Section 6.5.2/Simultaneous Heating and Cooling Limitation
ASHRAE 90.1 Section 6.5.2 prohibits HVAC systems that simultaneously heat and cool the same airstream, with specific exceptions for VAV systems with DDC controls and small-zone airflow.
HVAC systems must not provide simultaneous heating and cooling to a zone unless they meet one of the specific exceptions. Prohibited configurations include constant-volume reheat systems, dual-duct mixing systems, and multizone systems that mix hot and cold air. The primary exception permits zone-level reheat or recool in VAV systems with DDC controls when the airflow to the zone has been reduced to the larger of: 20% of the zone peak primary supply airflow, the minimum ventilation requirement per ASHRAE 62.1, or any code-required minimum (such as exhaust makeup). Additional exceptions exist for zones with specific humidity requirements (data centers, labs), radiant heating/cooling systems, and zones where the simultaneous heating and cooling energy use is recovered by a heat recovery system.
Why this section exists
Simultaneously heating and cooling the same airstream is inherently wasteful, as energy is spent on both heating and cooling with much of it canceling out. A constant-volume reheat system that cools air to 55 degrees then reheats it to 72 degrees wastes all the energy used for the unnecessary overcooling plus the reheat energy. This provision drives the industry toward VAV systems that reduce airflow to zones before adding reheat, minimizing the simultaneous energy waste. The 20% minimum airflow exception allows limited reheat for zones that need it for comfort while ensuring the system operates at reduced airflow first. This section, along with Section 6.5.3 (air system design), forms the core of HVAC energy efficiency in the prescriptive compliance path.
What plan reviewers look for
Plan reviewers check the HVAC system type on the mechanical drawings and specifications. For any system with zone-level reheat (hot water coils, electric reheat), they verify the system is VAV with DDC controls and that the minimum airflow setpoints comply with the 20% or ventilation minimum threshold. For dual-duct or multizone systems, they check that the mixing is controlled to avoid simultaneous heating and cooling. They verify the control sequence describes the VAV box minimum airflow settings and the conditions under which reheat activates.