HRV Sizing Calculator

Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs) are mechanical ventilation systems recovering thermal energy from exhaust air to precondition incoming outdoor air, significantly reducing heating and cooling loads associated with ventilation while maintaining acceptable indoor air quality. These systems are essential in energy-efficient building design, particularly in climates with extreme temperatures where conditioning outdoor air cost is substantial. HRVs and ERVs provide continuous balanced ventilation (equal supply and exhaust airflows), replacing uncontrolled infiltration and exfiltration with controlled, filtered, energy-efficient outdoor air delivery.

HRV vs. ERV Distinction: HRVs transfer only sensible heat (temperature) between exhaust and outdoor air streams using aluminum or plastic plate heat exchangers allowing thermal conduction but preventing moisture transfer. Ideal for cold, dry climates where indoor humidity control is desired during winter—HRV prevents moisture from humid indoor air transferring to dry incoming outdoor air, maintaining comfortable humidity levels. ERVs transfer both sensible and latent heat (moisture) using desiccant-coated enthalpy wheels or permeable membranes transferring water vapor with temperature. Preferred in hot-humid climates (summer dehumidification benefit) and spaces with high indoor moisture generation (bathrooms, kitchens, swimming pools) where moisture recovery during winter reduces excessive dryness.

Heat Exchanger Effectiveness and Types: Sensible effectiveness εs measures temperature recovery: εs = (Tsupply - Toutdoor) / (Texhaust - Toutdoor). For ERVs, total effectiveness εt includes both sensible and latent recovery. Residential HRVs typically achieve 60-75% sensible effectiveness, high-performance units 80-90%, best passive house systems exceed 90%. Commercial ERVs range 65-80% total effectiveness. Heat exchanger types include: counter-flow plate (80-95% effectiveness, highest performance, common in passive house), cross-flow plate (60-75%, lower cost, standard residential), rotary enthalpy wheels (70-85%, widely used commercial ERVs), heat pipes/run-around coils (50-65%, useful when ducts not adjacent), and fixed plate ERVs with permeable membranes (65-75%, no moving parts). Higher effectiveness means greater energy recovery but also higher first costs, larger space requirements, and increased pressure drop.

Sizing and Energy Savings: Size HRVs/ERVs per ASHRAE 62.2 residential (Qtot = 0.15 × Afloor + 3.5 × (Nbr + 1)) or ASHRAE 62.1 commercial ventilation rate procedure. Example: 185m², 3-bedroom home requires 41.75 L/s (89 CFM), add 10-15% for duct losses, select HRV rated 45-50 L/s. Commercial units range 500-50,000 CFM. Proper sizing avoids undersizing (inadequate ventilation, IAQ problems) and oversizing (excessive first cost, poor part-load efficiency). Energy savings substantial—residential system with 45 L/s, 75% effectiveness, 30°C winter temperature difference recovers 1,218W continuous heating (6,090 kWh annually). Subtracting fan power (50-80W) and maintenance, net annual energy savings 5,500-6,000 kWh. Energy recovery periods typically range 2.6-7.5 years. Commercial buildings with higher ventilation rates have shorter recovery periods (2-5 years).

Frost Control and ASHRAE 90.1 Requirements: When outdoor air below -10°C to -25°C, moisture condenses and freezes on cold side of heat exchanger, blocking airflow and reducing effectiveness. Frost control strategies include defrost cycles (periodically close outdoor damper, run exhaust through both sides to melt ice, 5-10 minutes every 30-60 minutes when <-15°C), preheating outdoor air (electric/hydronic coil), recirculating warm supply air, or modulating airflow. High-performance passive house HRVs operate effectively to -30°C or colder. ASHRAE 90.1 Section 6.5.6.1 mandates energy recovery for systems exceeding thresholds varying by climate zone and percent outdoor air—Climate Zone 3B requires ERV if outdoor air >70% of supply and capacity >11,000 L/s; Climate Zone 6A requires ERV if outdoor air >30% of supply and capacity >2,350 L/s.

Control Integration and Maintenance: Demand-controlled ventilation using CO₂ sensors modulates fan speed based on occupancy, reducing ventilation during low-occupancy periods (saving fan energy). Economizer lockout disables energy recovery when outdoor conditions favorable for free cooling, allowing 100% outdoor air bypass. Schedule-based control reduces ventilation to code minimums during unoccupied hours. Airflow balancing maintains equal supply and exhaust flows (±5-10%) ensuring neutral building pressure and optimal heat exchanger performance. Maintenance includes regular filter changes (3-6 months residential, 1-3 months commercial), heat exchanger core cleaning (annually residential, semi-annually commercial), condensate drain inspection, and fan inspection. HRV/ERV lifespan typically 15-25 years with proper maintenance.

Standards Reference: ASHRAE 62.2 specifies residential ventilation requirements. ASHRAE 62.1 governs commercial ventilation. ASHRAE 90.1 Section 6.5.6.1 mandates energy recovery for specific applications by climate zone and system capacity. Passive House Institute (PHI) certification requires ventilation heat recovery efficiency $\geq$75% for extreme climates.