Heating, Ventilating, Air-Conditioning (HVAC) System Types: Direct Refrigerant & All-Air Systems

Heating, Ventilating, Air-Conditioning (HVAC) System Types: Direct Refrigerant and All-Air Systems

from MEEB 9th ed.

Way to classify HVAC systems = by the media used to transfer heat.

Thousands of liquids and gases can be used, three must common in building applications are air, water and refrigerant.

Four main system classifications:

Direct refrigerant systems

All air systems

air and water systems (see separate post)

all water systems (see separate post)

(last three, the heating/cooling production equipment is typically located centrally in a large building.) (in direct refrigerant system, the machine is usually located adjacent to the zone(s) it serves.)

The air handling components = served centrally or floor by floor

Direct refrigerant systems = These nearly eliminate the distribution trees of air or water, relying instead on a heating/cooling device adjacent to or within the space to be served. Prevalent in skin dominated buildings with extensive perimeter zones. tending to be in smaller buildings.

All air systems = Air is the only heat transfer media used between the mechanical room (central station) and the zones it serves, and because air holds much less heat per unit volume than water, the distribution trees for this class are quite thick.

Higher velocities = can be used to reduce duct sizes, but this generates more noise and higher friction resulting in more energy used by fans. Should be used sparingly where space limitations are extreme.

All air systems provide Overall best comfort = Quantities of air moved through the central station(s) are heated or cooled, humidity controlled, filtered, and freshened with outdoor air, all under controlled conditions.

All air, single zone system = The common, small building forced air system, controlled by a single thermostat.

All air, single duct, variable air volume (VAV) system = Most popular large building system of the 1990s. Its single duct requires less building volume for distribution than multi duct systems, and the variation of air volume flow rate (rather than of air temp.) saves energy relative to the single duct with reheat.

Central station = Supplies at normal velocity either a heated or a cooled stream of air.

Automatic volume controls = linked to each zone's thermostat, adjust the volume admitted to that zone, within an air terminal diffuser, often located above a suspended ceiling.

All air Fan powered VAV system = This variation allows individual units to heat when the main supply system is cooling, it might serve perimeter zones. In such a case, the cool air is reduced to a minimum for IAQ and the unit's fan draws additional air from a ceiling (or floor) plenum, heating it as required.

All air multi zone system = Each zone has an individual, centrally conditioned air-stream, the total distribution tree volume grows to astonishing size with only a few zones.

Central station = produces both warm and cool air streams, which are mixed at the central location to suit each zone.

Application = More likely to be found on medium size buildings, or on larger building in which smaller central stations are located on each floor.

Return = The single return air stream collects air from all zones.

All air Single duct with reheat system = This system, along with VAV, has the smallest distribution tree of this class. because at each zone the only object added to the duct is a small reheat coil (heat provided by steam, hot water, or electric resistance.

central station = provides a single stream of cold air that must be cold enough to meet the maximum cooling demand of any one zone, all other zones reheat this air as needed.

Cold weather = outdoor air at temp's as low as 38 deg F can be used, the colder the single central airstream, the less air need be circulated.

Not ideal for buildings with large interior zones in the U.S., central airstream needs cooling most of the time, then much energy is spent to reheat the airstream at most zones.

All air Double duct, constant volume systems = Two complete distribution trees are required, at the height of summer the cooling airstream does all the work, whereas in the coldest winter conditions the heating airstream carries the load. Air from these two streams are mixed at each zone's air terminals to meet heating/cooling requirements.

Temperature and volume can be controlled, thus this system offers better comfort under reduced load conditions than does the single-duct, variable volume system.

Cost = Much more expensive to install, consumes much building volume for the two distribution trees and is usually more energy consuming than the single duct, variable volume system that has largely replaced it.