all air, all water, and air-water hvac systems

4/27/16 MEC 522 Extra Credit Assignment All Air System 





Advantages o Most commonly used because controlling air properties is most easily accomplished directly by all air systems (No need to transfer energy from water back to air o Better for maintaining safe indoor air quality o For Single Zone, they are lowest upfront costs, easiest to maintain, and simplest to design o Terminal Reheat and Dual Duct Systems are terminally controlled which leads to increased system flexibility and adaptability o Multi Zone systems – Adequately condition several zones simultaneously o Variable Volume (VAV) systems – air supply can be shifted from one zone to another throughout the day, which reduces the design capacity of the HVAC equipment  Also, a big advantage of VAV systems is the low initial and operating costs compared to other systems because only a single run of ducts is necessary and the air control at the air terminal is simple o An economizer can be used in mild seasons to take advantage of outdoor air temperatures to reduce energy costs Disadvantages o Air is not an efficient heat transfer medium o Extensive ductwork is necessary, which leads to higher building costs o For Multi Zone systems, multiple supply air ducts are necessary, which is expensive and space consuming o VAV Systems – With reduced supply air flow, air ventilation is limited and can be problematic for the indoor air quality Applications o Best used when ductwork is easily accommodated o Used most often when indoor air quality is a main concern o Single Zone – One control device (thermostat) located in the zone controls operation  One family residential Units  Simplest of All Central HVAC All Air Systems o Multi Zone – Larger Buildings with multiple zones demanding different air characteristics  Best suited for applications with high sensible heat loads and limited ventilation requirements o

All Water System







Advantages o Water is a much more effective heat transfer medium, so the distribution pipes (As opposed to ducts) can be much smaller, which reduces upfront costs and saves space o Big advantage is the flexibility for adaptation to many building module requirements and for remodeling work o All water systems can be installed in existing buildings with minimal interference with occupied space Disadvantages o The energy that is carried by water must be transferred to the air, which makes more sophisticated delivery devices necessary o Limited ventilation, no humidification is provided o Fan noise from the fan-coil unit may be a concern o Cooling towers use large quantities of water o Water cooled systems require more maintenance than air cooled systems Applications o Most applicable where individual space temperature control is desired without close control of humidity o For full air-conditioning purposes, the fan-coil delivery device is most commonly used o For all water heating-only systems, delivery devices include baseboard radiators, convectors, unit heaters, and radiant floors. o For all water cooling-only systems, which are very rare, valence units are used as the delivery device, which is a ceiling located counterpart of a baseboard radiator

Air-Water System 



Advantages o Incorporates the main benefits of all-air and all-water systems into a hybrid system  Most notably the space saving of all-water systems and the outdoor ventilation benefits of all-air systems  Most of the space load is dealt with by heated or cooled water (80-90%) and about 10-20% is carried out by heated or cooled air, which is just enough to meet ventilation demands o The air system can provide humidification typically needed during heating seasons o Reduced quantity or air leads to higher velocity of air during distribution, which also reduces space requirements Disadvantages o Because most of the energy is carried by water, there still needs to be a large amount of energy transfer to air in order to facilitate the specific requirements o Requires specially trained staff for maintenance o Humidity can’t be as tightly controlled



o Controls are more complicated than all-air systems o Induction and fan coil units require frequent in-space maintenance Applications o Due to space saving and lower operating costs, these systems are especially beneficial for high-rise structures o Also commonly used in hospitals, schools, office buildings, hotels, apartment houses

References: Textbook: Heating, Ventilating and Air Conditioning, 6th Edition. McQuiston, Parker, Spitler Article: HVAC Components and Systems, Vital Signs Curriculum Materials Project, Walter Grodzik and Richard Furst