Ventilation Requirements and EUI

After setting up a baseline thermal model , the pervious EUI study was mostly concerned with upgrading the thermal properties of the building envelope. In this exercise, we will be looking at the impact of different ventilation control strategies and thermostat settings on energy use intensity. In addition, this exercise will help you to better understand the spatial requirements for different ventilation systems.

Task A: Ventilation Controls and Temperature Setpoints

First, we’ll look at performative impacts of changing ventilation strategies. Explore a series of control strategies to reduce ventilation-related loads and relax temperature setpoints. Pick the most promising options that you want to implement going forward and document them via an EUI comparison chart and separate descriptions of each additional upgrade. Submit in one PDF document that also includes Task B of this exercise.

Figure 1: Continuation of the EUI comparison chart for temperature setpoint adjustments and ventilation upgrades

Figure 1 shows a continuation of the EUI study from EUI Study - Envelope Upgrade for the Medium Office in Boston. The three new iterations are described in Table 1.

Table 1: Description of building upgrades from Figure 1

Task B: Ventilation Requirements and Duct Sizing

At this point we will determine the spatial dimensions required for your building design from Task A. We will be comparing two different HVAC systems, a Dedicated Outside Air (DOAS) and a Variable Air Volume (VAV). The former only delivers the minimum outside air ventilation requirement to the building. heating and cooling loads are met via seperate components. A VAV is an an all-air system, meaning that heating and cooling are also delivered via the circulated air. DOAS typically require smaller shafts than all-air systems, such as Variable Air Volume (VAV) systems.

For the exercise, you will need an Excel spreadsheet that allows you to perform simple duct sizing calculations. To determine peak heating and cooling loads for the VAV system, you will need the final thermal model from Task A.

Part 1: Dedicated Outdoor Air System

Step 1 : In the Excel spreadsheet, define your program area, program, and your occupant density. For the occupant density, you can either define your own value or use the standard value from the ASHRAE 62.1-2022 standard.

Step 2: Input the air flow rates. You can either use the default values from the ASHRAE standard or define your own.

Step 3: The spreadsheet then utilizes a conventional air velocity (8 m/s) and safety factor (1.15) to size the ductwork. Note the required area for the shaft. This area corresponds to area required on the roof for a shaft providing fresh outdoor air to the building. You will also need a similarly sized shaft for the exhaust air, to be exhausted out of the building. The shafts should ideally be square shaped (1:1) but can have up to a maximum aspect ratio of 4:1.

Figure 2: Required inputs (highlighted in gold) to size a dedicated outside air system using the ventilation requirements spreadsheet. Supply air and exhaust shaft sizes are given along with a representative section. Horizontal ducts are sized by dividing the total shaft area required by the number of floors (if floors are about the same area).

Part 2: All-Air System

Step 1: In the Excel spreadsheet, define your peak heating and cooling load. These values are taken from your thermal energy model (see figure 3). You also need to input your thermostat heating and cooling setpoint as defined in your energy model.

Figure 3: The peak heating and cooling loads can be found in the Climate Studio Results tab. Under Building Results, you can refer to the Peak Heating and Peak Cooling Week to identify the maximum load. Record this value.

Step 2: The spreadsheet then utilizes a conventional air velocity and safety factor to size the ductwork. Note the required area for the shaft. This area corresponds to area required on the roof for a shaft providing the full volume of conditioned supply air to the building. You will also need a similarly sized shaft for the return air, to be both recirculated and exhausted out of the building.

Figure 4: Required inputs (highlighted in gold) to size a variable air volume system using the ventilation requirements spreadsheet. Supply air and exhaust shaft sizes are given along with a representative section. Horizontal ducts are sized by dividing the total shaft area required by the number of floors (if floors are about the same area).

Part 3: Draw section diagrams for DOAS and VAV

Step 1: Now you are asked to produce a simple section through your building in which the supply air shaft is clearly drawn . Please indicate the dimensions of the shaft. Be sure to include both a supply and return air shaft. Note, that you can specify more than one par of shafts that can be located at different entry points into the building. The sum of the cross-sections of all shaft pairs has ot correspond to the results for Figure 3 and 4.

Step 2: On every floor of your building, in section, draw the horizontal ductwork that provides air to every floor. The size of these horizontal branches corresponds to the floor area served relative to the total and can be approximate.

Step 3: Repeat these steps for both systems options (dedicated outdoor air and all-air). Submit a PDF (also including Task A) that lists the shaft area requirements for both systems as well as two section cut drawings through your building (one with each system). Please see images below for an example section of both systems for the typical DOE medium sized office building.

Figure 5: Section through typical medium sized office building with supply and return air shafts shown for the all-air system.

Figure 6: Section through typical medium sized office building with supply and exhaust air shafts shown for the dedicated outdoor air system (DOAS).

Related Video Tutorial/Handbook Chapter