CO2- and Humidity-Controlled Demand Ventilation

Controlling air volume by actual occupancy and air quality rather than by schedule or design maxima.

How does CO2-based demand-controlled ventilation work?

Conventional air handling systems often deliver constant air volumes on a timer or even run permanently at the design value for full occupancy. With demand-controlled ventilation (DCV), CO2 and humidity sensors in the zones measure actual air quality. When the CO2 concentration rises due to people present, the control increases the air flow via variable speed drives on the fans and variable air volume dampers in the ducts; in empty rooms it reduces it to a minimum. The effect is twofold: heat demand for the supply air falls in proportion to the air volume, while fan electricity falls disproportionately. According to the affinity laws, fan power scales with the cube of the air flow, so halving the air volume cuts electricity demand to around one eighth. In buildings with variable occupancy, the average air flow is only 40 to 60 % of the design value.

Which buildings benefit from demand-controlled ventilation?

The measure targets non-residential buildings with strongly fluctuating occupancy: schools and lecture halls, offices, conference rooms and assembly buildings. The prerequisites are fans in the AHU that can be driven by variable speed drives and a freely programmable DDC controller into which the sensor values can be integrated. In older systems, retrofitting the variable speed drive is often the largest part of the investment. Systems with constant air requirements for process or hygiene reasons, such as cleanrooms and operating theatres, are unsuitable. A side benefit in schools: classrooms receive the full air volume exactly when they are occupied, which measurably improves concentration.

What does demand-controlled ventilation actually deliver?

Ventilation energy falls by 25 to 50 %. The saving is calculated from the previous ventilation energy demand and the relative average air flow, raised to an exponent between 1 (heat) and 3 (fan electricity). A school with 80,000 kWh/a ventilation energy whose average air flow drops to 50 % saves around 51,000 kWh/a of heat and electricity. Sensors and controls cost €5,000 to €25,000 as a retrofit depending on plant size. With a service life of 15 years, the measure pays back in 2 to 5 years and is thus one of the most economical interventions on existing ventilation systems.

Energy carrier

Erdgas, Heizöl, Fernwärme, Strom → Erdgas, Heizöl, Fernwärme, Strom

Savings potential

25-50 % Lüftungsenergie

Worked example

A school has so far run its ventilation system on a fixed schedule with a constant air volume, regardless of whether classrooms are occupied. The ventilation energy demand from heat for the supply air and electricity for the fans is 80,000 kWh per year. After retrofitting CO2 sensors in the zones and demand-based control with variable speed drives, the average air flow drops to 50 % of the design value, because rooms are only minimally ventilated during breaks, free periods and outside lesson times. With an average exponent of 1.5 for the mix of thermal and electrical savings, the result is: E_saved = 80,000 × (1 − 0.5^1.5) = 80,000 × (1 − 0.354) = 80,000 × 0.646 = 51,680 kWh/a. The school therefore saves around 51,000 kWh of heat and electricity per year while still receiving the full air volume at full occupancy.

Investment & payback

Sensoren + Regelung Nachrüst: 5.000-25.000 € je nach Anlagengröße · Service life: 15 a · Payback: 2-5 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much energy does CO2-controlled ventilation save?

CO2-based demand-controlled ventilation reduces ventilation energy by 25 to 50 %. In buildings with variable occupancy, the average air flow drops to 40 to 60 % of the design value. Since fan electricity falls with the cube of the air flow, the electricity saving is disproportionately large. A school with 80,000 kWh/a ventilation energy saves around 51,000 kWh per year.

Can CO2 sensors be retrofitted to an existing ventilation system?

Yes, provided the fans can be driven by variable speed drives and the DDC controller can be freely programmed. NDIR-type CO2 sensors are installed per zone and connected to the controller via Modbus or an analogue signal. In older systems with uncontrolled fans, a variable speed drive must also be retrofitted, which often accounts for the largest part of the investment.

How much does it cost to retrofit demand-controlled ventilation?

Sensors and controls cost €5,000 to €25,000 as a retrofit, depending on plant size and number of zones. With a service life of 15 years, the measure pays back in 2 to 5 years. It is therefore one of the most economical interventions on existing AHUs, particularly in schools, lecture halls and conference areas with strongly fluctuating occupancy.

What CO2 level counts as the limit for good indoor air quality?

CO2 concentrations below 1,000 ppm are regarded as the benchmark for good indoor air quality. ÖNORM EN 16798-3 classifies indoor air quality into categories, with the control typically increasing the air volume at 800 to 1,000 ppm. Values above 1,500 ppm are considered hygienically inadequate and noticeably impair concentration and performance.

Which buildings are unsuitable for demand-controlled ventilation?

Demand-controlled ventilation is unsuitable for systems that require a constant air change for process or hygiene reasons, such as cleanrooms, operating theatres or laboratories with fume cupboards. The saving potential is also low in buildings with uniformly high occupancy throughout operating hours. The benefit is greatest with strongly fluctuating occupancy, as in schools, offices and assembly buildings.

Related measures

Standards & sources

Metering points in the EDM Toolbox

CO2-Sensoren (NDIR) sind das Herzstück - mehrere pro Zone. LineMetrics LM2 mit CO2-Sensor oder externe Modbus-Sensoren. Trendvergleich Belegung vs. Luftstrom als Dashboard.

All measures