# Demand controlled ventilation

Demand controlled ventilation (DCV) is a feedback control method to maintain indoor air quality that automatically adjusts the ventilation rate provided to a space in response to changes in conditions such as occupant number or indoor pollutant concentration. The most common indoor pollutants monitored in DCV systems are carbon dioxide and humidity. This control strategy is mainly intended to reduce the energy used by heating, ventilation, and air conditioning (HVAC) systems compared to those of buildings that use open-loop controls with constant ventilation rates.

## When to use DCV

Standard HVAC system design uses fixed airflow rates to calculate the outdoor air (OA) required in a space. These airflow rates are determined by mechanical code and vary based on expected occupancy and space use. This process of supplying fixed airflow to a space ensures that sufficient OA is present in that space when it is occupied. However, such spaces are not always fully occupied; in these cases, energy is wasted as the HVAC system processes more OA than is necessary for the space occupants. Demand control ventilation is an attractive alternative to standard design in these situations because DCV systems only supply the outdoor airflow necessary to serve the occupants in a space. Therefore, the above-described energy is not wasted in this system type.

## DCV application in different system types

DCV is primarily used in variable-air-volume (VAV) systems. In DCV VAV systems, airflow to a zone is modulated to control the temperature and outdoor airflow to the space. Using the pollutant levels measured in a zone, the system’s controller sets the zone’s minimum airflow requirement to dilute the pollutant concentration. Such a control sequence is supported by a pollutant sensor (e.g. carbon dioxide sensor), a variable frequency drive (VFD) on the fan supplying the zone, individual VAV boxes with reheat serving each space in the zone, and airflow measuring stations.
Research has been conducted on the application of DCV in constant-air-volume (CAV) systems. Although CAV systems cannot modulate airflow, researchers have experimented with running CAV system equipment intermittently to reduce energy consumption. In this proposed system, the HVAC equipment is to run continuously when the space is occupied, then cycle on and off to maintain indoor air quality during inoccupancy.

## Carbon dioxide sensing

Carbon dioxide levels measured in a space are commonly used to control DCV systems because CO$_2$ level is generally proportional to the level of bioeffluents, or occupant generated pollutants, in a space. Carbon dioxide sensors monitor carbon dioxide levels in a space by strategic placement. The placement of the sensors should be able to provide an accurate representation of the space, usually placed in a return duct or on the wall. As the sensor reads the increasing amount of carbon dioxide levels in a space, the ventilation increases to dilute the levels. When the space is unoccupied, the sensor reads normal levels, and continues to supply the unoccupied airflow rate. This rate is determined by the building owner standards, along with the designer and ASHRAE Standard 62.1.

## Codes & standards

Common reference codes and standards for ventilation:
* International Mechanical Code (IMC) Chapter 4: Ventilation
* International Organization for Standardization (ISO) International Classification for Standards (ICS) 91.140.30: Ventilation and air-conditioning systems
* American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) 62.1 & 62.2: The standards for Ventilation and Indoor Air Quality

## Examples of estimating occupancy

* Ticket sales
* Timed schedules
* Positive control gates
* Motion sensors (various technologies including: Audible sound, inaudible sound, infrared)
* Gas detection (CO$_2$) In a survey on Norwegian schools, using CO$_2$ sensors for DCV was found to reduce energy consumption by 62% when compared with a constant air volume (CAV) ventilation system.
* Security equipment data share (including people counting video software)
* Inference from other system sensors/equipment, like smart meters