# Predictive Model for the Optimized Mixed-Air Temperature of a Single-Duct VAV System
## 1 Introduction  
As global warming accelerates due to greenhouse gas emissions, efforts to reduce these emissions are required. Building energy use accounts for approximately 25% of the total energy used, and this is increasing in South Korea. Most energy consumption is consumed by HVAC systems [1]. One method for saving energy in HVAC (heating, ventilating, and air conditioning) system is to improve the operating method. The economizer control of a HVAC system is an energy-efficient measure by improving the operating method that introduces outdoor air to save cooling energy when the outdoor-air temperature is sufficiently low. There are two types of economizer control: dry-bulb temperature control and enthalpy control. Each control adjusts the outdoor-air intake ratio by comparing indoor and outdoor temperatures or enthalpy, and at this time, the mixed air set-value is constant [2]. Cooling energy can be saved when the HVAC system is operated by an economizer control. When the system is operated by enthalpy control, more energy can be saved compared to dry-bulb temperature control [3; 4; 5]. Various studies are being conducted on saving energy through the use of economizers. [PERSON] et al. evaluated the control methods of various economizers while considering the mixed-air temperature, outdoor-air intake ratio, and cooling-energy consumption. In addition, economizer controls in various climate environments have been evaluated in terms oftheir cooling-energy consumption [6]. [PERSON] et al. evaluated energy consumption by economizer controls in six climate zones in China using a simulation [7]. [PERSON] et al. simulated the optimal economizer operating range for cost reduction caused by mechanical cooling and humidification and confirmed that energy was saved through an experiments [8]. In addition, studies on mixed-air temperature control in economizers has been conducted and is as follows: [PERSON] et al. set up a steady-state energy model of the AHU system; they derived the optimal supply air temperature set point value for minimizing energy costs when controlling the economizer [9]. [PERSON] et al. analyzed the relationship between the mixed-air temperature and energy consumption using the BIN method and a simulation; it was confirmed that the set point value of the mixed-air temperature at which energy consumption was minimized was not constant depending on the operating conditions. Accordingly, an optimal economizer control that adjusts the mixed-air temperature variable according to the load using an ANN (Artificial Neural Network) was presented [10] As such, the economizer is an energy-saving operation method, and the mixed air temperature set-point is controlled constantly in the existing control. Although various studies have been conducted on the control of economizers, there is insufficient research dealing with economizer optimal control that control the mixed air temperature set-point in consideration of real-time dynamic conditions (outdoor air condition, indoor occupancy etc.) in building. For economizer optimal control, it is necessary to predict the future state by considering all applicable conditions based on the current state. ANNs are neural network-based learning algorithms used in biology that were developed by [PERSON] and [PERSON]. An ANN can be used for analyses regardless of the variables involved because empirical inferences based on the learned data are possible without theoretical explanations. An ANN consists of an input layer that receives input data and delivers data, a hidden layer that processes input values and produces results, and an output layer that calculates the system output values according to the input values and system states. Each layer is composed of nodes, and the result is calculated by the links between the nodes and the transfer function [11; 12]. ANN can process the relationships between non-linear variables accurately and quickly and is most widely used in the field of building performance and energy, and various related studies have been conducted.