Radiant and Forced Air
Radiant and forced air HVAC systems comprise the two basic methods of conditioning the temperature of an interior space. The traditional radiator best represents radiant HVAC systems; radiant systems transfer hot or cool temperatures through a series of pipes or tubes carrying hot or cold water.
Such tubes are usually concealed beneath floors or behind walls. In the case of a radiator, the pipes are visible as part of a stand-alone unit.
Forced air systems use ductwork and fans to circulate air in and out of a building. Once the air is sucked from a building, the furnace coils or air conditioner coils condition the air, alter its temperature and send it back into the duct system.
Single Zone and Multiple Zone HVAC Systems
The terms single zone and multiple zone refer to the amount and method of control available to an individual HVAC system. Single zone HVAC systems have a single control panel for the system, which might span across multiple rooms or floors.
Multiple zone HVAC systems feature variable controls, similar to a programmable sprinkler system; different zones, such as rooms or floors, can experience various temperatures according to the programmer's preference. Alternately, the single zone system often results in some rooms being too hot while others are too cold.
Despite its disadvantages, the single zone system requires less equipment and is generally less expensive than the multiple zone system. Multiple zone systems allow a zone's occupants to alter the temperature of the room without affecting other zones.
For example, the occupants of a zone receiving afternoon sun can lower their room's temperature without affecting the temperature of a zone receiving afternoon shade.
Constant Volume and Variable Volume Systems
The terms constant volume and variable volume refer to the volume of air that flows through the HVAC system. Constant volume systems deliver a steady, constant volume of air through ducts.
Variable volume systems occasionally adjust the volume of air flowing through the ducts to affect a room's temperature. A constant volume HVAC system alters a space's temperature by cooling or heating the air flowing through the ducts.
A variable volume system alters a space's temperature by reducing or increasing the flow of heated or cooled air into the space.