HRV Sizing and Performance Guide

Introduction

Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs) are essential mechanical ventilation systems that transfer heat between outgoing stale air and incoming fresh outdoor air, reducing heating and cooling loads by 50-80% while maintaining good indoor air quality. HRVs transfer only sensible heat with 60-85% efficiency, while ERVs transfer both sensible and latent heat (moisture) with 60-75% total efficiency.

Why This Sizing Matters

Accurate HRV/ERV sizing is crucial for:

• Energy Efficiency: Maximizing heat recovery to reduce HVAC heating and cooling loads by 50-80%.
• Indoor Air Quality: Ensuring adequate fresh air ventilation rates per ASHRAE 62.1 requirements.
• System Balance: Maintaining balanced supply and exhaust airflows to prevent pressure imbalances.
• Code Compliance: Meeting ASHRAE 90.1 energy recovery requirements for systems with high outdoor air fractions.

The Fundamental Challenge

The primary challenge in HRV sizing lies in properly calculating ventilation requirements using the ASHRAE 62.1 combined method ($Q = R_p \times N + R_a \times A$), then selecting equipment that can deliver the required airflow against system pressure drop while maintaining high heat recovery efficiency. Additionally, choosing between HRV (sensible heat only) and ERV (sensible plus latent heat) depends on climate conditions—HRVs suit dry cold climates while ERVs perform better in humid climates where moisture recovery is beneficial. Balancing supply and exhaust airflows within ±20% is critical for proper operation; imbalance causes building pressure problems and reduces recovery efficiency.

What You'll Learn

In this comprehensive guide, you will learn:

• The ASHRAE 62.1 ventilation rate procedure for calculating outdoor air requirements.
• How to size HRV/ERV units based on airflow, pressure drop, and efficiency.
• The differences between HRV and ERV systems and selection criteria for different climates.
• Energy recovery calculations and annual energy savings analysis.
• Step-by-step examples applying ASHRAE 90.1 and EN 13053 heat recovery requirements.

Quick Answer: How to Size an HRV System

Size HRV systems based on ASHRAE 62.1 ventilation requirements, then calculate energy recovery potential.

Core Formula (ASHRAE 62.1)

Where:

• $Q$ = Required ventilation rate (L/s)
• $R_p$ = Air circulation rate per person (L/s per person)
• $N$ = Number of occupants
• $R_a$ = Air exchange rate per area (L/s per $\text{m}^2$)
• $A$ = Floor area ($\text{m}^2$)

Additional Formulas

Heat Recovery Rate:

Where:

• $Q_{\text{recovery}}$ = heat recovery rate (kW)
• $\eta$ = heat recovery efficiency (decimal, typically 0.60-0.85)
• $Q$ = ventilation airflow rate (m³/s)
• $\rho$ = air density (1.204 kg/m³ at sea level, 20°C)
• $c_p$ = specific heat capacity (1.005 kJ/kg·K)
• $\Delta T$ = temperature difference between indoor and outdoor air (°C)

Annual Energy Savings:

Where:

• $E$ = annual energy savings (kWh/year)
• $Q_{\text{recovery}}$ = heat recovery rate (kW)
• $H$ = operating hours per year (typically 8,760 for continuous operation)
• $\text{COP}$ = coefficient of performance (typically 3.0 for heat pumps, 0.8-0.95 for gas furnaces)

Efficiency Rating:

Where:

• $\eta$ = sensible heat recovery efficiency (decimal)
• $T_{\text{supply}}$ = supply air temperature after heat recovery (°C)
• $T_{\text{outdoor}}$ = outdoor air temperature (°C)
• $T_{\text{indoor}}$ = indoor air temperature (°C)

Worked Example

Reference Table

Key Standards

Sizing Calculation

Ventilation Rate

The required air supply movement rate uses the ASHRAE 62.1 combined method:

Where:

• $Q$ = airflow supply rate (L/s)
• $R_p$ = atmosphere circulation rate per person (L/s per person)
• $N$ = number of occupants
• $R_a$ = ventilation air exchange rate per area (L/s per $\text{m}^2$)
• $A$ = floor area ($\text{m}^2$)

Airflow Balance

For balanced airflow, supply and exhaust flows should be approximately equal:

Typical acceptable ratio: 0.8-1.2 (supply vs. exhaust)

Unit Selection

Select HRV/ERV based on:

1. Required airflow
2. Available space
3. Productivity requirements
4. Budget constraints

Performance Calculation

Heat Recovery Rate

Where:

• $Q_{\text{recovery}}$ = heat recovery rate (kW)
• $\eta$ = heat recovery output ratio (decimal)
• $Q$ = fresh air movement rate (m³/s)
• $\rho$ = air supply density (kg/m³)
• $c_p \approx 1.005$ kJ/kg·K (specific heat)
• $\Delta T$ = temperature difference (°C)

Energy Savings

Where:

• $E_{\text{savings}}$ = energy savings (kWh/year)
• $Q_{\text{recovery}}$ = heat recovery rate (kW)
• $H$ = operating hours (hours/year)
• $\text{COP}$ = coefficient of performance

Efficiency Ratings

Sensible Heat Recovery Efficiency

Where:

• $\eta_s$ = sensible yield (decimal)
• $T_{\text{supply}}$ = supply airflow heat (°C)
• $T_{\text{outdoor}}$ = outdoor atmosphere thermal value (°C)
• $T_{\text{indoor}}$ = indoor ventilation air degree (°C)

Typical Efficiency Values

Worked Example

Evaluate HRV sizing and performance for an office:

• Floor area: 100 $\text{m}^2$
• Occupancy: 10 people
• Ceiling height: 2.7 m
• Design heat level difference: 25°C
• HRV productivity: 75%

Step 1: Calculate Ventilation Rate

Using ASHRAE 62.1 combined method ($R_p = 5$ L/s per person, $R_a = 0.6$ L/s per $\text{m}^2$):

Step 2: Calculate Heat Recovery Rate

Step 3: Calculate Annual Energy Savings

Assuming 8,760 hours/year and COP = 3:

Step 4: Calculate Airflow Ratio

For balanced HRV operation, supply and exhaust should be equal:

This is within the acceptable range of 0.8-1.2.

Step 5: Calculate ACH

This is low. Consider increasing fresh air supply rate to meet minimum ACH requirements.

Design Guidelines

Airflow Balance

• Positive pressure (supply > exhaust): Prevents infiltration
• Negative pressure (supply < exhaust): Prevents exfiltration
• Balanced (supply = exhaust): Ideal for most applications

Location and Installation

1. Central location - Minimize ductwork
2. Accessible - For maintenance
3. Protected - From weather and damage
4. Adequate space - For service clearance

Controls

1. On/Off control - Simple, low cost
2. Multi-speed - Better control, moderate cost
3. Variable speed - Optimal control, higher cost
4. Demand control - Based on CO₂ or occupancy

How Do You Improve Energy Efficiency with?

Annual Energy Cost Savings

Payback Period

Typical Payback Periods

Maintenance

Regular Maintenance

1. Filter replacement - Every 3-6 months
2. Core cleaning - Annually
3. Fan inspection - Annually
4. Control check - Annually

Efficiency Degradation

Without proper maintenance, output ratio can decrease by 10-20% per year.

Standards and References

• EN 13053: Air supply circulation for buildings - Airflow handling units
• ASHRAE 90.1: Energy Standard for Buildings
• EN 308: Heat exchangers - Test procedures

Our airflow calculations follow industry standards for optimal system performance.

Our airflow calculations follow industry standards for optimal system performance.

Conclusion

Proper sizing and selection of HRVs/ERVs can significantly reduce energy consumption while maintaining good indoor air quality. By calculating heat recovery rates and energy savings, engineers can optimize system design and demonstrate cost-effectiveness.

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Key Takeaways

• Calculate ventilation rate using ASHRAE 62.1 combined method: $Q = R_p \times N + R_a \times A$ where $R_p$ is ventilation rate per person (L/s per person), $N$ is number of occupants, $R_a$ is ventilation rate per area (L/s per $\text{m}^2$), and $A$ is floor area ($\text{m}^2$). This ventilation rate determines the HRV capacity required for adequate indoor air quality.

• Select HRV type based on climate conditions: HRV (Heat Recovery Ventilator) for dry cold climates—transfers sensible heat only with 60-85% efficiency. ERV (Energy Recovery Ventilator) for humid climates—transfers both sensible and latent heat (moisture) with 60-75% total efficiency. HRVs are ideal when dehumidification is needed; ERVs are better when humidification is beneficial in winter.

• Add 20-30% safety margin to calculated airflow: Apply safety factors to account for filter loading (10-15%), duct friction losses (5-10%), and control tolerance (5%). Total safety margin: $Q_{\text{size}} = Q \times 1.20$ to $Q \times 1.30$. Always round up to nearest standard HRV size.

• Ensure balanced airflow with supply/exhaust ratio: Maintain ratio $\frac{Q_{\text{supply}}}{Q_{\text{exhaust}}} = 0.8$ to $1.2$ for balanced operation. Balanced airflow prevents building pressure issues and maintains heat recovery efficiency. Imbalance greater than ±20% causes pressure problems and reduces recovery efficiency.

• Select efficiency rating meeting ASHRAE 90.1 minimums: Climate zones 3-4 require $\geq 60\%$ sensible recovery, zones 5-6 require $\geq 65\%$, zones 7-8 require $\geq 70\%$. Efficiency is calculated as $\eta = \frac{T_{\text{supply}} - T_{\text{outdoor}}}{T_{\text{indoor}} - T_{\text{outdoor}}}$. Higher efficiency units cost 30-50% more but save proportionally more energy.

• Calculate energy savings and payback period: Annual energy savings: $E = \frac{Q_{\text{recovery}} \times H}{\text{COP}}$ where $Q_{\text{recovery}} = \eta \times Q \times \rho \times c_p \times \Delta T$. Payback period: $\text{Payback} = \frac{\text{Initial Cost}}{\text{Annual Savings}}$. Typical payback: 2-5 years depending on climate, efficiency rating, and energy costs.

Further Learning

• Duct Sizing Guide - Sizing distribution ductwork for HRV systems
• Duct Pressure Loss Guide - Calculating system pressure drops
• Fresh Air Flow Guide - Ventilation rate calculations
• HRV Sizing Calculator - Interactive calculator for HRV sizing

References & Standards

Primary Standards

ASHRAE 90.1-2022 Energy Standard for Buildings. Section 6.5.6.1 mandates energy recovery for exhaust air systems with ≥70% outdoor air or ≥5,000 CFM in climate zones 3-8. Table 6.5.6.1-1 specifies minimum sensible recovery effectiveness: 50% for zones 3-4, 50% for zone 5, 60% for zones 6-8. Exception (h) allows bypass dampers for economizer operation.

ASHRAE Standard 62.1-2022 Ventilation for Acceptable Indoor Air Quality. Section 6.2 provides the Ventilation Rate Procedure used for HRV sizing. Table 6.2.2.1 lists outdoor air rates per person (Rp) and per area (Ra) for all occupancy categories. Section 6.2.5 covers zone air distribution effectiveness.

EN 13053:2019 Ventilation for buildings — Air handling units — Rating and performance for units, components and sections. Section 6 specifies heat recovery efficiency testing at rated conditions; Section 7 covers pressure drop requirements; Annex B provides calculation methods for annual energy savings.

Supporting Standards & Guidelines

EN 308:1997 Heat exchangers — Test procedures for establishing performance of air to air and flue gases heat recovery devices. Section 5 defines thermal efficiency calculation; Section 6 specifies test conditions for sensible and latent heat recovery measurement.

EN 13141-7:2021 Ventilation for buildings — Performance testing of components/products for residential ventilation — Part 7: Performance testing of mechanical supply and exhaust ventilation units. Section 6.5 covers defrost testing procedures; Section 6.7 specifies efficiency measurement at various outdoor temperatures.

Further Reading

• ASHRAE Technical Resources - American Society of Heating, Refrigerating and Air-Conditioning Engineers resources
• SMACNA HVAC Systems Duct Design Manual - Industry-standard duct design manual for sizing and construction practices

Note: Standards and codes are regularly updated. Always verify you're using the current adopted edition applicable to your project's location. Consult with local authorities having jurisdiction (AHJ) for specific requirements.

Disclaimer: This guide provides general technical information based on international ventilation standards. Always verify calculations with applicable local codes and consult licensed professionals for actual installations. Ventilation system design should only be performed by qualified professionals. Component ratings and specifications may vary by manufacturer.