Naturally Conditioned, Air-conditioned and Mixed Mode Buildings

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Buildings are usually classified into three categories, namely air-conditioned, naturally conditioned and mixed mode or hybrid buildings. Description is as follows:

Naturally Conditioned Spaces of Buildings  

Naturally conditioned spaces are those where the thermal conditions of the space are regulated primarily by the occupants through the opening and closing of windows. It is specifically noted that the windows must be easy to access and operate, and sometimes also called free running buildings. 

Naturally ventilated (NV), naturally conditioned (NC), unconditioned, and non-conditioned buildings or spaces are the similar terms very frequently used in the context of thermal environment conditions in non-air-conditioned buildings. Their meanings have a significant overlap with some differences. As per the previous studies conducted by de Dear and Brager in 2002, the definition of naturally conditioned buildings is one of the most contentious issues including as defined in section 5.3 of ASHRAE Standard 55. As suggested by the authors , the term naturally conditioned can be used considering the following circumstances:
  • Naturally conditioned spaces are where the thermal conditions of the space are regulated primarily by the occupants through the opening and closing of windows. It is specifically noted that the windows must be easy to access and operate.
  • Spaces can have a heating system, but the method does not apply when it is in operation.
  • Spaces cannot have a mechanical cooling system (e.g. refrigerated air-conditioning, radiant cooling, or desiccant cooling).
  • Spaces can have mechanical ventilation with unconditioned air, but the opening and closing of windows must be the primary means of regulating thermal conditions.
  • Occupants of spaces must be engaged in near sedentary activity (1-1.3 met), and must be able to freely adapt their clothing to the indoor and/or outdoor thermal conditions.

Air-conditioned Buildings (AC)

Air-conditioned buildings (AC) are those that equipped with Heating, Ventilation and Air-conditioning (HVAC) system and also known as climate controlled buildings. The indoor environment in air-conditioned buildings is controlled by Heating, Ventilation and Air-Conditioning (HVAC) system to satisfy the need of thermal comfort of the occupants. Heating systems are commonly being used in the cold climate where the primary requirement is space heating (increase room temperature than the outdoor temperature) whereas cooling systems are employed in warm climatic conditions to lower the indoor temperature as compared to the outdoor temperature. Since room temperature in air-conditioned buildings is maintained at certain thermostat that results in a narrow bandwidth of room temperature. The study of thermal comfort performed in climate chamber is carried out at steady state conditions while field study of thermal comfort is conducted under transient thermal conditions. The field study of thermal comfort is a person-environment approach and it considers the effect of adaptive processes on comfort temperature of individual or group of subjects.

The use of air-conditioning is increasing and it is also expected to go up in further buildings. Air-conditioning is an energy intensive process and needs to be studied for evaluating the optimum thermal comfort and energy conservation. 

Use of air-conditioning systems is becoming popular in modern offices, IT spaces, institutes, hotels, hospitals and other buildings due to urbanization, civilization and high expectation to live and work in conditioned environment. The selection of air-conditioning system depends upon the climatic condition, building type, and usage. Buildings can use either central air-conditioning system while or unitary/packaged air-conditioners to maintain desired indoor thermal environment. Also, occupants may have controls such as thermostat set point control, air speed and different cooling modes as per their requirements and availability of such controls.

Mixed Mode or Hybrid Buildings 

The buildings which are operated sometimes on air-conditioning mode and remaining time on naturally conditioned mode are called mixed mode or hybrid buildings. 

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