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Calcimator

Setpoint Optimization Calculator

Energy savings from supply air temperature reset.

About this calculator

This calculator combines two independent HVAC energy-saving strategies: VFD speed reduction on a fan or pump, and supply air temperature (SAT) reset. The VFD side applies the fan/pump affinity laws -- power scales with the cube of speed -- so the Reduced Speed input dominates VFD Power Savings: raising Reduced Speed toward Current Speed shrinks the gap between currentSpeed^3 and reducedSpeed^3, and therefore shrinks the savings, while lowering Reduced Speed widens that gap and grows the savings. That is why the calculator's own helpText notes "even small reductions yield large savings" -- a 20% cut in speed (reducedSpeedPct going from 100 to 80) yields roughly 49% power savings, because 0.8 cubed is 0.512.

The SAT reset side is unrelated arithmetic: it applies a flat 1.8% cooling-energy savings per degree the Reset SAT setpoint sits above the Current SAT setpoint, multiplied by Annual Cooling Cost, and the two savings streams (VFD Savings and SAT Reset Savings) are simply added together in Total Combined Savings without interacting. Annual Run Hours, Electricity Rate, Current SAT, Reset SAT, and Annual Cooling Cost each feed only their own half of the calculation and have no effect on the other half's savings figures -- adjusting the SAT reset inputs never changes VFD Power Savings, and adjusting the VFD inputs never changes SAT Reset Savings.

Inputs

%
%
°F
°F

Results

VFD power savings (kW)

9.78

Power reduction (%)

48.8

Annual VFD savings ($)

$7,043.00

Current power (kW)20.05
Reduced power (kW)10.26
Annual kWh saved58,694
Estimated VFD cost ($)$5,625.00
VFD payback (years)0.8
Flow reduction (%)20
SAT reset savings ($/yr)$4,500.00
Total combined savings ($)$11,543.00
Pressure reduction (%)36%
How to Use This Calculator
  1. Enter Fan/pump motor (HP), Current speed (%), and Reduced speed (%).
  2. Set Annual run hours, Electricity rate ($/kWh), and Current SAT setpoint (°F).
  3. Adjust Reset SAT setpoint (°F), Annual cooling cost ($) as needed.
  4. Review VFD power savings (kW), Power reduction (%), and Annual VFD savings ($) ($).
  5. Use Current power (kW) and Reduced power (kW) to inform your decision.

How the result changes with Reduced speed (%)

Reduced speed (%)VFD power savings (kW)Power reduction (%)Annual VFD savings ($)
4018.7693.6$13,509.00
6015.7278.4$11,315.00
10000$0.00

What each input means

Fan/pump motor (HP)
Motor horsepower of the fan or pump to be controlled by VFD.
Current speed (%)
Current operating speed as percentage of full speed.
Reduced speed (%)
Target reduced speed. Affinity law: Power = (Speed)³. Even small reductions yield large savings.
Annual run hours
Hours per year the fan/pump operates. 24/7 = 8,760; typical office = 3,000-5,000.
Electricity rate ($/kWh)
Blended electricity rate in dollars per kilowatt-hour.
Current SAT setpoint (°F)
Current supply air temperature setpoint. Typical design: 55°F.
Reset SAT setpoint (°F)
Optimized (higher) SAT during low cooling load. Each +1°F saves ~1.5-2.5% cooling energy.
Annual cooling cost ($)
Annual cooling energy cost for SAT reset savings calculation.

What each result means

VFD power savings (kW)
Instantaneous power reduction from speed reduction per the affinity law (Power ~ Speed³).
Power reduction (%)
Percentage power reduction. A 20% speed cut saves ~49% power.
Annual VFD savings ($)
Annual dollar savings from VFD speed reduction.
Current power (kW)
Motor power consumption at the current speed.
Reduced power (kW)
Motor power consumption at the reduced speed.
Annual kWh saved
Total kilowatt-hours saved per year from VFD speed reduction.
Estimated VFD cost ($)
Approximate VFD purchase and installation cost (~$225/HP).
VFD payback (years)
Simple payback period for VFD installation.
Flow reduction (%)
Percentage reduction in flow (linear with speed per affinity law).
SAT reset savings ($/yr)
Annual cooling cost savings from raising the supply air temperature setpoint.
Total combined savings ($)
VFD savings + SAT reset savings combined.
Pressure reduction (%)
Percentage reduction in developed pressure at the reduced speed (pressure scales with the square of speed per the affinity law).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Fan/pump motor (HP) = 25, Current speed (%) = 100, Reduced speed (%) = 80, Annual run hours = 6000 = 8 input(s) provided
  2. Calculate VFD power savings
    VFD power savings = currentPowerKW - reducedPowerKW
    9.78 = 9.78
  3. Calculate Power reduction
    48.8 = 48.8
  4. Calculate Annual VFD savings
    Annual VFD savings = annualKWhSaved * electricityRate
    7043 = $7,043
  5. Calculate Current power
    Current power = fullLoadKW * pow(currentSpeedRatio, 3)
    20.05 = 20.05
  6. Calculate Reduced power
    Reduced power = fullLoadKW * pow(reducedSpeedRatio, 3)
    10.26 = 10.26

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does a small speed reduction save so much power?

Because fan and pump power follows the affinity law Power is proportional to Speed cubed, not speed directly. Reducing speed from 100% to 80% (a 20% cut) drops power to 0.8 cubed, or about 51.2% of the original -- roughly 49% savings -- which is why the calculator's own guidance notes that even modest speed reductions yield disproportionately large power savings.

Does raising the reduced-speed target increase or decrease my savings?

Decrease. VFD Power Savings is the current speed's cubed power minus the reduced speed's cubed power, so pushing Reduced Speed closer to Current Speed shrinks that gap and lowers the savings. The largest savings come from running the fan or pump as slow as the application allows, not from a modest trim.

Do the SAT reset inputs affect the VFD savings, or vice versa?

No -- the two calculations are entirely independent and are only added together at the end, in Total Combined Savings. Current SAT, Reset SAT, and Annual Cooling Cost never appear in the VFD Power Savings or Annual VFD Savings formulas, and Motor HP, Current Speed, and Reduced Speed never appear in the SAT Reset Savings formula.

How is the SAT reset savings estimated?

The calculator applies a flat 1.8% cooling-energy savings for each degree Fahrenheit the Reset SAT setpoint sits above the Current SAT setpoint, then multiplies that percentage by Annual Cooling Cost. It is a fixed rate assumption (a conservative mid-range figure per the calculator's own documentation), not a value derived from the building's actual load profile.

What does the VFD payback period tell me that annual savings doesn't?

VFD Payback divides Estimated VFD Cost (about $225 per motor horsepower, covering the drive and installation) by Annual VFD Savings, showing how many years it takes the VFD installation to pay for itself from the speed-reduction savings alone -- it does not include the SAT reset savings, since SAT reset requires no VFD hardware investment.

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