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