Czech building regulations have progressively tightened envelope airtightness requirements since the implementation of EU Directive 2010/31/EU on energy performance of buildings. Nearly-zero-energy building (nZEB) standards now apply to most new construction and major renovation projects. Airtight envelopes, while thermally efficient, eliminate the incidental air exchange that older, leakier buildings relied upon to dilute CO₂, humidity, and pollutants. Mechanical ventilation fills that gap.
There are three distinct categories of mechanical ventilation used in Czech residential and light-commercial buildings, each with different pressure characteristics, equipment requirements, and performance profiles.
1. Exhaust-Only Ventilation (MEV)
Mechanical exhaust ventilation removes stale air from wet and polluted rooms — typically kitchens, bathrooms, and toilets — through dedicated extract ducts connected to a central fan or individual room fans. Fresh air enters passively through purpose-made trickle vents or controlled gaps in window frames.
Operating principle
The fan creates a slight negative pressure relative to outdoors. This draws outside air in through any available opening, prioritising trickle vents when they are correctly sized and positioned. The negative pressure also prevents odours and moisture from migrating toward dry habitable rooms.
Applicability under Czech norms
ČSN EN 16798-3 classifies exhaust-only systems as suitable for residential buildings when the building envelope is not excessively airtight — typically where n50 (air changes per hour at 50 Pa pressure difference) exceeds 1.0 h⁻¹. Below this threshold, relying on infiltration to supply fresh air produces uncontrolled and insufficient inflow in mild weather, and excessive cold draughts in winter when pressure differences are driven by wind and thermal buoyancy.
Limitations
Exhaust-only systems recover no heat from extracted air. In a Czech climate, where heating degree days in Prague average around 3,400 °C-day per year, this represents a significant thermal penalty. Every cubic metre of warmed indoor air exhausted to outside must be replaced by cold outdoor air, which must then be heated.
2. Supply-Only Ventilation
Supply-only systems mechanically introduce filtered outdoor air into habitable rooms while relying on positive pressure to push stale air out through exhaust openings, which may be passive grilles or extract fans. This configuration is less common in Czech residential buildings but appears in specific commercial applications.
Operating principle
A central air handling unit draws in outdoor air, passes it through at least a coarse filter (minimum G4 or ePM10 50% under ISO 16890), conditions it if required, and distributes it through supply ducts to living rooms and bedrooms. Stale air migrates toward bathrooms and kitchens, where it exits through passive openings.
Applicability
Supply-only systems are sometimes used in commercial buildings where controlled positive pressurisation is technically desirable — for example, to prevent ingress of outdoor pollutants into clean environments. In residential settings, positive pressure risks driving moisture-laden indoor air into the building structure if any component of the envelope is permeable to airflow, potentially causing interstitial condensation.
3. Balanced Mechanical Ventilation (MVHR / HRV)
Balanced mechanical ventilation with heat recovery is the technically preferred solution for airtight Czech buildings. Equal volumes of air are supplied to habitable rooms and extracted from wet rooms simultaneously. A heat exchanger transfers thermal energy from the warm extract stream to the cold supply stream before the extract is discharged to outside.
Operating principle
A central unit contains two fans (supply and extract), a heat exchanger core, and typically two filter stages — a coarser filter on the supply side to protect the heat exchanger and a finer filter to protect indoor air quality. The unit is connected to two separate duct networks: one for supply air, one for extract. These networks do not mix in the central unit — air-to-air heat exchange takes place across a solid membrane or through a rotating thermal wheel, not through commingling of airstreams.
Heat recovery efficiency
Counterflow plate heat exchangers in quality residential MVHR units achieve 75–92% thermal efficiency under standard test conditions (EN 308). Enthalpy (moisture-recovering) variants transfer latent heat as well as sensible heat, which is particularly relevant when outdoor humidity is very low in winter and indoor RH maintenance matters. Rotary heat exchangers typically reach slightly higher efficiency values (up to 85–88%) but carry a small risk of cross-contamination between exhaust and supply airstreams, which is relevant in buildings with occupants sensitive to airborne allergens.
Fresh air rates under Czech norms
ČSN EN 16798-1 (the Czech adoption of EN 16798-1) defines minimum outdoor air supply rates by room type and occupancy category. For Category II residential buildings, the default design value is 0.5 room air changes per hour (h⁻¹) for bedrooms and living areas, with a minimum of 15 m³/h per person as the lower bound. Kitchens require a minimum extract rate of 50–100 m³/h during cooking. These figures set the design flow rates that the balanced system must be capable of delivering.
Choosing Between System Types
The selection of system type for a specific building is driven by three primary factors: the airtightness of the building envelope, the available budget for installation and operation, and the acceptable level of heat loss through ventilation.
- Buildings with n50 above 3 h⁻¹ may adequately use exhaust-only ventilation with trickle vents, subject to noise and draught assessment.
- Buildings with n50 below 1.5 h⁻¹ — which includes all new Czech construction built to current nZEB standards — require balanced ventilation to maintain both adequate fresh-air supply and predictable internal pressure conditions.
- MVHR adds 15–30% to ventilation installation cost compared to simple exhaust systems but recovers 75–90% of ventilation heat loss, typically paying back this premium through reduced heating costs within 5–8 years under Czech energy price conditions (based on gas at approximately 2.0 CZK/kWh and electricity at approximately 6.0 CZK/kWh as of 2025 Czech ERO data).
Duct Design Considerations
Regardless of system type, duct sizing is a critical design variable. Undersized ducts increase fan pressure and energy consumption; they also generate aerodynamic noise that is difficult to attenuate after installation. Czech practice typically targets duct velocities below 3 m/s in main distribution runs and below 2 m/s in final branches to occupied rooms, with total system specific fan power (SFP) below 0.45 W/(m³/h) for residential applications per SFP class SFP4 defined in EN 13779.
References
- SZKT — Society of Heating, Cooling and Sanitary Engineering
- TZB-info.cz — technical building services reference
- ČSN EN 16798-1: Energy performance of buildings — ventilation for buildings — Part 1 (Czech adoption 2020)
- ČSN EN 16798-3: Ventilation for buildings — Part 3: For residential buildings
- EU Directive 2010/31/EU on the energy performance of buildings
