The heat is generated in the air-conditioned space from various sources. To maintain the comfort conditions inside the room the total heat generated inside the room per hour should be removed completely.
Here are various sources of heat that affect HVAC designing and heat load calculations, these are:
1) Heat gained by the walls: The walls of the room gain heat from the sun by way of conduction. The amount of heat depends on the wall material and its alignment with respect to sun. If the wall of the room is exposed to the west direction, it will gain maximum heat between 2 to 5 pm. The southern wall will gain maximum heat in the mid-day between 12 to 2 pm. The heat gained by the wall facing north direction is the least. The heat gained by the walls in day-time gets stored in them, and it is released into the rooms at the night time thus causing excessive heating of the room. If the walls of the room are insulated the amount of heat gained by them reduces drastically.
2) Heat gained by the roof and partitions: If the roof is exposed directly to the sun, it absorbs maximum heat. If there is other room above the air-conditioned room, then the amount of heat gained by the roof reduces. The heat gained by the partitions of the room depends upon the type of partition.
3) Heat gained by the windows: Windows of the room are exposed directly to the surrounding and the heat from the sun enters the room by radiation. As in the case of the walls, the heat gained by the rooms through windows depends on their alignment. If there are sufficient curtains on the windows and the external awning the amount of heat gained by radiation reduces. The type of glass doors on the windows also affects the amount of heat gained through the windows by radiation.
4) Heat generated by the people: The people inside the room generate lots of heat. The heat dissipated by working people is more than from sitting people.
5) Heat generated by the electrical appliances: Heat is generated by electrical appliances like lights, motors, coffeemakers, electronic equipments, etc. should also be considered for heat load calculations, which is also called cooling load calculations.
6) Heat gain from outside air: Outside air is normally at a greater temperature than the room temperature. When this air comes inside the room, it brings certain amount of heat along with it.
A good HVAC designer will thoroughly consider all the sources of heat inside the room and find out the total amount of heat generated inside the room per hour. These days it has become easier for engineers to design HVAC as various softwares are available which can also be integrated with AutoCAD. Based on these heat load calculations, they will suggest you the air-conditioning system of proper capacity for your room.