HVAC BASICS
Air Handling System
Components/Layout:
A: Dampers
H
B: Filters
C: Cooling Coil B C D E
D: Heating Coil F
E: Humidifier
F: Supply Fan
G: Diffusers
H: Return Fan
G
Air Flow:
Air enters the system from the outside through an air duct and is mixed just before the filters with return air from the
space. The mixed air is pulled through a set of filters (B) where impurities like particulates and gaseous
Pollutants are removed. As the air moves forward, it passes over a set of cooling coils (C), where the air is cooled, and
depending on the dew point of the passing air, may be dehumidified. The air then passes over a set of heating coils (D)
where it is heated to the required temperature if necessary. Following the heating coil the air will pass over a set of
humidification tubes (E). If the relative humidity of the passing air is too low these tubes will add moisture to the air as it
passes. After the humidifier, the air is pulled into the supply fan (F), which then pushes it through the supply ductwork,
through the diffusers (G) and into the space. The return fan (H) pulls air from the space and pulls it through the return
ductwork. Finally, the air is either exhausted to the outside or returned to the unit to be mixed with new outside air and start
the cycle all over again.
Refrigerant/Heating Supply:
The cooling coils are fed from a source of cooling, such as chillers, a cooling tower, or DX unit. The refrigerant enters
the cooling coils, absorbs heat from the air passing the coils, and returns to its source to desorb the heat that it took in. The
heating coils are fed from a heat source such as a boiler or electricity. The heat from these coils is transferred to the air
passing over it.
, System Variations:
1. A system may have multiple sets of filters to remove different levels of impurities.
2. The arrangement of the cooling coil and heating coil may vary from unit to unit.
3. A cooling coil may not be present in every unit or may be before the outside air dampers for pre-
cooling.
4. A heating coil may not be present in every unit.
5. The main heating coil may be farther down the line in the ductwork in what is called a reheat. As a reheat the coil
will heat air for a specific space just before it is discharged from the ducts.
6. A system may utilize a bypass setup where the air can be diverted past the coil(s) if it does not need to be treated.
7. Humidification tubes may not be present in every unit or may be farther down in the ductwork just before the
diffusers.
8. A return fan may not be present on every unit if the supply fan creates enough of a draw to pull the air back
through the space.
Heating/Cooling System
Components/Layout:
A: Dampers
G
B: Filters
C: Heating Coil B C D E
D: Cooling Coil F
F
E: Humidifier
F:Supply Fan
G: Return Fan
Air Flow:
This type of unit is most common in environments where moisture removal from the air is not much of a concern.
Starting at the outside air intake, air enters the outside air duct and is mixed with return air from the space just before the
filters (B). This mixed air is pulled through a set of filters where impurities are removed. The air then passes over a set of
heating coils (C) where it is heated to the required temperature if necessary. Next the air passes over a set of cooling coils
(D); here it is sensibly cooled to the desired temperature if necessary. After the cooling coil the air passes over a set of
humidification tubes. If the relative humidity of the air is too low these tubes will add moisture to the air as it passes. After
the humidifiers, the air is pulled into the supply fan (F), which pushes it through the supply ductwork, through the diffusers
and into the space. The return fan (G) will pull in air from the space and push it through the return ductwork. The air is
either exhausted or returned to the unit to be mixed with outside air and start the cycle all over again.
Refrigerant/Heating Supply:
The cooling coils are fed from a source of cooling, such as chillers, a cooling tower, or DX unit. The refrigerant enters
the cooling coils, absorbs heat from the air passing the coils, and returns to the source to desorb the heat that it took in.
The heating coils are fed from a heat source such as a boiler or electricity. The heat from these coils is transferred to the air
passing over it.
2 © Susan 2024
, System Variations:
1. A system may have multiple sets of filters to remove different levels of impurities.
2. A cooling coil may be before the outside air dampers for pre-cooling.
3. The system may utilize a bypass setup where the air can be diverted past the coil(s) if it does not need to be
treated.
4. Humidification tubes may not be present in every unit or may be farther down in the ductwork just before the
diffusers.
5. A return fan may not be present on every unit if the supply fan creates enough of a draw to pull the air back
through the system.
Preheat System
Components/Layout:
A: Dampers
G
B: Filters
C: Cooling Coil H B H C D E
D: Heating Coil
F
E: Humidifier F: F
Supply Fan
G: Return Fan
H: Preheat Coils
Air Flow:
This system is a standard air handling unit that utilizes a preheat coil (H). This coil should only operate during the
winter months, if the outside air is below freezing. The goal of this coil is to preheat the incoming outside or mixed air to
prevent the cooling coil (C) from freezing. These units are typically found in colder regions where there is greater risk
of low winter temperatures. Starting at the outside air intake, air enters the outside air duct and is heated by the pre-heat
coil. The air is then mixed with return air from the space just before the filters
(B). This mixed air is pulled through a set of filters where impurities are removed. As the air moves it passes over a set
of cooling coils (C), here the air is cooled and depending on the dew point of the passing air, may be dehumidified. The air
then passes over a set of heating coils (D) where it is heated to the required temperature if necessary. Following the heating coil the air
will pass over a set of humidification tubes (E). If the relative humidity of the passing air is too low these tubes will add moisture to
the air as it passes. After humidification, the air is pulled through the supply fan (F), which will push it through the supply
ductwork, through the diffusers and into the space. The return fan (G) then pulls in air from the space and pushes it through the
return ductwork. The air is either exhausted or returned to the unit to be mixed with outside air and start the cycle all over again.
Refrigerant/Coolant Cycle:
The cooling coils are fed from a source of cooling, such as chillers, a cooling tower, or DX unit. The refrigerant enters
the cooling coils, absorbs heat from the passing air, and returns to its source to desorb the heat that it took in.
The heating coils are fed from a heat source such as a boiler or electricity. The heat from these coils is transferred to the air
passing over it.
3 © Susan 2024
Air Handling System
Components/Layout:
A: Dampers
H
B: Filters
C: Cooling Coil B C D E
D: Heating Coil F
E: Humidifier
F: Supply Fan
G: Diffusers
H: Return Fan
G
Air Flow:
Air enters the system from the outside through an air duct and is mixed just before the filters with return air from the
space. The mixed air is pulled through a set of filters (B) where impurities like particulates and gaseous
Pollutants are removed. As the air moves forward, it passes over a set of cooling coils (C), where the air is cooled, and
depending on the dew point of the passing air, may be dehumidified. The air then passes over a set of heating coils (D)
where it is heated to the required temperature if necessary. Following the heating coil the air will pass over a set of
humidification tubes (E). If the relative humidity of the passing air is too low these tubes will add moisture to the air as it
passes. After the humidifier, the air is pulled into the supply fan (F), which then pushes it through the supply ductwork,
through the diffusers (G) and into the space. The return fan (H) pulls air from the space and pulls it through the return
ductwork. Finally, the air is either exhausted to the outside or returned to the unit to be mixed with new outside air and start
the cycle all over again.
Refrigerant/Heating Supply:
The cooling coils are fed from a source of cooling, such as chillers, a cooling tower, or DX unit. The refrigerant enters
the cooling coils, absorbs heat from the air passing the coils, and returns to its source to desorb the heat that it took in. The
heating coils are fed from a heat source such as a boiler or electricity. The heat from these coils is transferred to the air
passing over it.
, System Variations:
1. A system may have multiple sets of filters to remove different levels of impurities.
2. The arrangement of the cooling coil and heating coil may vary from unit to unit.
3. A cooling coil may not be present in every unit or may be before the outside air dampers for pre-
cooling.
4. A heating coil may not be present in every unit.
5. The main heating coil may be farther down the line in the ductwork in what is called a reheat. As a reheat the coil
will heat air for a specific space just before it is discharged from the ducts.
6. A system may utilize a bypass setup where the air can be diverted past the coil(s) if it does not need to be treated.
7. Humidification tubes may not be present in every unit or may be farther down in the ductwork just before the
diffusers.
8. A return fan may not be present on every unit if the supply fan creates enough of a draw to pull the air back
through the space.
Heating/Cooling System
Components/Layout:
A: Dampers
G
B: Filters
C: Heating Coil B C D E
D: Cooling Coil F
F
E: Humidifier
F:Supply Fan
G: Return Fan
Air Flow:
This type of unit is most common in environments where moisture removal from the air is not much of a concern.
Starting at the outside air intake, air enters the outside air duct and is mixed with return air from the space just before the
filters (B). This mixed air is pulled through a set of filters where impurities are removed. The air then passes over a set of
heating coils (C) where it is heated to the required temperature if necessary. Next the air passes over a set of cooling coils
(D); here it is sensibly cooled to the desired temperature if necessary. After the cooling coil the air passes over a set of
humidification tubes. If the relative humidity of the air is too low these tubes will add moisture to the air as it passes. After
the humidifiers, the air is pulled into the supply fan (F), which pushes it through the supply ductwork, through the diffusers
and into the space. The return fan (G) will pull in air from the space and push it through the return ductwork. The air is
either exhausted or returned to the unit to be mixed with outside air and start the cycle all over again.
Refrigerant/Heating Supply:
The cooling coils are fed from a source of cooling, such as chillers, a cooling tower, or DX unit. The refrigerant enters
the cooling coils, absorbs heat from the air passing the coils, and returns to the source to desorb the heat that it took in.
The heating coils are fed from a heat source such as a boiler or electricity. The heat from these coils is transferred to the air
passing over it.
2 © Susan 2024
, System Variations:
1. A system may have multiple sets of filters to remove different levels of impurities.
2. A cooling coil may be before the outside air dampers for pre-cooling.
3. The system may utilize a bypass setup where the air can be diverted past the coil(s) if it does not need to be
treated.
4. Humidification tubes may not be present in every unit or may be farther down in the ductwork just before the
diffusers.
5. A return fan may not be present on every unit if the supply fan creates enough of a draw to pull the air back
through the system.
Preheat System
Components/Layout:
A: Dampers
G
B: Filters
C: Cooling Coil H B H C D E
D: Heating Coil
F
E: Humidifier F: F
Supply Fan
G: Return Fan
H: Preheat Coils
Air Flow:
This system is a standard air handling unit that utilizes a preheat coil (H). This coil should only operate during the
winter months, if the outside air is below freezing. The goal of this coil is to preheat the incoming outside or mixed air to
prevent the cooling coil (C) from freezing. These units are typically found in colder regions where there is greater risk
of low winter temperatures. Starting at the outside air intake, air enters the outside air duct and is heated by the pre-heat
coil. The air is then mixed with return air from the space just before the filters
(B). This mixed air is pulled through a set of filters where impurities are removed. As the air moves it passes over a set
of cooling coils (C), here the air is cooled and depending on the dew point of the passing air, may be dehumidified. The air
then passes over a set of heating coils (D) where it is heated to the required temperature if necessary. Following the heating coil the air
will pass over a set of humidification tubes (E). If the relative humidity of the passing air is too low these tubes will add moisture to
the air as it passes. After humidification, the air is pulled through the supply fan (F), which will push it through the supply
ductwork, through the diffusers and into the space. The return fan (G) then pulls in air from the space and pushes it through the
return ductwork. The air is either exhausted or returned to the unit to be mixed with outside air and start the cycle all over again.
Refrigerant/Coolant Cycle:
The cooling coils are fed from a source of cooling, such as chillers, a cooling tower, or DX unit. The refrigerant enters
the cooling coils, absorbs heat from the passing air, and returns to its source to desorb the heat that it took in.
The heating coils are fed from a heat source such as a boiler or electricity. The heat from these coils is transferred to the air
passing over it.
3 © Susan 2024
