On June 23, 2026, the RESNET® Standards Development Committee (SDC 300) approved two new Standards Interpretations clarifying how Dwelling Unit Mechanical Ventilation Systems (DUMVS) are evaluated under both the RESNET HERS® Standards and ANSI/RESNET/ICC 301. This IR applies to all homes and may be used immediately. RESNET will enforce this interpretation on all homes with permit dates on or after July 23, 2026. This bulletin explains both the interpretations and the practical implications for HERS Raters, Rating Field Inspectors (RFIs), HERS Modelers, Quality Assurance Designees (QADs), Providers, and builders. Why This Interpretation Was Needed Many homes satisfy dwelling unit mechanical ventilation requirements by using a continuously operating exhaust fan. In practice, builders frequently install an ON-OFF wall switch so the ventilation system can be temporarily disabled during unusual conditions such as wildfire smoke, pesticide application, or other periods of poor outdoor air quality. This approach to provide ventilation override controls has long been accepted by ENERGY STAR®, ASHRAE 62.2, and many code jurisdictions, as a means to provide good indoor air quality. However, revisions to the definition of a DUMVS in successive editions of ANSI/RESNET/ICC 301 created uncertainty about how these installations impact HERS Ratings and Energy Rating Index (ERI) Ratings. Interpretation #1 — RESNET HERS Ratings For HERS Ratings with permit dates on or after January 1, 2022, HERS Addendum 53 applies together with ANSI/RESNET/ICC 301-2019. Under this interpretation: A ventilation system with a labeled ON-OFF wall switch that lacks a protective cover does not qualify as a DUMVS because those editions expressly state that an occupant-operated switch is not considered automated, continuous, or programmed. If a wall switch is labeled and has a protective cover then it is considered an override control, which is acceptable. Therefore, a ventilation system with a labeled ON-OFF wall switch that has a protective cover does qualify as a DUMVS. The same interpretation applies to HERS Ratings using ANSI/RESNET/ICC 301-2022 and ANSI/RESNET/ICC 301-2025. Interpretation #2 — ERI Ratings For ERI Ratings, the applicable edition of ANSI/RESNET/ICC 301 determines the outcome. The typical use case here is for HERS Raters doing a code compliance “ERI” per 2021 IECC or 2024 IECC section R406, but not for a HERS rating. The Committee concluded: For the 2021 IECC, ANSI/RESNET/ICC 301-2019 (unamended) is required, and a ventilation system with a labeled ON-OFF wall switch qualifies. For the 2021 IECC where the jurisdiction allows Addendum B to ANSI/RESNET/ICC 301-2019, a ventilation system with a labeled ON-OFF wall switch also qualifies. For 2024 IECC, ANSI/RESNET/ICC 301-2022 is required. A ventilation system with a labeled ON-OFF wall switch and no protective cover no longer qualifies because that edition expressly states that an occupant-operated switch is not considered automated, continuous, or programmed. How This Impacts the ERI / HERS Index The Standard does not require a DUMVS to be present, but when one is present its energy consumption is accounted for in the rating. If the system does not meet the definition of a DUMVS, its fan energy is not accounted for. This can cause a change in the HERS Index or ERI, if the rating benefits from the energy savings associated with a high efficiency ventilation system. The recent IR’s have highlighted the need to further clarify the definition of a DUMVS and how a Rater is to evaluate whether a ventilation system complies with such a definition. Improvements will be proposed for the 2028 edition of ANSI/RESNET/ICC 301. These interpretations do not create new technical requirements. Instead, they clarify how existing requirements should be interpreted. As a result: A builder is free to install any control strategy desired. However, if the installed controls do not meet the interpreted definition of a DUMVS, the HERS Rater cannot recognize that ventilation system as a qualifying dwelling-unit mechanical ventilation system for the HERS Index calculation and/or ERI calculation. Similarly, a ventilation system may comply with local building code or ASHRAE 62.2 while still failing to qualify as a DUMVS for HERS purposes because different programs can have different purposes when evaluating building performance. Access the Interpretations No. 301-2019-042 Exhaust-Only Ventilation Fan WH Mechanical Ventilation Controls: This interpretation applies to RESNET HERS Ratings completed using the 2019, 2022, or 2025 editions of Standard 301. No. 301-2019-043 Exhaust-Only Ventilation Fan WH Mechanical Ventilation Controls: This interpretation applies to ERI Ratings completed using the 2019, 2022, or 2025 editions of Standard 301. Frequently Asked Questions In addition to explaining the new interpretations, we want to answer some of the related ventilation questions that we are frequently asked about. Why does a normal wall switch prevent the system from being considered DUMVS? A normal wall switch allows occupants to disable continuous ventilation during ordinary daily use. The Committee intended override controls to be used only occasionally and only after a deliberate action. Is labeling the switch sufficient to be considered DUMVS? For HERS ratings – no. A separate ON-OFF wall switch must also include an operable protective cover to prevent accidental use. For ERI ratings – it depends on the edition. Under 301-2022 and 301-2025, no. Under earlier editions, yes. Does a system with a dedicated circuit breaker as its override control still qualify as DUMVS? Yes. A circuit breaker is considered an acceptable override control. Whether the breaker is dedicated solely to ventilation or also serves another bathroom load does not affect its acceptability. If the circuit breaker is the only way to turn off the ventilation system, do I need to photograph it? No. If there is no separate manual ON-OFF control, the interpretation does not require documenting the circuit breaker as a ventilation control. My ventilation fan has integrated controls that can reduce airflow to zero. Is that acceptable? Yes. Integrated manufacturer controls are considered acceptable override controls because they are part of the qualifying ventilation control system. Can I use the thermostat’s “Fan On” or “Circulate” mode? No. Those functions neither qualify as automated control devices nor as permitted override controls. Questions regarding these interpretations may be directed to technicalsupport@resnet.us.
