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:

  1. VAV/Air Cooled Chiller/Gas Boiler

  2. Packaged Unit/Gas Furnace

  3. Fan Coil System/Water Cooled Chiller/Electric Boiler

  4. 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.

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