Supply Air Temperature Calculator

The sensible heat equation solved for supply temperature, room temperature, airflow, or load.

Solve For
Supply Air Temp
—
°F
ΔT (Room − SAT)
—
°F
Sens. Factor (1.08 adj)
—
altitude-corrected
Formula
Q = 1.08 × CFM × ΔT  (sea level)

Where:
 · Q = sensible heat (BTU/hr)
 · 1.08 = 0.24 BTU/lb·°F × 0.075 lb/ft³ × 60 min/hr
 · CFM = supply airflow
 · ΔT = Room Temp − Supply Air Temp (°F); + = cooling, − = heating

At elevation, 1.08 is reduced proportionally to air density. At 5,000 ft ≈ 0.90.
Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This supply air temperature calculator works the sensible heat equation in any direction. Pick what you want to solve for — supply air temperature (SAT), room temperature, airflow, or sensible load — enter the other three quantities, and the tool returns the answer along with the room-to-supply temperature difference and the altitude-corrected sensible factor.

It is useful for checking a design supply air temperature, finding the airflow a coil must deliver to meet a sensible load, or back-calculating a room temperature from measured conditions. An elevation input corrects the sensible factor for air density so results stay valid above sea level.

Formula & Method
Sensible heatQ (BTU/hr) = 1.08 × CFM × ΔT
Supply air tempSAT = Room − Q ÷ (1.08 × CFM)
Required airflowCFM = Q ÷ (1.08 × ΔT)
Sensible factor1.08 = 0.24 · 0.075 · 60  (sea level)

The sensible heat equation is the standard airside relationship in ASHRAE Fundamentals. The constant 1.08 is the dry-air specific heat (0.24 BTU/lb·°F) × standard density (0.075 lb/ft³) × 60 min/hr, the long-standing design convention. ASHRAE Fundamentals Ch. 18 uses 1.10, which adds the water vapor in typical air (cp ≈ 0.244) — about 2% more heat per CFM. ΔT is room dry-bulb minus supply dry-bulb: positive for cooling, negative for heating. Airflow is actual CFM at the site. At elevation the factor is scaled by the barometric pressure ratio (1 − 6.8754×10⁻⁶·ft)⁵⋅²⁵⁵⁹, falling to about 0.90 near 5,000 ft.

Frequently Asked Questions
What is a typical supply air temperature for cooling?
Most commercial cooling systems supply air around 52 to 58 degrees F, with 55 degrees F a common design point. Supplying much colder than about 48 degrees F risks condensation on diffusers and drafts unless the system is designed for low-temperature air; supplying warmer than about 60 degrees F usually means more airflow is needed to meet the load. When solving for supply air temperature, this tool flags cooling supply temperatures below 48 or above 60 degrees F.
What does the constant 1.08 represent?
The 1.08 in Q = 1.08 times CFM times delta-T is the product of the specific heat of dry air (0.24 BTU per pound per degree F), standard air density (0.075 pounds per cubic foot), and 60 minutes per hour. ASHRAE Fundamentals uses 1.10, which includes the water vapor in typical air (specific heat about 0.244), so 1.08 runs about 2 percent conservative on capacity. It applies to sea-level standard air; at altitude the value drops in proportion to air density, which this calculator corrects for using site elevation.
How do I size CFM from a known load and delta-T?
Rearrange the sensible heat equation: CFM = Q divided by (1.08 times delta-T), where delta-T is room temperature minus supply air temperature. For example, a 43,200 BTU/hr sensible load with a 20 degree F delta-T needs about 2,000 CFM at sea level. Select the CFM solve mode and the tool does this for you, including altitude correction.
Why does altitude reduce cooling and heating capacity per CFM?
Air gets less dense as elevation rises, so each cubic foot per minute carries less mass and less heat. The sensible factor falls from 1.08 at sea level to roughly 0.90 at 5,000 feet. If you ignore this, a system sized at sea-level assumptions will move too little heat at altitude, so the elevation input scales the factor by the barometric pressure ratio.
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