Field calculator
Sensible heat calculator (1.08 x CFM x delta-T)
The sensible heat equation is one of the most-used relationships in HVAC, tying airflow and temperature to heat. The sensible heat in BTU per hour equals 1.08 times the airflow in CFM times the temperature difference in degrees Fahrenheit. Enter the CFM and the delta-T (supply to return, or across a coil) to get the BTU/hr and the equivalent tons at 12,000 BTU/hr per ton. The 1.08 constant is for standard air at sea level and combines air density with its specific heat, so it shifts with altitude, temperature, and humidity. This is the sensible (dry, temperature-change) heat only; the latent heat that removes moisture from the air is a separate calculation, which is why a coil's total capacity is more than this number in a humid space. Use it to check whether the airflow matches the load, to estimate the heat a duct delivers, or to run a quick delta-T diagnostic on a system, and confirm equipment selection against a full Manual J load calculation.
Result
Sensible heat from airflow: BTU/hr = 1.08 × CFM × the temperature difference (delta-T in degrees F). Enter the airflow and the supply-to-return (or across-coil) temperature difference. The 1.08 constant is for standard air at sea level (it combines air density and specific heat), so it shifts with altitude, temperature, and humidity. This is the sensible (dry, temperature) heat only; the latent heat that removes moisture is a separate calculation. Use this to check airflow against a load, to estimate the heat a duct carries, or to size a quick delta-T diagnostic, and confirm equipment sizing against a full Manual J load calculation.
anvilfield.com/calculators/sensible-heat-airflow-btu-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Sensible heat FAQ
What is a Manual J load calculation?
It is ACCA's room-by-room accounting of a home's heat gain and loss, used to size heating and cooling equipment from the building itself. It tallies the envelope, the windows by orientation, infiltration, ventilation, and the people and appliances inside, then splits the result into sensible and latent load for equipment selection.
Why is oversizing an AC bad?
An oversized AC cools the air fast and shuts off before it runs long enough to remove moisture, leaving the house cold and clammy above comfortable humidity. The short cycling also wastes energy and wears the compressor early, because the start is the least efficient part of every cycle. Right-sizing fixes all of it.
What design temperature do you use for a load calc?
Use the 1 percent cooling and 99 percent heating design conditions from ASHRAE, tabulated in Manual J, not the record high or low. The 1 percent dry-bulb is exceeded only about 88 hours a year. Sizing to the once-a-decade extreme oversizes the equipment for the thousands of normal hours.
What is the difference between sensible and latent load?
Sensible load is heat that changes air temperature, what a thermometer reads. Latent load is the heat in moisture, the energy to condense water vapor out of the air. Manual J figures them separately because equipment removes them separately. A high-sensible unit on a humid house cools but never dries it, which is the usual humidity complaint.
How many square feet per ton should I use to size an AC?
None as a sizing method. The square-feet-per-ton habit, often quoted around 400 to 600 square feet per ton, ignores the windows, insulation, orientation, and air leakage that actually set the load, and it almost always oversizes. Run a Manual J. Use a per-ton figure only as a rough sanity check against the calculated answer.