HVAC systems can generally be separated into 2 categories: convective (also referred to as all-air) and radiant systems. Before diving into the HVAC systems, let us talk about the 3 main mechanisms of thermal transfer: conduction, convection and radiation. We experience heat in our day to day life through these transfer mechanisms.
Conduction: the process by which heat moves from the hotter end to the colder end of an object. You primarily feel heat through conduction when you touch a boiling pot with your hand. The heat moves from the hotter end (boiling pot) to the colder end (your hand).
Convection: heat transfer from one place to another through movement of fluid. You are experiencing convection when you feel a cool breeze in a hot summer afternoon or freezing from a gust of chilly wind in the winter. Air is a fluid. The wind which is moving air is removing heat from your body in both of these scenarios.
Radiation: emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. The sun is heating up earth through radiation. When you are walking under the sun you are gaining heat from the sun through radiative heat exchange. Once you are under a shade you immediately feels less hot because the radiation is cut off by the shade.
Convection is the dominant mechanism used in HVAC system for either heating or cooling (removing heat) the building occupants. The two examples we had in our previous posts (illustrations below) are convective systems. They condition the air, throw it into the space and through the conditioned-air convectively heats or cools the occupants.
In radiant systems the primary heat mechanism used for heating and cooling the occupants is radiant heat exchange. Active surfaces are either heated or cooled and they are exchanging heat with the occupants radiantly as shown in the illustration below. For heating, the radiant surfaces are radiatively heating the occupants while for cooling, the heat moves the other way from the occupants to the radiant surfaces as the occupants get cooled. Usually water is used to actively heat or cool the surfaces.
Heating - hot water is supplied to heat up the surfaces and radiantly heat up the occupants. As the occupants are heated, the water loses heat and is recirculated to be reheated by a heater which can be a boiler or a heat pump.
Cooling - chilled water is supplied to cool the surface and radiantly remove heat from the occupants. As the occupants are cooled, the water gains heat and is recirculated to be chilled by a chiller.
For heating radiant system, resistance heater powered by electricity can also be embedded in the surface to heat it up.
Now that we have a conceptual understanding of the two different types of HVAC system. We will look into the differences between the two types of HVAC systems and their different system configurations in our next post.
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