Technology
Adding demand-controlled ventilation to a building without a BMS
Demand-controlled ventilation does not always need a retrofit BMS. What the ventilation unit must be able to do, and what sensors, controls and commissioning involve.
Yes. A building without a building management system (BMS) can get demand-controlled ventilation (DCV) without a retrofit BMS project, as long as the ventilation unit can change its airflow. If the fan runs at one speed and the outdoor air damper sits in a fixed position, a controller has nothing to adjust, and the first job is the unit.
The US Department of Energy's federal technology alert on CO2-based DCV (DOE/EE-0293, listed in a library catalog as March 2004) says the technique is suitable only when there is a means of automatically adjusting the ventilation air supply. It lists properly functioning modulating dampers as necessary and says pneumatic controls will need to be replaced with electronic or direct digital controls. It defines the technique as two parts: CO2 sensors that monitor CO2 levels inside the building, and an air-handling system that uses their data to regulate the outdoor air admitted.
Do you need a retrofit BMS for demand-controlled ventilation?
The definition above names no BMS. The alert says CO2-based DCV capability can be added to an existing system by installing sensors, and that most HVAC equipment suppliers were offering systems designed to accept CO2 sensor readings. Its examples feed the sensors into an existing control system, and it does not discuss buildings with no building automation at all.
A fan manufacturer's data sheet for a standalone demand ventilation controller lists a 0 to 10 V sensor input (CO2, temperature and humidity sensors are given as examples), minimum and maximum fan speed limits, a 0 to 10 V or PWM fan output and an input for remote enable from a BMS. So the logic that turns a CO2 reading into an airflow can sit in the unit's own controller, in a standalone controller or in a BMS.
A retrofit BMS project is a bigger decision than ventilation alone, and in some buildings the law decides whether building automation is required. Article 13 of Directive (EU) 2024/1275 tells member states to set requirements for building automation and control systems in larger non-residential buildings, where technically and economically feasible. National law sets the details, covered in the article on the EPBD in NL, DE, FR, BE, AT and IE.
The work in five steps
- Find out what the unit can do. Check whether the fan speed is variable, whether the outdoor air damper is motorized and can hold intermediate positions, whether zone dampers or variable-air-volume boxes exist, and what controls the unit today (pneumatic, electronic or digital). Alberta's Design & Technology Series #20 on demand control ventilation (August 2025) notes that in systems running on 100% outdoor air the fans and variable-air-volume boxes have a limited turn-down ratio, which limits how well DCV works. In mixed air systems the critical zone sets the outdoor air fraction, and an unbalanced design over-ventilates the other zones.
- Fix the basics first. The DOE alert advises studying the HVAC system and troubleshooting existing ventilation problems before the retrofit, because for CO2 control to work all other HVAC-related systems must be in good operating order. It also warns that a building that was chronically under-ventilated might use more energy after DCV.
- Place the sensors. CO2 sensors go in the occupied space. ASHRAE 62.1-2022 addendum ab asks for 3 to 6 ft (0.9 to 1.8 m) above the floor, with at least one per ventilation zone and one per 5,000 ft² (460 m²). The DOE alert generally prefers wall-mounted sensors to duct-mounted ones, says to avoid spots where people breathe directly on them, and says to cover every CO2 zone, because high CO2 in one zone may not show up in the readings from the adjacent zone. It also notes that installation costs for wireless sensors are minimal beyond the equipment itself. Sensor quality varies: in an LBNL check of 208 sensors in 34 buildings, 40% were off by more than 75 ppm at about 1,010 ppm.
- Add the control on the unit. The reading has to reach something that changes the airflow: a variable speed drive on the fan, a motorized outdoor air damper or zone dampers. The DOE alert calls the control algorithm the most critical aspect of the project and says variable-air-volume systems may need fairly complex calculations and control algorithms. Set a minimum airflow. A REHVA Journal article puts typical DCV airflow at 30 to 80% of maximum, depending on the design and the minimum ventilation rate. Keep a way for staff to override the automatic control. ASHRAE addendum f, approved on 30 September 2025, adds emergency modes to Standard 62.1-2022 that may be started manually or by a sensed value, and one of them disables DCV and sets outdoor air to the designer's maximum.
- Commission and verify. REHVA's checks include confirming that all CO2 sensors read the same ppm in an empty building, overriding the room signal on each DCV unit to force maximum and minimum airflow while recording airflow and opening rate, testing in summer and winter, and reviewing after about a year. It says an opening rate of 40 to 80% is a favorable range. The DOE alert adds a few days of comparison against a handheld CO2 monitor and a check of sensor readings against outdoor air during a several-hour unoccupied period.
What to expect from the savings
Published figures depend on the building. ASHRAE research project RP-1747 reported 7 to 44% HVAC energy savings for CO2-based DCV against non-DCV baselines, and 10 to 30% in simulations of an office building with realistic occupancy schedules. A REHVA Journal article says the potential of DCV is seldom met in real-world evaluations. LBNL found that in California general offices DCV is cost effective only if ventilation rates without DCV are very high relative to code requirements. Whether or not a retrofit BMS is involved, measure energy use before and after the change and compare like with like.
The DOE alert gives the conditions where payback is best: variable-occupancy, high-density spaces, high heating or cooling demand and high utility rates. It also says retrofit cost depends on what type of control system the building has. Its dollar figures are from the early 2000s and are not repeated here. The article on demand-controlled ventilation covers the savings studies in more detail.
Nomestic measures indoor climate room by room, including CO2, temperature and humidity, and works with or without a BMS. The devices are wireless and battery-powered, so a room takes minutes to equip, and savings are verified by comparing measured data before and after a change. Details are on the ClimateIQ platform page.
