Field calculator
Mixed air temperature calculator (MAT)
When an air handler blends outdoor air with return air, the result is the mixed air temperature, the air that hits the heating or cooling coil. It is a weighted average: MAT equals the outdoor-air percentage times the outdoor temperature plus the return-air percentage times the return temperature, where the return fraction is 100 minus the outdoor fraction. Enter the outdoor temperature, the return temperature, and the outdoor-air percentage. This is a core check for testing and balancing and for economizer commissioning, because the measured mixed air should match what the damper position implies. Two cautions matter in the field. In cold weather a high outdoor-air fraction can pull the mixed air below freezing, tripping the freeze-stat or bursting a coil, so the low-limit control and the minimum outdoor-air setting matter. And real mixing boxes stratify, with cold and warm layers that do not fully blend, so a single sensor can read wrong; traverse across the duct. Confirm the design outdoor-air fraction and the readings against the balancing report and the engineer.
Result
Mixed air temperature: MAT = (percent outdoor air × outdoor temp) + (percent return air × return temp), where the return fraction is 100 minus the outdoor fraction. Enter the outdoor air temperature, the return air temperature, and the outdoor-air percentage, and the tool blends them. This is the temperature of the air leaving the mixing box ahead of the heating or cooling coil, and it is a core check for testing, balancing, and economizer commissioning. Two cautions: in cold weather a high outdoor-air fraction can pull the mixed air below freezing and trip the freeze-stat or burst a coil, so watch the low limit; and real mixing boxes stratify, so measure across the duct rather than at one point. Confirm the design outdoor-air fraction and the readings against the balancing report and the engineer.
anvilfield.com/calculators/mixed-air-temperature-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Mixed air temp 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.