Heat recovery ventilation (HRV) units — known in Czech technical literature as rekuperátory or jednotky s rekuperací tepla — are the central component of balanced mechanical ventilation in airtight buildings. Their function is to extract thermal energy from outgoing stale air and transfer it to incoming cold outdoor air, before the extract is discharged and the conditioned supply enters the occupied space.
In Czech climate conditions, where outdoor temperatures routinely drop below −10°C during January and February (the design outdoor winter temperature for Prague is −12°C per ČSN 73 0540-3), effective heat recovery reduces the ventilation heat loss fraction of the total building heat demand by 70–90%.
Heat Exchanger Core Types
Counterflow plate heat exchanger
The most widely used core type in residential HRV units is the counterflow plate exchanger, in which supply and extract airstreams pass on opposite sides of a thin corrugated plate in opposing directions. This counterflow arrangement creates a large average temperature difference across the entire length of the exchanger, enabling high heat transfer rates.
Thermal efficiency (temperature efficiency, η_t) is defined by EN 308 as:
η_t = (T_supply_out − T_supply_in) / (T_extract_in − T_supply_in)
Quality residential counterflow units achieve η_t of 80–92% at balanced flow rates, measured at 0°C outdoor temperature per EN 308. At higher outdoor temperatures, efficiency often rises slightly because the condensation risk on the extract side diminishes.
Rotary heat exchanger (thermal wheel)
A rotary exchanger uses a slowly rotating drum of hygroscopic or non-hygroscopic material that alternately absorbs heat (and moisture, in hygroscopic variants) from the warm extract stream, then releases it into the cold supply stream as the drum rotates. Rotary units typically achieve η_t of 83–88% at standard test conditions.
The key distinction from plate exchangers is the partial transfer of moisture (latent heat) and the small but non-zero rate of cross-contamination — test data from EN 308 annex C indicates cross-leakage rates between 1% and 5% for typical residential-grade rotary units. This is generally acceptable in standard residential applications but is a consideration for occupants with documented sensitivity to airborne allergens or where ventilation is used to dilute specific indoor pollutants.
Counter-crossflow and multi-pass arrangements
Some manufacturers combine counterflow and crossflow segments in the same core to produce a compact unit with higher efficiency than a single-pass crossflow design. These are structurally closer to counterflow plate units in performance, typically achieving 75–85% efficiency.
Frost Protection
At outdoor temperatures below approximately −3°C to −5°C, moisture from the extract airstream begins to condense and freeze on the cold surface of the heat exchanger core. Without a frost protection strategy, ice accumulates, restricts airflow through the core, and can eventually damage the heat exchanger.
Czech HRV installations must include one of the following frost protection methods:
Supply air preheating
An electric or hydronic preheater in the supply duct upstream of the heat exchanger raises the incoming outdoor air temperature to approximately −3°C or above before it enters the core, ensuring the extract side stays above the frost threshold. This approach is thermally clean but adds standby power consumption (typically 100–400 W for residential units during frost events).
Bypass or exhaust flow reduction
During frost risk conditions, the unit controller reduces the extract fan speed relative to supply, or opens a bypass damper that routes part of the supply air around the core. This reduces heat exchange efficiency during the frost event but protects the core without consuming additional electrical energy.
Ground-to-air heat exchanger (earth tube)
An underground duct (typically PVC or HDPE, 1.5–2.5 m burial depth) pre-conditions outdoor air using the stable ground temperature (approximately +8°C to +12°C year-round at 1.5 m depth in Czech soil). Air entering through the ground tube does not fall below frost risk temperatures for the HRV core, and requires no separate frost heater. Capital cost is higher than electric preheating but operating cost is near zero.
Sizing Principles
HRV unit selection begins with calculating the required ventilation flow rate for the building. Czech practice for residential buildings uses ČSN EN 16798-1 Category II values as the design basis — typically 0.5 h⁻¹ of the total volume of ventilated rooms. A 120 m² apartment with 2.7 m ceiling height has a ventilated volume of approximately 324 m³, requiring a design airflow of 162 m³/h.
HRV units are rated at specific airflow rates; performance data (efficiency, pressure drop, sound power level, SFP) should be read at the operating point closest to the building's design flow. Selection at a flow rate significantly below the unit's rated maximum improves efficiency but may not justify the additional cost of an oversized unit. Conversely, running a unit at or above its maximum rated flow raises fan noise and reduces efficiency due to increased bypass of the exchanger core under pressure.
Specific Fan Power
EN 13779 defines Specific Fan Power (SFP) as the total electrical power of supply and extract fans divided by the design air volume flow rate, expressed in W/(m³/h). For residential applications in the Czech Republic, the target is SFP ≤ 0.45 W/(m³/h), placing the unit in class SFP4. Systems achieving SFP ≤ 0.25 W/(m³/h) are classified SFP2 and qualify for higher energy-performance building certificates under Czech MPO methodology.
Maintenance Intervals
The primary maintenance task for an HRV unit is filter replacement. Czech manufacturers and the ČSN EN 16798-3 commentary recommend inspection every 3–6 months and replacement at least annually for supply-side filters, more frequently in locations with elevated outdoor particulate levels (road proximity, agricultural areas). Blocked filters increase system pressure, raise fan energy consumption, and reduce supply airflow below the design value — compromising indoor air quality without any visible warning to occupants.
The heat exchanger core itself requires annual inspection for fouling and, in units without bypass frost protection, cleaning of condensate drainage channels. Condensate — produced in the extract side of the core during winter — must drain freely to prevent water accumulation that can damage the core or promote microbial growth.
Energy Saving Potential
A correctly specified and maintained HRV system in a Czech climate will recover approximately 2,000–3,500 kWh of thermal energy per year for a 120 m² apartment ventilated at 160 m³/h, depending on the efficiency of the unit and the number of heating degree days at the building's location. At a gas heating price of approximately 2.0 CZK/kWh (2025), this represents an annual thermal saving of 4,000–7,000 CZK relative to an exhaust-only system with no heat recovery.
References
- TZB-info.cz: Heat recovery in residential buildings
- EN 308: Heat exchangers — test procedures for establishing performance of air-to-air and flue gases heat recovery devices
- ČSN EN 16798-1: Energy performance of buildings — ventilation for non-residential buildings
- ČSN 73 0540-3: Thermal protection of buildings — design temperature data
- EN 13779: Ventilation for non-residential buildings — performance requirements for ventilation and room-conditioning systems
