DCV Modelling

Background

Demand-controlled ventilation (DCV) is an energy-saving control strategy that reduces the rate at which outdoor air is delivered to a zone during periods of partial occupancy. In most commercial buildings ventilation is provided to deal with two types of indoor pollution: odours from people, and off-gassing from building components and furniture. When a space is vacant, it has no people pollution so the people-related ventilation rate is not needed. Many types of high-occupancy spaces, such as classrooms, multi-purpose rooms, theatres, conference rooms, or lobbies have ventilation designed for a high peak occupancy that rarely occurs.

 

Demand-controlled ventilation can provide significant HVAC energy savings by conditioning only the amount of ventilation air necessary to maintain good indoor air quality. It can be particularly effective in buildings or zones that have widely varying occupancy.

 

DCV is most successful in reducing ventilation rates, and so reducing fan energy use, when meeting the demands of indoor air quality (IAQ). It is not necessarily so successful, in energy terms, for high design cooling loads, where airflows will increase above the required IAQ minimum to meet space cooling loads. In such cases, the HVAC system would need to increase the supply air flow rate of the conditioned air to remove the excess heat from the space and so maintain the desired indoor setpoint temperature. This will happen regardless of the current IAQ requirements and/or occupancy levels. Therefore, the amount of outside air brought into the building could exceed the minimum airflow rate required for maintaining the indoor IAQ. In other words, even though DCV is designed to reduce ventilation rates to save energy, in this case, the need to cool the space down will be prioritised, and ventilation rates may increase regardless. Practically, this typically means that IAQ-driven DCV is most effective in ASHRAE climate zones 4 and above.

 

Schematic of a typical DCV system from Demand-Controlled Ventilation and Sustainability with permission from author Tom Lawrence and ASHRAE.

CO2 Control / DCV Modelling Example

DesignBuilder provides various options for modelling DCV. The examples below show how to model two common types of IAQP (Indoor Air Quality Procedure) system:

 

 

The examples below walk you through the steps involved in modelling both of these systems.

Model Set up

Create a base model

Step 1 - Create a new file located in London Gatwick and add a building to the site with a simple rectangular block having dimensions 30m x 20m.

 

Sub-divide the block into 2 zones, an East and a West zone as shown below. The East zone should have East-West length of 10m and the West zone a length of 20m.

 

Use default model data settings.

Detailed HVAC layout

Step 2 - Set the HVAC model option to “Detailed”.

 

Step 3 - Navigate to HVAC System level and load the “DOAS with DCV and FCU” Detailed HVAC template.

 

AHU

Step - 4 Navigate to the AHU and open the Edit dialog. On the Outdoor Air System tab of the Air Handling Unit dialog:

 

  1. Under the Recirculation header, set Minimum outdoor air flow rate to 0.

  2. Under the Demand Controlled Ventilation header, ensure that the Demand controlled ventilation checkbox is checked.

  3. Choose the 3-Indoor Air Quality Procedure (IAQP) for the System outdoor air method item.

  4. Press OK.

HVAC zones

Step 5 - Navigate to one of the HVAC zones in the HVAC zone group and open the HVAC zone dialog.

 

  1. On the General tab, under the CO2 and Contaminant Control header, check the CO2 and contaminant control checkbox.



  2. On the Target tab, apply the changes to all zones by:

     

    1. Selecting the 3-Multiple (modified settings only) option.

    2. Selecting both zones.

    3. Press OK

Zone terminal units

Step 6 - Update the terminal units:

 

a. Navigate to one of the CAV No Reheat terminal units and open the Edit dialog.

 

b. Check the Control on outdoor air flow checkbox.

Note: you can ignore the Per person ventilation mode child setting of the Control on outdoor air flow checkbox as it is only used when outdoor air ventilation rates are calculated based on a per person rate, and not when using the IAQP.

c. On the Target tab, apply the changes to all zones by:

 

  1. Selecting the 3-Multiple (modified settings only) option.

  2. Selecting both zones.

  3. Press OK

Note: the CAV No Reheat terminal unit used in this example provides pure CO2 control. Replacing this default CAV terminal with a VAV No Reheat terminal adds free cooling functionality. In this case you would have to change the AHU type to VAV to connect the VAV terminal.

Model Data Settings

Step 7 - Go to Site level and enable the Simulate CO2 concentration site level option.

 

 

Step 8 - At building level, on the HVAC tab, ensure that the Mechanical ventilation, On checkbox is checked.

 

In this case, the zone Outside air definition method setting is used for sizing using fresh air per person, area and ac/h.

 

Step 9 – At building level, confirm the CO2 setpoint settings on the Activity tab:

 

 

Step 10 – Run a simulation over the typical winter period with sub-hourly outputs requested.

Results

When the simulation has finished, check the following sub-hourly results in the Results Viewer:

 

 

These will allow you to check the indoor CO2 levels, the fresh air delivery rate in ac/h and the associated fan energy consumption.

 

You should see results similar to those below, with zone CO2 levels controlled to 900 ppm and the fan operating constantly to achieve this.

 

Model Set up

Create a base model

Step 1 - Create a new file located in London Gatwick and add a building to the site with a simple rectangular block having dimensions 30m x 20m.

 

Sub-divide the block into 2 zones, an East and a West zone as shown below. The East zone should have East-West length of 10m and the West zone a length of 20m.

 

Use default model data settings.

Detailed HVAC layout

Step 2 - Set the HVAC model option to “Detailed”.

 

Step 3 - Navigate to HVAC System level and load the “DOAS with DCV and FCU” Detailed HVAC template.

 

AHU

Step - 4 Navigate to the AHU and open the Edit dialog. On the Outdoor Air System tab of the Air Handling Unit dialog:

 

  1. Under the Recirculation header, switch off recirculation by unchecking the Recirculation, On checkbox.

  2. Press OK.

HVAC zones

Step 5 - Navigate to one of the HVAC zones in the HVAC zone group and open the HVAC zone dialog.

 

  1. On the General tab, under the CO2 and Contaminant Control header, check the CO2 and contaminant control checkbox.



  2. On the Target tab, apply the changes to all zones by:

     

    1. Selecting the 3-Multiple (modified settings only) option.

    2. Selecting both zones.

    3. Press OK

Zone terminal units

Step 6 - Update the terminal units:

 

a. Navigate to one of the CAV No Reheat terminal units and open the Edit dialog.

 

b. Check the Control on outdoor air flow checkbox.

Note: you can ignore the Per person ventilation mode child setting of the Control on outdoor air flow checkbox as it is only used when outdoor air ventilation rates are calculated based on a per person rate, and not when using the IAQP.

c. On the Target tab, apply the changes to all zones by:

 

  1. Selecting the 3-Multiple (modified settings only) option.

  2. Selecting both zones.

  3. Press OK

Note: the CAV No Reheat terminal unit used in this example provides pure CO2 control. Replacing this default CAV terminal with a VAV No Reheat terminal adds free cooling functionality. In this case you would have to change the AHU type to VAV to connect the VAV terminal.

Model Data Settings

Step 7 - Go to Site level and enable the Simulate CO2 concentration site level option.

 

 

Step 8 - At building level, on the HVAC tab, ensure that the Mechanical ventilation, On checkbox is checked.

In DesignBuilder v7.3:

Step 8.a - Select the 7-Indoor air quality procedure Outside air definition method.

 

In DesignBuilder v8.0:

Step 8.b - Check the Indoor air quality procedure checkbox.

Step 9 – At building level, confirm the CO2 setpoint settings on the Activity tab:

 

 

Step 10 – Run a simulation over the typical winter period with sub-hourly outputs requested.

Results

When the simulation has finished, check the following sub-hourly results in the Results Viewer:

 

 

These will allow you to check the indoor CO2 levels, the fresh air delivery rate in ac/h and the associated fan energy consumption.

 

You should see results similar to those below, with zone CO2 levels controlled to 900 ppm and the fan operating for only a few hours in the afternoon to achieve this.

 

Comparing the results for the two types of IAQP system, it is clear that the Zone IAQP method uses much less fan energy than the AHU IAQP method to achieve similar levels of comfort. This is because the fan only operates to provide fresh air to the zones to meet the IAQP CO2 setpoint of 900 ppm.