In this short podcast, Bryan answers a question submitted to HVAC School by an aspiring licensed architect who wanted to learn more about the many different types of HVAC systems.

The three main buckets of HVAC systems are air-to-air, water-source, and air-to-water. Air-to-air systems move air around to remove heat from one space, and that heat is rejected to the air somewhere else. Water-source systems move water around the building and use water as the heat rejection medium. Air-water hybrid systems condition the load with water and may use air for ventilation; water or air may be used as the rejection medium. Systems may be direct-exchange (DX) and may transfer heat directly to refrigerant, or they may flow the air or water over a coil with water or glycol, utilizing a secondary fluid. Systems may also have separate indoor and outdoor architecture (split systems), or all components may be rolled into a single box (package unit). Package units include window units, PTACs, and RTUs.

When it comes to forced-air systems, constant air volume (CAV) systems maintain the same volume of airflow (though the temperatures will change). Variable air volume (VAV) systems use one stream of cold air in a main duct, and each zone has a VAV box that functions as a damper to control zones individually. Dual duct systems have one cold duct and one warm duct that run parallel to each other and mix at each zone. Packaged rooftop units (RTUs) are self-contained with ducts that run down into the space and are common in retail spaces.

Air-to-water systems use fan coil units (FCUs) fed with chilled or hot water. Air moves locally inside the space, so there is less ductwork and good zone control, but there are many units to manage.

Chillers make chilled water, and that water is pumped around the building and sent to individual air handler units (AHUs). These are highly efficient and have large amounts of piping. They need mechanical rooms and dedicated personnel to maintain them.

Variable refrigerant flow (VRF) systems are DX systems that are becoming more popular and consist of multiple indoor units with one or more outdoor units. Some of these can be used for heat recovery, meaning one space can be cooled while another is heated.

Heat pump types include air-source, water-source, and ground-source. Air-source heat pumps absorb heat from the air via one unit and reject it via the other; the outdoor and indoor units can swap functions. Water-source heat pumps are common in commercial applications and have multiple heat pumps tied into a water loop that tries to stay within a given temperature range via boilers and cooling towers. Ground-source or geothermal heat pumps pick up heat from the earth’s stable temperature and are highly efficient, but they have high installation costs.

Passive systems come in all sorts of varieties and reduce the HVAC system’s loads but don’t replace HVAC systems in North America. Mechanical systems consist of straight-cool (air conditioner with electric heat), furnaces (gas, propane, or oil combustion), or heat pump (reversible air conditioners) systems.

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