IEER (Integrated Energy Efficiency Ratio) measures the cooling efficiency of commercial air conditioning equipment across four part-load conditions, weighted to reflect typical building operation. Unlike EER, which tests at a single full-load condition, IEER captures how efficiently a unit performs when it's running at 25%, 50%, 75%, and 100% capacity. Since commercial systems operate at part load the vast majority of the time, IEER is a far better predictor of real-world energy consumption than EER alone.
IEER applies to commercial unitary equipment over 65,000 BTU/h, including rooftop units (RTUs), split systems, and variable refrigerant flow (VRF) systems. If you're specifying or purchasing commercial HVAC, IEER is the number to compare.
The IEER Formula
IEER uses a weighted average of EER measurements at four operating points:
IEER = (0.02 x EER at 100%) + (0.617 x EER at 75%) + (0.238 x EER at 50%) + (0.125 x EER at 25%)
The weightings reflect the percentage of annual operating hours a typical commercial building spends at each load level:
The 100% load condition gets only 2% weight. This is a critical insight — the system's full-load EER barely matters in the IEER calculation. A unit that excels at 75% and 50% load will have a much higher IEER than one that's only efficient at full blast. This heavily favors variable-capacity equipment.
IEER vs EER: Why IEER Matters
A fixed-speed commercial RTU might rate 11 EER and 12 IEER. A variable-speed VRF system might rate 10 EER but 22 IEER. The IEER gap reveals how much better the VRF system performs during the 98% of the year when it's not at full load.
What's a Good IEER?
ASHRAE 90.1 IEER Requirements
ASHRAE Standard 90.1 (Energy Standard for Buildings) sets minimum IEER requirements for commercial equipment. These vary by capacity and equipment type:
These are minimum code requirements. High-performance buildings typically specify IEER 15-20+ to achieve green building certifications like LEED.
IEER and Building Energy Codes
IEER is referenced in several important standards and codes. ASHRAE 90.1 sets the baseline. The International Energy Conservation Code (IECC) references 90.1 for commercial buildings. LEED and other green building certifications often require exceeding 90.1 minimums by 10-30%, which pushes IEER requirements higher.
Many utility rebate programs for commercial buildings use IEER thresholds to qualify equipment for incentives. Rebates of $50-$200 per ton of cooling are common for units exceeding minimum IEER by 10-20%.
Key Takeaways
- IEER weights part-load performance heavily — 100% load gets only 2% weight, while 75% load gets 61.7%
- IEER is always higher than EER for the same unit because part-load operation is more efficient
- Good IEER is 15+. Excellent is 22+. Variable-capacity systems dominate the high end.
- IEER applies to commercial equipment over 65,000 BTU/h — not residential systems
- Variable-speed/VRF systems can achieve IEER 20-30+ compared to 12-15 for fixed-speed RTUs
- ASHRAE 90.1 sets minimum IEER requirements, with typical ranges of 11-13 depending on capacity
- For building owners, IEER is the best predictor of actual cooling energy costs in commercial applications
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