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Why Sensible Recovery Efficiency Doesn’t Tell the Whole Story
Reading Time: 4 Minutes
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When specifying ventilation equipment, few metrics are scrutinized more closely than Sensible Recovery Efficiency (SRE). Yet despite its prevalence on submittals and spec sheets, SRE often fails to tell the whole story. That’s especially true when comparing heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs), two system types with overlapping applications but distinct performance behaviors.
The challenge for engineers and HVAC contractors isn’t just evaluating a single number. It’s knowing how that number was derived, under what test conditions, and whether it reflects real-world performance. This article explores how to accurately compare HRVs and ERVs, why SRE should not be viewed in isolation, and what data professionals can trust when designing balanced ventilation systems.
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Understanding the Complexity of Comparing HRVs and ERVs
On paper, comparing HRVs and ERVs can appear straightforward. But identical SRE values on two units don't necessarily indicate equivalent performance. Critical context is often missing such as testing airflow, latent recovery, power consumption, fan efficacy, and defrost operation.
Performance ratings alone don’t tell the full story; engineers may inadvertently compare units at different ratings conditions, leading to equipment choices that underperform in the field.
Core Performance Metrics to Consider
To make informed product comparisons, it’s important to evaluate HRVs and ERVs using a consistent set of metrics:
Sensible Recovery Efficiency (SRE)
Sensible Recovery Efficiency (SRE) is a key performance metric, certified by the Home Ventilating Institute (HVI), that measures how effectively an HRV or ERV transfers temperature between the exhaust and supply air streams. Unlike basic recovery metrics, SRE accounts for multiple system characteristics to provide a more complete view of a unit’s sensible heat recovery performance.
SRE reflects the net sensible energy recovered by the supply air, adjusted for factors such as:
Electrical consumption
Heat loss or gain through the unit casing
Air leakage
Airflow imbalance between the two airstreams
Energy used for defrost during low-temperature operation
SRE is expressed as a percentage of the total recoverable sensible energy (plus exhaust fan energy) and is used to evaluate and compare HRV/ERV performance during the heating season.
Total Recovery Efficiency (TRE)
Total Recovery Efficiency, or TRE, represents the combined effectiveness of a ventilation unit in transferring both sensible (temperature) and latent (moisture) energy, also known as enthalpy, between the exhaust and supply air streams.
TRE is particularly relevant in warm and humid climates, where both forms of energy play a significant role in comfort and energy use.
TRE reflects the net total energy recovered by the supply airstream, adjusted for electrical consumption, case heat loss or gain, air leakage, and airflow mass imbalance, expressed as a percentage of the total recoverable energy plus exhaust fan energy.
As a certified HVI metric, TRE provides a meaningful basis for comparing cooling season performance across ERVs and HRVs, making it a vital specification when designing systems for year-round energy efficiency and occupant comfort. (HVI)
Adjusted Sensible Recovery Efficiency (ASRE) and Adjusted Total Recovery Efficiency (ATRE)
The Adjusted SRE and TRE (ASRE & ATRE) are the same as SRE & TRE, respectively, but they do not factor in the fan power consumption. These values should be used for energy modeling when wattage for air movement is separately accounted for in the energy model.
Latent Recovery (ERVs only)
Latent effectiveness measures how well an energy recovery ventilator (ERV) transfers moisture between the airstreams. This capability is especially important in humid climates or tightly sealed homes, where excess humidity can contribute to indoor air quality issues, occupant discomfort, and potential mold growth.
On product spec sheets, this performance is often expressed as Latent Recovery/Net Moisture Transfer (NMT), the amount of moisture recovered divided by the moisture exhausted, corrected for the effects of cross-leakage. An NMT value of 0 indicates no net moisture transfer, while a value of 1.0 indicates complete transfer. HVI provides NMT ratings for both Heating Season Performance and Very Low Temperature tests, making it a key indicator of an ERV’s ability to assist with humidification or dehumidification under specific operating conditions.
Although sometimes overlooked, latent recovery should be a critical consideration when designing comfort, energy efficiency, and moisture control.
Fan Efficacy (cfm/W)
Fan efficacy, expressed in cubic feet per minute per watt (cfm/W), quantifies the electrical efficiency of a ventilation unit. Higher fan efficacy means more airflow is delivered for each watt of electricity consumed. Certain codes and energy programs require a defined cfm/W, underscoring the importance of electrical efficiency in sustainable system design.
Source: (HVI)
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Testing Standards and Why They Matter
When performance values are listed without identifying the testing methodology, it’s difficult to validate or compare products accurately. Understanding the test standard behind the published ratings is essential.
CAN/CSA C439 is a North American standard that governs the testing and performance reporting of residential heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs). It establishes uniform testing conditions for key performance metrics, including Sensible Recovery Efficiency (SRE), latent effectiveness, and fan power consumption. This standard ensures consistency and comparability across manufacturers. CSA C439 is the required testing protocol for products certified by the Home Ventilating Institute (HVI) and is also mandated for ENERGY STAR® certification in Canada.
Source: CSA Group
HVI Certification is administered by the Home Ventilating Institute (HVI) and provides third-party verification of performance data for residential ventilation products. The certification process relies on standardized testing methods outlined in CSA C439, ensuring consistency across manufacturers. Products that are HVI Certified include verified data on key performance metrics such as airflow, Sensible Recovery Efficiency (SRE), fan efficacy, and more, offering engineers and HVAC contractors a reliable basis for specification and comparison.
Source: HVI Certified Directory
Other Metrics and Standards
Many standards define metrics like the ones listed above, but only the ones from CAN/CSA 439 can accurately depict the characteristics of the whole H&ERV system. While Sensible, Latent and Total Effectiveness defined in standards like ASHRAE 84 and AHRI 1060 provide a good representation of the recovery characteristics of a heat or energy recovery core it does not account for system characteristics such as frost control strategies, fan power consumption, or other system losses. As a result, these effectiveness ratings alone do not provide an accurate comparison between residential ventilation systems, where there is more in play than just the core effectiveness.
Source: ASHRAE 84-2024 | ASHRAE Store
Sensible Recovery Efficiency (SRE) remains a valuable metric, but only when considered alongside other certified data points like TRE, latent recovery, and fan efficacy. Equally important is understanding how those values were derived and whether they reflect complete system performance.
In Part Two, we’ll apply these principles to real-world product comparisons, exploring what certified testing reveals about performance differences and why they matter to engineers and contractors in the field.
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References
HVI. Certified Product Directory. https://www.hvicertified.org
AHRI. Standard 1060 – Performance Rating of Air-to-Air Heat Exchangers for Energy Recovery Ventilation Equipment. https://www.ahrinet.org