air, and hot water (or steam) in the heating coils heats the air, in order to maintain the desired temperature of the supply [4]. The supply and return fans assist in moving the air for heat exchange, as well as circulating it in the HVAC system at the required flow rate [5]. Several components are part of a typical system, i.e., the chiller, the boiler, the supply and return fans, and the water pump that consumes a lot of energy [6].

Direct expansion ventilation units are becoming more commonly used central air-conditioning technical solutions, in which a refrigerant is directly delivered to the cooling (and heating) coil [7]. These systems have the potential to save cooling and heating energy use, since they do not require any water pumps for their operation, compared to water-based central air conditioning systems [8,9].

Developers are working really hard to minimalize the energy consumption of their developed devices; however, there are many imperfections in the actual available product catalogues, technical data, and technical support service systems, especially for the annual energy designing provided by the ventilation producers for building service and energy design engineers [10,11]. Therefore, it would be particularly significant to have measured and recorded data obtained from field studies [12,13], which may be utilized in the course of design work, and which would allow a proper estimation of the expected realizable annual energy consumption of air handling elements in the function of the temperature and relative humidity of ambient and indoor air and operating parameters [14].

Stamatescu et. al [15] presented the implementation and evaluation of a data mining methodology based on collected data from a more than one-year operation. The case study was carried out on four AHUs of a modern campus building for preliminary decision support for facility managers. The results are useful for deriving the behavior of each piece of equipment in various mode of operation and can be built upon for fault detection or energy efficiency applications. The imperfection of their work is the missing data for air condition parameters (temperature and humidity) between the coils and mixing box; before and after the fans, which cannot be neglected, since the electrical motor of the fans increases the air temperature and decreases the relative humidity; and the air volume flow rate, which changes during the operation. All these missing parameters have a significant effect on the energy efficiency of the ventilation system.

Hong et. al [16] conducted a case study on a running AHU for data-driven predictive model development. In order to develop the optimal model, input variables, the number of neurons and hidden layers, and the period of the training data set were considered. The results and conclusions presented for the one-year field study could have much better reflected the reality from the view point of energy performance, if further temperature and relative humidity sensors had been placed between the coils and humidifier element, before and after the fans. Only focusing on energy performance data recording is not enough, since the desired indoor air quality and thermal comfort are also significant parameters that need to be considered. To draw a more exact conclusion from this point of view, the CO2 parameter should also have been monitored and recorded in the outdoor air inlet (OA) and supply air outlet (SA) sections in the investigated AHU.

Based on a literature review of the field, there are some case studies in which the heat recovery unit has also been considered in the ventilation system. Noussan et. al [17] presented results obtained from an operation data analysis of an AHU serving a large university classroom. The main drivers of energy consumption are highlighted, and the classroom occupancy is found to have a significant importance in the energy balance of the system. The availability of historical operation data allowed a comparison of the actual operation of the AHU and the expected performance from nominal parameters to be performed. Calculations were made considering the operation analysis of the heat recovery unit over different years; however, the existing system does not include any heat or energy recovery devices, so there are no exact measured data from this point of view.

Bareschino et. al [18] compared three alternative hygroscopic materials for desiccant wheels considering the operation of the air handling unit they are installed in. Their results demonstrated that a primary energy saving of about 20%, 29%, and 15% can be reached with silica-gel, milgo, and zeolite-rich tuff desiccant wheel-based air handling units, respectively. The results were given based on a simulation and there is no exact measured data, which would be significant for making precise and clear energetic conclusions.

In this work, a field study is carried out on an existing, operating ventilation system that includes an air-to-air rotary heat wheel, a mixing box element, and a direct expansion cooling coil connected to a variable refrigerant volume outdoor unit. One of the main objectives of the present paper is to investigate the cooling energy performance and thermal behavior of each air handling component separately. To achieve this, an advanced data recording and remote monitoring system was considerately developed by building managemen<sup>t</sup> system-based software. The system includes an electricity energy meter installed in the outdoor unit, as well as temperature, humidity, air velocity, and CO2 sensors placed in the inlet and outlet section of all the air handling elements that have an effect on the cooling process. The purpose of the CO2 measurements was to investigate the CO2 cross-contamination, which occurs from the exhaust air flow to the supply air flow in the air-to-air rotary heat wheel, resulting in indoor air quality degradation. The novelty of this research is the accurate determination of the seasonal e ffectiveness and the energy saving impact of the heat wheel on the electric energy consumption of the outdoor unit. Moreover, the relative average and maximum value of CO2 cross-contamination in the rotary heat recovery using the developed measurement system in the cooling period are presented. A further innovation in this study is the analytical evaluation method developed, which shows a good agreemen<sup>t</sup> between the calculated and measured energy consumption.
