ARE346P: HVAC Design Tyler Bolinger
Final Design Report Priscilla Williams
Our project was to size components for a VAV system. A variable-air-volume (VAV) system controls the
dry-bulb temperature in a zone by varying the supply-airflow rate rather than the supply-air
temperature. This is done through a space thermostat at a set point that sends a control signal to a VAV
box which then controls the flow with dampers. If the room temperature is greater than the set point
temperature, the dampers are opened to increase the supply-airflow rate. If the room temperature is
less than the set point temperature, the dampers are closed to decrease the supply-airflow rate. If there
is no difference in the room temperature and set point temperature, the dampers do not change. At
times, different zones of a building will experience peak loads while other zones do not. When this
happens, the zones experiencing peak loads will borrow the extra air from the other zones. VAV
systems typically cool buildings and do not provide simultaneous heating and cooling in different zones.
When simultaneous heating and cooling is needed, secondary systems provide the required heat.
The ASHRAE Handbook describes several advantages and design precautions for VAV systems. For
example, VAV can offer inexpensive temperature control when combined with perimeter heating
systems. Also, the variation in loads can be made useful VAV systems. In addition, these systems are self
balancing and easy and inexpensive to change zones for new loads or users. But when designing these
systems, special care must be taken to ensure acceptable operation at off-peak supply-airflow rates.
Also, fan controls should be considered to reduce power consumption and noise, but only for systems
where this option is economical. Finally, in zones with fluctuating loads, heating or reheat should be
considered to prevent excessive space humidity.
Many HVAC systems use too much energy. Built-up VAV systems can improve system performance,
energy efficiency, cost effectiveness, and occupant comfort of these systems by following design
recommendations made based on findings of a three-year study of five California office buildings.
Careful sizing of VAV boxes, minimizing VAV box minimum airflow setpoints, and controlling VAV boxes
using a “dual maximum” logic can significantly save fan and reheat energy because typical systems
operate at a higher than necessary airflow.
Determine heating and cooling loads for all zones
Size VAV boxes for all zones
Size Reheaters for all zones
The building in question consists of two levels with five zones each (North, East, South, West, Core). A
diagram is shown in Figure 1.
Figure 1: A two story office building with five zones will have a VAV system.
A sample VAV schematic of one floor is shown in Figure 2.
Figure 2: VAV system schematic for one level.
Based on the heating and cooling loads of the entire building, shown in the attached spreadsheet, the
conditions of each zone are established. The VAV boxes are sized first given that the outside air is cooled
to 55 degrees Fahrenheit before passing through the VAV box and reheater. Based on this, and the 75
degree set point of the room, the necessary maximum volumetric airflow can be assessed. This is the
size of the VAV box. If we assume that the whole building will be run be one ACU, for the sake of
simplicity, we can also size the ACU components for heating and cooling loads.
With the VAV boxes sized, we can sum all the airflow rates to calculate the total airflow rate through the
ACU. We will assume 50/50 mixing of the outdoor and return air to reduce the load on the reheating
units while keeping minimum fresh air requirements. For heating loads in the winter, after mixing, the
supply air will be at 50 degrees. A heating coil will heat the air up to about 72 degrees. The air will then
pass through an evaporative cooler, until saturation, which happens at 55 degrees. This air will pass
through the ducts to the VAV boxes and reheaters where it will be heated to the room set point of 75
degrees and 50% relative humidity. Heating loads will be read off of a psychrometric chart.
For cooling loads, it’s a little simpler. Outdoor and return air mix at 85 degrees, then pass through a
cooling coil until saturation at 55 degrees. After this temperature is reached, the reheater will heat the
air to the set temperature of 75 degrees. Again, the cooling loads will be read off the psychrometric
chart. These charts will be given in the data.
Our first step was to size the VAV boxes. As stated before, we did this via the peak cooling loads for
each zone. The cooling load for the plenum on each floor was distributed throughout the 5 zones on
each floor for the sake of convenience. The required maximum flowrate for each zone was calculated
using equation (1):
Q = macp(t2 – t1) (1)
The mass flowrate was found, then converted to a volumetric flowrate, assuming a density of 0.078
lbm/cf for air. Delta T was taken as 20 degrees difference between the duct and the zone. Table 1 lists
the peak cooling load and resulting volumetric flowrate for each zone’s VAV box.
Table 1: Peak cooling Loads for each zone and the respective volumetric flowrates.
Zone Flowrate Q cool
South Level 1 2381 cfm 53 KBTU/Hr
East Level 1 2337 cfm 52 KBTU/Hr
North Level 1 1356 cfm 30 KBTU/Hr
West Level 1 2228 cfm 50 KBTU/Hr
Core Level 1 1584 cfm 35 KBTU/Hr
South Level 2 2854 cfm 63 KBTU/Hr
East Level 2 2520 cfm 56 KBTU/Hr
North Level 2 1634 cfm 36 KBTU/Hr
West Level 2 2483 cfm 55 KBTU/Hr
Core Level 2 2944 cfm 65 KBTU/Hr
With these values, we can calculate the total CFM passing through the ACU. The sum is roughly 22,300
CFM. With the flowrate, we can size the heating and cooling components of the ACU. From a
psychromentric chart, we determine that the difference in enthalpy for the heating coil is about
5btu/lbm Using equation (2):
SQs + SQL = ma(h2-h1) (2)
We can conclude that the max load for the heating coil is about 520 Kbtu/hr.
Passing through the evaporative cooler is an adiabatic process, so there is no load to calculate.
The cooling load is calculated in the exact same way. The enthalpy difference over the cooling coil from
the psych chart is about 10 or 11 btu/lbm, we will assume 10. Given the same volumetric flowrate, this
will require a cooling load twice that of the heating. The calculated load on the cooling coil is about
The reheater boxes only activate once the VAV dampers have reached the minimum fresh air flowrate
required for each zone. The enthalpy difference is always the same (5 btu/lbm), therefore, the heating
load is dependent on the flowrate through the reheater. With 50% mixing, the actual flowrate will be
twice the required minimum for fresh air. Table 2 shows the loads calculated for each zone.
Table 2: Reheat loads for each zone and the respective minimum flowrates.
Zone Minimum CFM Q reheat (kBtu)
South Level 1 328 7.675
East Level 1 328 7.675
North Level 1 328 7.675
West Level 1 328 7.675
Core Level 1 1692 39.59
South Level 2 296 6.926
East Level 2 296 6.926
North Level 2 296 6.926
West Level 2 296 6.926
Core Level 2 1870 43.758
We can conclude that there are endless possibilities for this system to heat and cool. Levels of cooling
and heating as well as mixing levels of the return and supply air can be adjusted based on design
parameters in order to reduce required loads in this system. With more extensive analyzation, the total
cooling and heating load using a different combination could be minimized. To make design less
complicated, we have assumed many variables and simply calculated the loads based on these
conditions. But based on this model of design, the system could be sized for any desired indoor
Charts and Data
Figure 1: Cooling Loads
Figure 2: Heating Loads
Table 3: Zone Loads
Table 4: Fresh air Reqs.