Back bone of HVAC design for buildings are areas are building construction, building usage, and building location.

Human comfort (depends on sex, age, health etc,. ),  activity like normal, heavy activity, applications such as auditorium, operation theatre, etc., design of buliling or structure, location of bulding, psychrometric condtions etc,. 

In my thinking there are two parts; (1) Accurate load calculation / estimate, (2) Component selection,  

Dr.S.Ravindran

actually your question is based on the thermodynamics( psychrometry) human comfort, your going to design the HVAC, THE FOLLOWING FACTOR WILL BE FACTORS TO BE CONSIDERED   (1) DRY BULB TEMPERATURE (2) SEATING CAPACITY (3) RELATIVE HUMIDITY OF AIR

HVAC design procedure need not fully fall in domain of Mechanical Engineering.  Though it involves knowledge of  heat transfer, effect of weather and solar radiation on above, human response to ambient, statistics, chemistry, biology, civil engg., duct design, refrigeration system design/selection, fan and pump selection..so on and so forth.  Thus, it is  inderdisciplinary in nature.

Human comfort by controlling air quality, temperature and relative humidity. To achieve these you  have understand thermodynamics, fluid mechanics heat tranfer  ect...

Basically, its spine is : Maximum Human Comfort with Minimum Energy spent. As Mr. Jignesh said it is interdisciplinary. To achieve this obviously we should focus on every factor which accounts; as most of the people mentioned in above comments. 

Syazwan , there  are many publications on HVAC not a difficult problem to master...gen y typical problem

Back bone of HVAC design is Refrigeration and airconditioning system.Cooling load calculation is more important for the evaluation of HVAC.

As the name indicates backbone for HVAC is Heating Ventilation and Air Conditioning.  The Heating and Cooling Loads, Flow of  Air, Heat Dissipation systems, Human Comforts viz., Comfort Temperature, Relative Humidity and Speed of Air are the important factors.

Syazwan what do you mean by backbone  is it Fundamentals of HVAC--to get a better information just write to some of the maufacturers of  products and ask for notes.

The basis of the HVAC are heat transfer, fluid mechanics, thermodynamics and solar radiation.

 thanks for the response. i get a new knowledge from your response. 

HVAC&R is a challenging field because you need to understand the basic subjects first. You must have thorough knowledge of thermodynamics,heat transfer and fluid mechanics.But among all these,heat transfer is the subject which will be widely useful in all the aspects and components of refrigeration and air conditioning.

Having said that; thermodynamics is important in laying down the foundations of this subject and fluid mechanics is important simply because refrigeration system is a flow system.In this universe every living thing has a flow system within it and wherever life exists then there will be a flow system in the vicinity of that location(Rivers etc). So that makes fluid mechanics important in every field;not just refrigeration.

So overall all the mentioned subjects are important but the most important from refrigeration point of view is heat transfer. Thats what i believe.

Thank you. ..and good luck

The heart of the HVAC&R industry is the compressor which moves the refrigerant around the system and so compressor design and thermodynamics are also vital and very important beside heat transfer( specially heat exchangers design)

Syazwan,

Do not forget the V in the term HVAC. Indoor climate may be modified by proper ventilation, which is mandatory in order to provide a good working ambience in a room. Clean, fresh air in proper q'ties is as important as controlling the temperature.

Study of energy required for human comfort and ventilation in building is the backbone of HVAC design.

Actually HVAC system design is based on the principles of thermodynamics, fluid mechanics, and heat transfer, psychrometry. which is the backbone to the HVAC design.

For HVAC, pshchrometry is the back bone.  Other principles are included in it.  

Regards, Dr.S.Ravindran