HVAC load calculators

Every tool in this section

• Load calculation

  Manual J Load Calculator

  Residential heating and cooling design loads, sensible and latent. Whole-house on the free page; room by room in the Pro plan.

  The free form takes a ZIP code and a short description of the house and returns the whole-house load with its breakdown. The room-by-room calculation, with window orientation and measured R-values, returns BTU/h per room and is part of the Pro plan.

  Based on ACCA Manual J 8th Edition

• Design conditions

  HVAC Design Temperatures by ZIP

  Cooling and heating design dry-bulb temperatures and the IECC climate zone for a US ZIP code.

  Maps ZIP → county → climate zone and returns approximate 1% / 0.4% cooling and 99% / 99.6% heating values for that zone - the same lookup the load calculators use.

  Based on IECC 2021 climate zones

• Load density

  BTU per Square Foot Calculator

  Cooling and heating BTU/h per square foot by climate zone, from the whole-house load engine.

  Puts Manual J engine results next to the 20 to 30 BTU per sq ft trade rule of thumb for each climate zone and marks the zones where the two land furthest apart.

  Based on ACCA Manual J 8th Edition

• Heating load

  Garage Heater Size Calculator

  Heat loss of an attached or detached garage held at a shop temperature, in BTU/h and kW.

  Counts walls, ceiling, the overhead door, windows, the slab edge and air leakage. Returns the electric heater kW and the gas heater input for the efficiency you enter.

  Based on U x A x dT heat loss, BuildSolver engine defaults

• Space load

  Sunroom BTU Calculator

  Heating and cooling BTU/h for a sunroom, with the solar gain through the wall glass and a glass roof.

  Takes glass area by side, glass type and roof type, and shows how much of the cooling load comes through the glass. The glazed side with the highest direction factor sets that factor for all of the wall glass. A simplified estimate for one space. Room-by-room Manual J for the whole house is part of the Pro and Team plans.

  Based on U x A x dT and solar gain, BuildSolver engine defaults

• Space load

  Basement BTU Calculator

  Heating and cooling BTU/h for a finished basement, with the below-grade walls and floor split out.

  Applies below-grade wall U-factors and floor F-factors by depth and insulation as a stated assumption, plus the above-grade wall, windows and air leakage.

  Based on Seattle Energy Code Table A104.1 below-grade defaults

• Space load

  Room Addition BTU Calculator

  Heating and cooling BTU/h for a new bedroom, family room or bonus room, separate from the house load.

  Compares the addition's load to the spare capacity you enter for the existing system, as a rough check. Room-by-room Manual J for the whole house with the addition is part of the Pro and Team plans.

  Based on U x A x dT and solar gain, BuildSolver engine defaults

• Space load

  Mini Split Sizing Calculator

  Heating and cooling BTU/h for one zone under a single ductless head.

  Stops at the load in BTU/h and leaves the head to you. Matching a unit to that load is Manual S, part of the Pro and Team plans, and a multi-zone split needs room-by-room Manual J.

  Based on U x A x dT and solar gain, BuildSolver engine defaults

• Replacement

  Replacement AC Sizing Calculator

  Whole-house load for a replacement from a description of the house, with no brand lock-in. Matching a condenser and coil to that load under Manual S is part of the Pro plan.

  For the changeout where the old unit's nameplate is the only documentation on site. Brand-agnostic: the load is the load, whichever manufacturer's line you quote from.

  Based on ACCA Manual J 8th Edition

• Retrofit

  Retrofit Sizing Without a Blueprint

  For houses that never had drawings, or whose drawings are three renovations out of date.

  Describe what you can see and measure on a walkthrough - floor area, window count and what the attic looks like. Type it in from your phone in the driveway.

  Based on ACCA Manual J 8th Edition

• New construction

  Manual J for New Construction

  Preliminary load numbers during design development, before drawings are stamped.

  Useful when the framing plan is still moving: re-run the load as glazing, orientation and insulation spec firm up, so the mechanical budget stops being a guess.

  Based on ACCA Manual J 8th Edition

• Permit

  When a Permit Needs a Stamped Manual J

  Where the line falls between sales-phase sizing and permit-grade submittal.

  What AHJs typically ask for, which jurisdictions require ACCA-approved software for the permit load (BuildSolver's reports are built for the sales phase), and when to hand the job to a PE.

  Based on IECC R403 / local AHJ practice