Plenum
HVAC sizing calculators
A room estimate, a block-load tonnage, a heating and cooling load that is an estimate and not Manual J, a duct diameter, a mini-split layout, a moist-air state point, a superheat or subcooling reading, and a CFM estimate from air changes or a duct velocity. Each one says what it is not.
BTU Calculator
BTU calculator for room heating and cooling. ENERGY STAR chart, climate zone, sun, people, and kitchen. An estimate, not a Manual J.
AC Tonnage Calculator
AC tonnage calculator from floor area and climate zone. A block-load estimate. Get a Manual J before you buy equipment.
Duct Size Calculator
Duct calculator using equal friction. Round size, rectangular equivalent, and velocity from the ASHRAE friction equation.
Mini Split Calculator
Mini split calculator for head sizes and the outdoor unit. Room estimates rolled up to catalog sizes. Not a Manual J.
Psychrometric Calculator
Psychrometric calculator for dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio, enthalpy, and specific volume. An ASHRAE-style estimate, not a Manual J and not a certified chart.
Superheat & Subcooling Calculator
Superheat and subcooling from line pressure and temperature for R-410A, R-32, R-454B, R-22, R-134a, and R-407C. Diagnostic PT arithmetic, not a charging amount.
CFM Calculator
CFM calculator for room air changes, a fan’s ACH, duct velocity, and BTU or tons to airflow. Estimates, not Manual J or Manual D.
HVAC Load Calculator
Heating and cooling load from U·A·ΔT, infiltration, and the gains you type. An estimate, not Manual J. No equipment model.
Furnace Size Calculator
Furnace input versus output from a heating load and AFUE, plus electric kilowatts and airflow. An estimate, not Manual J. Not gas piping or venting.
How these calculators work
The room BTU tool starts from the ENERGY STAR room-air-conditioner chart, which assumes an 8 ft ceiling. Shaded rooms drop 10%, sunny rooms add 10%, each person past two adds 600 BTU/h, and a kitchen adds 4,000. ENERGY STAR then rounds a window unit down to the next 1,000. A ceiling other than 8 ft, and the poor/average/good insulation multiplier, are this site’s adjustments and are labeled that way. Heating uses a planning BTU per square foot by DOE climate zone. DOE defines the zones. It does not publish those rates.
Whole-house tonnage is area times a planning cooling rate for the zone, divided by 12,000. Mini-split heads use the same room estimate and round up to a catalog size. The outdoor unit is sized from the sum of the unrounded room loads, not from the sum of the oversized heads. Duct size is the equal-friction method: Altshul–Tsal and Darcy–Weisbach at standard air, with Huebscher’s round-to-rectangular equivalent and the velocity written out. Buy equipment from a Manual J, not from these pages.
The psychrometric calculator is a moist-air state point: dry-bulb with relative humidity, wet-bulb, dew point, or humidity ratio. Saturation pressure is the Hyland–Wexler liquid-water equation ASHRAE prints, with no enhancement factor. It reports humidity ratio, enthalpy, specific volume, vapor pressure, and degree of saturation. It is an engineering estimate, not a certified chart and not equipment selection.
The superheat and subcooling calculator looks up a dew point or a bubble point from an embedded PT table and subtracts. R-410A, R-32, R-22, R-134a, and R-407C come from CoolProp. R-454B comes from the Chemours Opteon XL41 chart. It describes the reading. It does not say how much refrigerant to add. Charge to the equipment manufacturer’s chart. Handling refrigerant requires EPA Section 608 certification.
The CFM calculator turns air changes and room volume into airflow, a fan’s CFM back into air changes, duct velocity times area, and a sensible-heat CFM from BTU and temperature difference. The 1.08 factor is standard air at sea level. Results are estimates, not Manual J or Manual D. The air-change presets are guidelines, not code. The load page is a steady-state U·A·ΔT estimate. It shows the assumptions and does not name an equipment model. The furnace page turns that heating load into input versus output by AFUE. An estimate, not Manual J.