When to use: Use to estimate annual energy and cost savings from implementing supply-air-temperature (SAT) and chilled-water-temperature (CHWT) reset sequences (ASHRAE Guideline 36). Raising CHWT improves chiller efficiency (~1.5% per °F), while raising SAT reduces reheat and chiller load (~1% per °F). This is a degree-based rule-of-thumb screening estimate — note that aggressive SAT reset raises airflow and can increase fan energy, partially offsetting cooling savings, so validate large resets with hourly simulation.
This tool estimates the annual energy savings and cost reduction from implementing supply air temperature (SAT) reset and chilled water temperature (CHWT) reset sequences per ASHRAE Guideline 36 and ASHRAE 90.1. Engineers use it to quantify the energy performance gap between fixed-setpoint and reset-control strategies during design and retro-commissioning.
SAT reset raises the supply air temperature setpoint from a fixed low value (e.g., 55°F) toward a higher value (e.g., 60–65°F) when zone cooling demands are low, reducing both chiller load and reheat energy. The savings factor used is approximately 1% of baseline chiller energy per °F of SAT raise, per ASHRAE 90.1 Appendix G guidance.
CHWT reset raises the chilled water supply temperature from a fixed low value (e.g., 44°F) toward a higher value (e.g., 48–52°F) when chiller load is low, improving chiller COP by approximately 1.5–2% per °F of CHWT raise. Baseline annual chiller energy = tons × kW/ton × equivalent full-load hours (EFLH). Both savings factors are degree-based rule-of-thumb estimates appropriate for early design screening; detailed hourly simulation is recommended for large projects.
ASHRAE 90.1 Section 6.5.2.1 mandates chilled water temperature reset on new chilled water systems with a design capacity exceeding 300,000 BTU/hr. ASHRAE 90.1 Section 6.5.2.3 mandates supply air temperature reset on new single-duct VAV systems. ASHRAE Guideline 36 defines the specific reset sequences — for SAT reset, the setpoint typically resets up as the highest zone cooling demand decreases from 100% to 50%, and resets back down as demand increases. For CHWT reset, the setpoint resets up as chiller load drops below 50% of peak, limited by the highest zone cooling request.
SAT reset must be coordinated with VAV box minimum airflow setpoints: raising SAT at low load means zones need more airflow to meet their cooling setpoints, potentially increasing fan energy. The net savings depend on the balance between chiller savings and fan penalty, which varies with climate, building type, and VAV box configuration. For humid climates, excessive SAT reset can cause space humidity to rise above acceptable limits (ASHRAE 55 target 50–60% RH), requiring a cap on the maximum reset temperature. CHWT reset below 42°F risks coil icing on draw-through air handlers; above 56°F may fail to maintain SAT setpoints on hot days.
Enter the peak cooling load in tons, equivalent full-load hours (EFLH — typically 1200–2000 for commercial buildings in the US), the number of degrees of SAT and CHWT reset, the plant efficiency in kW/ton, and the electricity rate. The tool reports baseline chiller energy, CHWT reset savings (kWh), SAT reset savings (kWh), total savings (kWh), annual cost savings ($), and percentage of baseline saved. Use the results to justify capital investment in reset control programming or retro-commissioning.
Field studies from LBNL and PNNL consistently find 5–15% annual chiller energy savings from CHWT reset, with larger savings in mild climates where the chiller frequently runs at part load. The savings factor of 1.5–2% per °F is a rule-of-thumb; actual savings depend on chiller part-load performance curve, condenser water temperature, and the distribution of annual cooling hours by load fraction.
SAT reset generally increases fan energy slightly because higher supply air temperature means less temperature differential per unit of airflow, requiring more airflow to meet zone cooling loads. However, the reduction in chiller and reheat energy typically outweighs the fan penalty by a factor of 3–5x in typical commercial buildings, making SAT reset a net energy saver in most cases.
EFLH is the number of hours at full design cooling load that would produce the same annual energy consumption as the actual varying load profile. For commercial buildings in the US, EFLH typically ranges from 800 (mild climate, low occupancy) to 2500 (hot climate, 24/7 operation). ASHRAE 90.1 Appendix G provides climate-specific EFLH values for energy simulation reference buildings.
ASHRAE 90.1 Section 6.5.2.3 requires SAT reset on new single-duct VAV systems with design airflow exceeding 1000 CFM per unit. The reset must raise the SAT setpoint by at least 5°F from the design minimum as the system approaches minimum supply fan speed. Most jurisdictions adopting ASHRAE 90.1 2016 or later require this reset.
CHWT and SAT reset are complementary: CHWT reset improves chiller efficiency, while SAT reset reduces the cooling load on the chiller. When both are implemented together, the SAT reset reduces the cooling demand placed on the chiller, allowing the chiller to run at even lower load where CHWT reset provides greater efficiency improvement, compounding the total savings beyond the sum of individual savings.
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