How Right-Sizing Saved Over $5 Million in a 600-Unit Multifamily High-Rise
A 600-unit multifamily high-rise in Chicago, Illinois (ASHRAE Climate Zone 5A) was originally designed with 2-ton heat pumps for every apartment. On review, that decision traced back to a 25 % “safety factor” applied on top of already conservative load calculations. Correcting it led to a smaller geothermal system, lower energy bills, and lifetime savings exceeding $5 million.
How the Oversizing Happened
Typical 1-bedroom MFHR units in Zone 5 require between ¾ and 1¼ tons of cooling. In this project, design calculations ranged from 1.0 to 1.4 tons—but a 25 % “safety factor” bumped the calculations over 1.5 tons, leading to the selection of 2-ton units. The logic was well-intentioned, and the reasoning was that the units were intended for techy-savvy students with excess electronics and prepared for extreme weather. However, the original assumptions were already conservative on schedules, glazing, high plug-loads, with almost no load diversity.
The Right-Sizing Process
After re-evaluating the load model, it became clear that the extra capacity provided no real benefit. We re-selected:
- 1.0-ton units for mid-floor interiors
- 1.5-ton units for corner and top floors
This small shift rippled through the entire mechanical system—smaller heat pumps required smaller piping, pumps, and wells, improving system balance and efficiency.
Cascading Benefits
- Reduced loop flow → smaller pumps and less horsepower
- Shorter pipe runs and smaller diameters
- Geothermal field downsized by 35 %
- Longer compressor cycles → less short-cycling and higher COP
Outcome: A leaner design saved roughly $3 million in first cost and continues to save about $250,000 per year in HVAC energy and maintenance—over $5 million over a 15-year lifespan.
Detailed Savings
| Category | Change Implemented | Estimated Savings |
|---|---|---|
| Heat pump equipment | 2.0 → 1.0–1.5 tons | $0.9 M |
| Piping & pumps | Reduced flow / smaller diameters | $2.0 M |
| Geothermal field | 35 % capacity reduction | $1.7 – 2.5 M |
| Electrical & controls | Smaller circuits & VFDs | $0.4 M |
| Total First-Cost Savings | — | ≈ $3 – 3.5 M |
| Annual Operating Savings | 50 % HVAC energy reduction | ≈ $0.25 M / yr |
| 15-Year Lifecycle Total | — | ≈ $5 – 6 M |
*Values based on 660,000 ft² multifamily building, Climate Zone 5 assumptions, and typical geothermal construction costs.
Lessons Learned
Across the industry, well-meaning safety margins accumulate at every step—load modeling, submittals, equipment selection, and controls. Each layer compounds cost and inefficiency. Right-sizing strips those layers back, aligning actual performance with design intent.
Efficiency isn’t about adding margin—it’s about precision. The difference between “safe” and “oversized” can be millions of dollars and decades of unnecessary energy use.
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