The Mechanical HVAC system in a building has a large impact on the quality of life for the occupants and the total energy consumption. Choosing the right HVAC system for a building is critical to the success of a project.
This article highlights how different mechanical systems can be easily compared using cove.tool in the early stages of design. The article includes a Large Office building prototype provided by Pacific Northwest National Laboratory (PNNL) located in Denver, CO (ASHRAE Climate Zone 5B) for analyzing the impact of different mechanical systems. Following table contains some key inputs used to create the Large Office model:
Total Area of Building | 498,588 sq ft |
Number of Floors | 12 |
Window Fractions | 29% on North, East, South and West |
Thermal Zoning | Single Zone Model |
Floor to Floor Height | 13 ft |
Thermostat Setpoints | 75°F Cooling, 70°F Heating |
Thermostat Setback | 85°F Cooling, 60°F Heating |
Lighting Power Density | 0.79 W/sq ft |
Equipment Power Density | 0.75 W/sq ft |
Domestic Hot Water Consumption Rate | 602,357 gal/year |
Occupied days | Monday – Friday |
Occupied hours | 8:00 am to 6:00 pm (10 hours per day) |
Daylight Sensors | Partial |
Occupancy Sensors | Partial |
Wall R value | 18.1 (h*sf*F/Btu) |
Roof R value | 31 |
Glazing U value | 0.31 (Btu/h*sf*F) |
Glazing SHGC | 0.38 |
Occupant Density | 275 sq ft / person |
The HVAC systems chosen for the analysis are:
VAV/Air Cooled Chiller/Gas Boiler
Packaged Unit/Gas Furnace
Fan Coil System/Water Cooled Chiller/Electric Boiler
VRF System/Air Cooled
Various performance parameters of the systems mentioned above are indicated in the table below:
Packaged Unit | VAV System | Fan Coil System | VRF System | |
Integrated Part Load Curve | Constant Speed Chiller | Variable Speed Chiller | Variable Speed Chiller | Absorption Chiller |
Heating System COP | 0.7 | 0.8 | 0.9 | 1.5 |
Cooling System COP | 3.5 | 3.5 | 3.8 | 3.75 |
The details about the performance curves of various chillers supported by cove.tool can be found in this article.
To see the impact of the different mechanical systems on the project, create the copies of the project and select the systems for the analysis from the 'System Type' dropdown from 'Building System' tab on 'Baseline Energy' page. The Energy Usage Intensity (EUI) breakdown for the selected systems can be seen below:
1. Packaged Unit/Gas Furnace
2. VAV/Air Cooled Chiller/Gas Boiler
3. Fan Coil System/Water Cooled Chiller/Electric Boiler
4. VRF System/Air Cooled
For a side-by-side comparison of the projects with different systems, the users can utilize the 'compare feature' of cove.tool. The following images shows the comparison for the 4 systems selected for analysis:
This feature is exceptionally useful when analyzing the impact of different systems on the heating and cooling energy of the conditioned space. This can be achieved by selecting the chosen parameters from the legends below the breakdown diagram.
Examples of this for Heating and Cooling Energy Comparison for the systems can be found in the image below:
In conclusion, the HVAC system selection is a crucial decision for any project because it directly contributes to the energy consumption and comfort of the occupants. cove.tool can make the selection process a lot easier and help to find the right fit in the early stages of design where a lot of system types are being considered for the project.