ASHRAE Journal - February 2022 - 46

TECHNICAL FEATURE
energy simulation software
EnergyPlus2 for seven climate
zone locations. Option
3a is when the same coils
and water loops of the base
(traditional system) is used.
Option 3b is when the combined
coils are designed to
meet the peak loads. Option
4b is identical to Option
3b but using heat pumps.
Dual-fuel heat pumps or
possible needed emergency
heaters are not included in
the cost as the needs of those
depend heavily on location.
In all options, the initial and
operation equipment costs
are equal and lower than the base.
Conclusion
The common design in traditional VAV systems, using
two separate chilled and hot water loops, reduces the
opportunity to achieve initial and operating cost savings.
Including heating and cooling sources in both loops can
make any of those loops provide heating or cooling when
needed, reducing the number of coils or cooling and
heating equipment to be installed.
In the first configuration option, when a common coil
is used instead of two coils, the required cooling and
heating by AHUs is provided with fewer coils, smaller
AHU, fewer pipes and valves and reduced fan energy
use. Compared to the traditional design, this option
reduces both initial and operating cost as indicated in
Table 1. If air-source heat pumps are used in the water
loop, as in Option 2, fewer cooling and heating sources
are required to be installed, further reducing the project's
initial cost.
Other possible improvements as in Option 3 are to
use the traditional two coils, each connected to a water
loop equipped with both cooling and heating sources so
it can provide heating or cooling as needed. Thus, the
combined coils and water loops are designed to meet
the peak loads, reducing the size of coils, AHU and pipe
diameters. It also reduces chiller, fan and pump energy
use. This option reduces significantly both initial and
operating cost as indicated in Table 1.
46
ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2022
TABLE 1 Cost estimation for several option configurations.
COST
EQUIPMENT
Coil + Valve
Initial
Cost of
Equipment
Listed
Pipe
Cooling or Heating
Equipment
Pumps
AHU Fan
Operation
Cost of
Equipment
Listed
Pump
Chillers/Boiler/HPs
OPTION 1
Base -
(40% to 50%)
Base -
(10% to 20%)
Base
Base -
(2% to 7%)
Base -
(20% to 30%)
Base -
(5% to 10%)
Base -
OPTION 2
Base -
(40% to 50%)
Base -
(10% to 20%)
Base -
(15% to 25%)
Base -
(2% to 7%)
Base -
(20% to 30%)
Base -
(5% to 10%)
Base -
(2% to 7% Boiler) (2% to 7% Boiler)
OPTION 3A
Base
Base
Base
Base
Base
Base -
(40% to 50%)
Base -
(10% to 20%)
OPTION 3B
Base -
(15% to 25%)
Base -
(20% to 30%)
Base
Base -
(15% to 25%)
Base -
(5% to 15%)
Base -
(10% to 15%)
Base -
(15% to 25%)
OPTION 4B
Base -
(15% to 25%)
Base -
(20% to 30%)
Base -
(25% to 35%)
Base -
(15% to 25%)
Base -
(5% to 15%)
Base -
(10% to 15%)
Base -
(15% to 25%)
Using air-source heat pumps instead of traditional
boilers and chillers as in Option 4 lowers the number of
equipment to be installed, further dropping the system's
initial cost.
All recommended options, with a simple changeover
temperature control and SAT temperature control
loop, provide huge energy and cost benefits for any
buildings designed to be conditioned by traditional
chilled-water VAV systems with air-cooled chillers.
Attention should be taken to increase the time when
both coils and loops operate during mechanical cooling-only
seasons or heating-only seasons. The control
can limit the coil valve opening to a maximum valve
position, so the loads can somewhat split between
the coils. This maximum value can dynamically
be adjusted based on the number of chillers, boilers
or heat pumps that operate in each loop. Also, it
requires careful examining of coil selections such as
number of rows of tubes, fins, coil face area, etc., and
optimal design of those parameters so the same coil
can provide effective cooling or heating in different
seasons.
References
1. ASHRAE Guideline 36-2018. High-Performance Sequences of
Operation for HVAC Systems.
2. DOE. 2018. " EnergyPlus. " Version 9.0.1. U.S. Department of
Energy. https://energyplus.net/
https://www.energyplus.net/ http://ashrae.org
ASHRAE Journal - February 2022

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