The basic operating principle
An HRV unit contains two air streams — one drawing stale indoor air out, one pulling fresh outdoor air in — that pass through a cross-flow or counter-flow heat exchanger without mixing. The outgoing warm air heats the incoming cold air through the exchanger plate, recovering between 70 % and 95 % of the thermal energy, depending on the unit type and outdoor temperature.
A simpler variant, the energy recovery ventilator (ERV), uses a membrane or enthalpy wheel that also transfers moisture — useful in dry winter conditions to avoid excessive humidity loss. ERVs are less common in Czech installations but are specified more often in airtight passive-house constructions where humidity control is critical.
Heat exchanger types
Cross-flow plate exchangers are the most common in residential Czech installations. They are straightforward to clean, tolerant of partial frosting in sub-zero temperatures and cost-effective. Their heat recovery efficiency peaks around 75–85 %.
Counter-flow exchangers are physically longer and achieve 85–95 % efficiency. They are found in compact wall-mounted decentralised units as well as in larger central units designed for whole-house ventilation in new-build family houses.
Rotary enthalpy wheels (rotating heat exchangers) achieve high efficiency with smaller pressure drops but involve moving parts and are typically reserved for commercial installations, not residential.
Decentralised vs centralised systems
The most relevant distinction for Czech apartment residents is between decentralised (room-by-room) units and centralised duct-based systems.
Decentralised wall units
A decentralised HRV installs through a single core hole in an exterior wall, roughly 160–200 mm in diameter. Each unit serves one room. They alternate between intake and exhaust cycles (push-pull) or run two fans simultaneously with a built-in heat exchanger. Because each room is treated independently, there are no ducts to route through ceilings — a significant advantage in existing panel-block apartments where duct installation would require ceiling demolition.
The limitation is that air is not distributed between rooms. A unit in a bedroom does not address CO₂ buildup in the living room. Multiple units are needed for whole-apartment coverage, which increases cost and requires core holes through external walls — requiring building management permission in most panelák blocks.
Centralised duct systems
A centralised HRV unit sits in a utility space — typically a storage room, bathroom ceiling or attic — and distributes air through a network of ducts to each room. One central unit handles the entire dwelling. This approach is standard in new-build Czech rodinný dům (detached family houses) and low-energy apartment buildings, where ducts can be integrated into construction.
Retrofitting a centralised system into an existing apartment is possible but involves significant construction work. The economics typically apply only when a full renovation is already planned.
Heat recovery efficiency and Czech winter conditions
Manufacturers publish heat recovery efficiency figures that apply under standard test conditions: indoor temperature typically 21 °C, outdoor temperature between -5 °C and 7 °C depending on the standard used. Czech winters in Prague average -2 °C to -4 °C in January; in highland Moravia and Bohemia temperatures regularly fall to -15 °C for extended periods.
At extreme cold, the moisture in outgoing air can freeze on the heat exchanger surface, reducing airflow and efficiency — a condition called frosting. Well-designed units manage this through a defrost cycle: the intake fan pauses briefly, allowing warm indoor air to melt the ice, before resuming operation. Not all decentralised units handle this gracefully. Checking that a unit is specified for outdoor temperatures to at least -15 °C before purchasing is important for installations in colder Czech regions.
Air flow rates and room sizing
Czech building standard ČSN EN 16798-1 (aligned with EU Directive 2010/31/EU) specifies minimum outdoor air supply rates for residential buildings. For habitable rooms — bedrooms, living rooms — the standard references 0.3–0.5 air changes per hour (ACH), with kitchens and bathrooms requiring local extraction of at least 25–50 litres per second during use.
For a 20 m² bedroom with a 2.7 m ceiling (volume = 54 m³), 0.4 ACH requires approximately 21 m³/h of fresh air. A single-room decentralised HRV rated at 30–50 m³/h on medium speed covers this comfortably with headroom for occupancy peaks.
Oversizing is not beneficial: running a unit at low speed reduces pressure drop, noise and energy use but may leave corners of the room with inadequate air exchange. Undersizing forces constant high-speed operation, which generates more noise and wears fans faster. Specifying the right unit size for the room volume is the most common point of error in DIY installations.
Noise levels
Noise is the main complaint from residents with HRV units — particularly decentralised push-pull units, which are located in the living space rather than remote in a utility room. Manufacturers quote noise levels in dB(A) at a given airflow rate. For a bedroom installation, a unit producing more than 30 dB(A) at its lowest operating speed is likely to disturb light sleepers. Well-reviewed units in the Czech market — Aerauliqa, Blauberg and Zehnder among them — publish measured noise curves rather than single-number claims. Checking these against tested flow rates, not just stated maximums, gives a more realistic picture.
Maintenance requirements
HRV units require regular filter cleaning or replacement. Most decentralised units use G3 or G4 pre-filters on both intake and exhaust to protect the heat exchanger from dust. In urban Czech environments, filter loading is typically faster than in rural areas — a filter that the manufacturer suggests cleaning every 90 days may need attention after 30–45 days in a building near a busy road during winter. Blocked filters reduce airflow and force the fan motor to work harder, increasing energy consumption and shortening fan life.
The heat exchanger core itself accumulates dust over 1–3 years. Most units allow the core to be removed and washed — a process that takes approximately 15–20 minutes and should be done annually in typical residential conditions.
Permissions and installation in panel flats
Installing a decentralised HRV in a Czech panelák apartment requires drilling through the external wall. In most cases, this needs approval from the bytové družstvo (housing cooperative) or SVJ (owners' association). The core hole must be filled and sealed correctly — both for weatherproofing and to ensure the building envelope remains intact, which is relevant for any insulation or fire safety certification the building holds.
It is worth checking whether the building's external insulation system (ETICS — external thermal composite system, known as zateplení) has a specification for ventilation penetrations. Some insulation contractors provide sleeve kits for retrofit ventilation holes; using them avoids thermal bridging at the wall penetration point.
