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BTU Logic

Tools / Economizer Hours

Economizer Hours

How many hours a year is the outdoor air at your site good enough for free cooling? Load the weather file for your location (EPW, TMY2, TMY3, or NSRDB-style CSV), pick a high-limit strategy, and set the supply-air setpoint. You get full and integrated free-cooling hours, split occupied and unoccupied, with every changeover strategy scored on the same hours so you can see what enthalpy changeover actually buys in your climate.

  • Five high-limit strategies: fixed or differential dry-bulb, fixed or differential enthalpy, and fixed enthalpy plus fixed dry-bulb
  • Full free cooling (outdoor air alone holds the supply setpoint) vs integrated (outdoor air helps, mechanical cooling makes up the rest)
  • Site elevation for the enthalpy math, filled in from the weather file when it carries one
  • IP or SI units
  • Strategy comparison and month-by-month tables, plus a CSV download
Free-cooling hour counts come from the weather file you provide and the changeover setpoints, supply-air setpoint, and occupied / unoccupied schedule you set. They are hours the outdoor air meets your high limit — not hours the building called for cooling, and not an energy saving. There is no load, damper, or control-sequence model here. Not a substitute for an energy model, commissioning data, or licensed HVAC design. Informational / preliminary use only; verify before design or utility-incentive decisions.
Input format

Changing format re-parses the loaded file immediately. Accepts EPW, TMY2, TMY3, and NSRDB-style CSV.

Get NSRDB / TMY weather data (NREL) · NREL SAM weather data

Lock out above an outdoor dry-bulb setpoint.

Occupied schedule

Default Mon–Fri 08:00–18:00 interval-beginning LST (8am through 6pm). EPW / TMY hour-ending 1–24 is converted to 0–23 on parse; NSRDB is already 0–23. Unchecked days are unoccupied. Start is inclusive, end is exclusive. If start is later than end, the window wraps overnight.

Typical-year files stitch months from different source years. Weekdays follow January 1 (from EPW DATA PERIODS when present, else Sunday) so month boundaries stay continuous. Use file Y/M/D only for a real calendar year.

These are outdoor-air availability hours, not savings.

An hour counts when the outdoor air passes the high limit — not when the building was actually calling for cooling. There is no load model here, so a January hour with no cooling load still counts as available. Use the numbers to compare changeover strategies and climates, not to predict kWh.

Load a weather file, set the changeover strategy, and press Count free-cooling hours.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms