Demand-Controlled Ventilation

What is demand-controlled ventilation and why does it matter?

Demand-controlled ventilation (DCV) is a strategy that adjusts the amount of fresh air supplied to a building based on real-time sensor readings rather than fixed schedules or occupancy assumptions. In a traditional ventilation system, air is either switched on or off, or runs at a constant rate regardless of whether spaces are occupied. DCV continuously monitors CO2 concentration and humidity, then modulates the ventilation fan speed or dampers to deliver only as much fresh air as needed. For a Slovak family house with variable occupancy (home offices that empty during the day, gathering spaces used intermittently, bedrooms needed only at night), DCV can reduce ventilation energy by 20-40% compared to always-on systems while maintaining indoor air quality and comfort.

How does CO2-based demand control work?

CO2 concentration serves as a proxy for occupancy. Humans exhale breath containing approximately 40,000 ppm CO2. Outdoor air averages 400 ppm. When a room is unoccupied, indoor CO2 drifts back toward outdoor levels. When people enter, CO2 rises. A DCV system uses one or more wall-mounted CO2 sensors to continuously measure this concentration. The sensor data is wired to a controller (often part of a building management system or a standalone smart thermostat) that compares the reading against programmed setpoints. When CO2 rises above the modulation threshold (typically 600-700 ppm), the controller increases fan speed or opens dampers. When CO2 falls below that point during unoccupied periods, ventilation is reduced to a minimum background rate (10-20% of full flow). This proportional response eliminates the binary on-off waste of conventional systems.

What control setpoints and schedules apply in Slovakia?

Modern building standards and passive-house certification guidelines recommend these setpoints: ventilation modulation begins at 600-700 ppm CO2, the target operating point is 800 ppm, and the maximum ceiling is 850 ppm averaged over an eight-hour window. Below 600 ppm, the system runs at minimum fresh air rate (typically 5-10 air changes per hour per person, as required by Slovak building code STN 73 0540). Above 850 ppm, the system should be at full capacity. This 250 ppm band (600-850) allows the controller time to respond smoothly without oscillation. In residential settings, occupancy schedules can override sensor signals: you might program 24-hour operation in bedrooms but time-of-day control in living spaces. Some systems add night setback, automatically reducing ventilation between 22:00 and 06:00 unless humidity signals otherwise.

How does humidity monitoring enhance demand control?

CO2 alone measures occupancy but not moisture generation. Cooking, showering, laundry, and drying create high humidity even when occupancy is low. A humidity sensor (typically targeting 40-60% relative humidity in winter, 50-65% in summer) acts as a safety override. If humidity approaches 70%, the system increases ventilation regardless of CO2 level, preventing condensation and mould. Conversely, in very dry periods or winter, limiting excessive ventilation below the CO2 setpoint conserves heat. Dual-sensor systems (CO2 plus humidity) are more robust than CO2-only: they catch moisture problems the occupancy sensor would miss and avoid over-ventilation on low-occupancy, high-humidity days like laundry day when nobody is home.

When is demand-controlled ventilation cost-effective in a Slovak house?

DCV is not universally justified. In a single-occupant apartment or elderly couple's stable-routine home where occupancy is predictable and changes slowly, energy savings from DCV are modest (5-10%) because the ventilation load is always present. DCV investment is best justified when occupancy swings exceed 50% between peak and off-peak periods. Examples: a multi-generational household where children and working adults are absent during school and work hours; a home office where the owner is absent 3-4 days weekly; a rental property or guest wing with unpredictable use. In such cases, payback on a EUR 1200-2000 retrofit investment occurs within 3-5 years through reduced heating and cooling of unnecessary ventilation air. Commercial buildings with more dramatic occupancy swings (open-plan offices, meeting rooms, shopping centres) see average energy savings of 38% and faster payback.

How does demand-controlled ventilation interact with heat recovery?

DCV and heat-recovery ventilation (MVHR) are complementary, not competing. An MVHR unit recovers 75-95% of heat from exhaust air but still operates at a constant or scheduled airflow. Overlaying DCV control on top of MVHR means the heat exchanger still runs whenever ventilation is needed, capturing heat at full efficiency, but the fan speed modulates based on occupancy demand. In winter, when DCV signals low occupancy and reduces airflow to 30% of design flow, the MVHR still recovers most of its heat percentage, but the absolute energy input to condition fresh air drops significantly. A DCV-controlled MVHR system in an airtight Slovak passive house reduces winter heating energy by 10-20% compared to constant-speed MVHR, without sacrificing the comfort and air quality the heat recovery provides.

What is the difference between demand-controlled and hybrid ventilation?

Hybrid ventilation uses natural (window-opening) and mechanical ventilation together, selecting one or the other based on outdoor temperature and wind. Balanced ventilation supplies and exhausts equal airflows via ducts. Demand-controlled ventilation is a control strategy that can be applied to either system. A hybrid DCV system might use natural ventilation during mild weather (spring, autumn) when outdoor CO2 and temperature are favourable, then switch to mechanical DCV in winter and summer extremes. A balanced-ventilation DCV system modulates the fans continuously based on CO2, running at reduced speed during low occupancy but never switching to natural air. The control approach makes the difference; the underlying ventilation type is separate.

Control Strategy	Trigger	Response Time	Best Use Case
Demand-Controlled (CO2-based)	CO2 concentration rises above setpoint	5-15 minutes to reach target	Homes with unpredictable occupancy, high occupancy swings
Scheduled (time-based)	Pre-programmed calendar / clock times	Instant (but often early or late)	Routine households, workplaces with fixed hours
Motion-Sensor Only	Occupancy detector activates	Immediate (within seconds)	Intermittent-use spaces (bathrooms, meeting rooms)
Hybrid (Temperature + CO2)	Outdoor conditions plus occupancy	Variable (natural or mechanical response)	Mild climates; Slovakia rarely suitable due to heating demand

What building systems can integrate demand-controlled ventilation?

DCV works with any mechanical ventilation source: exhaust-only systems (single-duct fans), balanced systems (supply and exhaust ducts), heat-recovery units (MVHR), or heat-pump-driven systems. The sensor wires and control signals connect to a building management system (BMS) or a simpler smart thermostat with wired or wireless inputs. In new construction, DCV is wired at commissioning; outlets and sensor nodes are planned during design. In retrofit applications, wireless CO2 sensors and battery-powered remote controllers are common, avoiding the cost and disruption of running new wiring through existing walls. Modern systems communicate via KNX, BACnet, or proprietary radio protocols, allowing individual room sensors to drive individual dampers (zoned DCV) or feed a central controller that modulates a single fan.

What are the maintenance and failure modes of demand-controlled ventilation?

CO2 sensors have a typical lifespan of 5-10 years before drift requires recalibration or replacement (cost: EUR 100-300 per sensor). Most sensors can auto-calibrate using outdoor air once per day (at night, when occupancy is low and CO2 should approach outdoor levels of 400 ppm). If a sensor fails or drifts high, the system will over-ventilate (excess energy waste but no comfort loss). If it drifts low, occupants may experience stuffiness and condensation (comfort loss, potential mould). Building management systems require annual checks of setpoints and sensor operation; many systems log CO2 data for audit trails and can alert the owner to sensor faults. In passive houses and airtight buildings, a non-functional DCV system can become a nuisance (too-high or too-low ventilation), but it does not cause structural damage as long as a minimum background ventilation rate remains active.

Typical CO2 Sensor Setpoint Range	Typical Human Comfort Response	Typical Energy Impact
< 600 ppm	Excellent, highly alert, no drowsiness	Minimum ventilation active; lowest energy
600-800 ppm	Good, normal office/home comfort	Ventilation ramping up; moderate energy
800-1000 ppm	Fair, some occupants notice mild stuffiness	Full ventilation typically active
> 1000 ppm	Poor, reduced cognitive function, headaches, fatigue	System at maximum; urgent need for additional fresh air

Is demand-controlled ventilation required by Slovak building code?

No. The Slovak Building Act (Act 25/2025 Z. z.) and energy-efficiency standards (STN 73 0540) do not mandate DCV for residential buildings. However, DCV is a recognised energy-saving measure and qualifies for green-building certifications (BREEAM, WELL). Passive-house standard (STN 73 0540 PASÍVNY DOM) recommends DCV as best practice for airtight homes but does not forbid constant-speed MVHR. In practice, DCV is becoming standard in new airtight houses and renovations because designers and owners recognise its energy benefit and cost payback in variable-occupancy contexts. Some renovation subsidies (e.g., Obnov Dom programme) prioritise projects that include DCV as part of comprehensive ventilation improvements.

What are common misconceptions about demand-controlled ventilation?

Misconception 1: "DCV eliminates the need for heat recovery." False. DCV modulates airflow; it does not replace heat recovery. You still need an MVHR or other heat-exchange method to capture energy from outgoing air. DCV simply optimises when and how much ventilation you use. Misconception 2: "CO2 sensors are fragile or require frequent replacement." False. Modern sensors are robust and last 5-10 years with minimal maintenance. Auto-calibration is common. Misconception 3: "DCV adds significant cost." Retrofit cost is EUR 1200-2000; new construction incremental cost is often under EUR 500 because sensor wiring is planned from the start. Payback is 3-5 years in suitable occupancy scenarios. Misconception 4: "DCV works only in commercial buildings." False. Residential benefits are highest in homes with irregular schedules, multiple occupants, or large variation in daily presence. Misconception 5: "DCV will make my home stuffy if the sensor breaks." Partly true: if a sensor fails and drifts high, over-ventilation occurs (uncomfortable but not dangerous). If it drifts low, stuffiness can develop. This is why maintenance and annual checks matter, and why backup schedules or motion sensors provide redundancy.