♻️ Energy Recovery Ventilators: How ERVs and HRVs Work, When to Use Them, and How to Size Them

The Energy Cost of Ventilation

Outdoor air ventilation is essential for indoor air quality, but it comes with an energy cost. Every cubic foot of outdoor air brought into a building must be conditioned — heated in winter, cooled and dehumidified in summer — before it reaches occupants. In a cold climate, a 50,000 ft² office building following ASHRAE 62.1 might require 15,000–20,000 CFM of outdoor air, which must be heated from -10°F to 70°F on the coldest days. The energy required to do this is substantial — often 30–40% of total HVAC energy consumption.

Energy recovery ventilation reduces this penalty by transferring heat (and in some systems, moisture) from the exhaust air stream to the incoming outdoor air stream before they are separated and delivered to the building. The exhaust air — already conditioned to room temperature and humidity — pre-conditions the outdoor air, dramatically reducing the load on the heating and cooling equipment.

HRV vs. ERV: The Critical Difference

Heat Recovery Ventilators (HRVs) transfer sensible heat only — temperature without moisture. In winter, the warm exhaust air heats the cold incoming outdoor air. In summer, the cool exhaust air cools the hot outdoor air. Moisture content of each air stream remains unchanged.

Energy Recovery Ventilators (ERVs) transfer both sensible heat and latent heat (moisture). In winter, the humidified exhaust air transfers both heat and moisture to the dry, cold incoming air — reducing both heating load and humidification load. In summer, the dehumidified exhaust air absorbs some moisture from the hot, humid outdoor air — reducing both cooling load and dehumidification load.

Which to use: In cold, dry climates, ERVs are preferable because retaining moisture in the exhaust air prevents the building from becoming excessively dry in winter. In mild climates with high summer humidity, ERVs are preferable because they help dehumidify incoming outdoor air. HRVs are better suited for climates where indoor humidity control is not critical — temperate climates with moderate humidity year-round — or for specific applications like pool buildings where high indoor humidity is desired and an ERV would transfer excess moisture to the incoming air.

Energy Recovery Device Types

Rotary enthalpy wheel (total energy wheel): A slowly rotating wheel with a hygroscopic desiccant coating sits in both the exhaust and supply air streams. As the wheel rotates, it alternately picks up heat and moisture from the exhaust air and transfers them to the incoming supply air. Highest effectiveness of all ERV types — sensible effectiveness typically 75–85%, total effectiveness (including latent) 65–75%. Risk of cross-contamination between exhaust and supply streams due to the rotating media. Not suitable for laboratories, hospitals, or other occupancies where exhaust air must be strictly separated from supply air.

Fixed-plate heat exchanger (sensible only): Two air streams flow through adjacent channels in a plate assembly, transferring sensible heat through the plate material without contact between streams. No cross-contamination — suitable for all occupancies. Sensible effectiveness typically 65–80%. Does not transfer moisture (HRV only). Requires frost control in very cold climates to prevent condensation and frost buildup at the cold end of the plates.

Membrane plate exchanger (total energy): Similar to fixed-plate but uses membrane materials that allow water vapor (but not air) to pass through. Achieves ERV performance (transfers both sensible heat and moisture) with the cross-contamination resistance of a fixed-plate design. Suitable for healthcare and other sensitive occupancies. Sensible effectiveness 70–80%, total effectiveness 60–70%.

Run-around coil system: Two separate coils — one in the exhaust duct, one in the supply duct — connected by a glycol loop. Heat is transferred from the exhaust coil to the glycol, which carries it to the supply coil. No cross-contamination possible because the two air streams are completely separate. Lower effectiveness than plate or wheel systems (typically 45–65% sensible only) but allows the exhaust and supply ducts to be in completely different locations — useful for buildings where exhaust and supply air handling units are far apart.

When Energy Codes Require Energy Recovery

ASHRAE Standard 90.1 Table 6.5.6.1 requires energy recovery for air-handling systems above certain airflow thresholds when the system operates more than 8,760 hours/year (continuous operation) or 4,380 hours/year, depending on climate zone and outdoor air percentage. The requirements have become more stringent with each code edition.

As a general rule of thumb: systems with design outdoor air flow rates above 70% of supply air flow (high outdoor air fraction systems) and airflows above approximately 3,000 CFM in climate zones with significant heating or cooling loads are likely to trigger energy recovery requirements. Energy modelers and code consultants should verify specific requirements against the adopted edition of 90.1 in your jurisdiction.

The International Energy Conservation Code (IECC) also references 90.1 recovery requirements for commercial buildings. California's Title 24 has its own energy recovery requirements that may differ from 90.1.

Sizing Energy Recovery Systems

ERV and HRV units are rated at specific airflow conditions. The key selection parameters:

Design airflow (CFM): Select the unit for the design ventilation outdoor air flow rate, not the total supply airflow. The ERV only handles the outdoor air and exhaust air streams, not recirculated air.

Effectiveness at design conditions: Manufacturer data provides sensible and total effectiveness at specific airflow rates and outdoor conditions. Effectiveness decreases at airflows above or below the rated conditions. Verify effectiveness at your actual operating airflow.

Pressure drop: The energy recovery media adds pressure drop to both the supply and exhaust air streams — typically 0.4–1.0 inches w.g. for each stream. This must be included in the fan pressure calculations for the air handling system.

Frost control: In cold climates, frost formation on the cold end of the heat exchanger reduces effectiveness and can block airflow. Frost control strategies include preheating the incoming outdoor air, bypassing a portion of the exhaust air past the recovery device, and periodic defrost cycles. Evaluate frost control requirements for your climate zone.

Calculating Energy Savings

Sensible heat recovery (BTU/hr) = 1.1 × CFM × ΔT × effectiveness_sensible

Latent heat recovery (BTU/hr) = 4,840 × CFM × Δw × effectiveness_latent (where Δw is humidity ratio difference in lb/lb)

For a quick estimate: an ERV with 75% total effectiveness on a 5,000 CFM outdoor air system in Chicago saves approximately 25–35% of the annual HVAC energy that would otherwise be spent conditioning outdoor air. For a large system, this translates to tens of thousands of dollars per year in energy savings — typically a payback period of 3–7 years for the ERV equipment and installation cost.