Air Sensible Heat (1.08 Rule)

Also known as 1.08 formula · CFM delta T · sensible cooling load

\dot{Q}_s = \rho_a c_a \dot{V} \, \Delta T

Worked example: 1.08 rule: 1000 cfm at 20 dF → 21,600 BTU/hr — press Try an example to run it live, then adjust anything.

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Air Sensible Heat (1.08 Rule) explained

This is the water formula wearing a different hat: mass flow times specific heat times temperature change, with air's properties instead of water's. Standard air weighs 0.075 lb/ft³ (1.2014 kg/m³) and holds 0.240 BTU/(lb·°F), so 1.08 = 60 min/hr × 0.075 lb/ft³ × 0.240 BTU/(lb·°F). A furnace moving 1,000 cfm across a 20 °F rise delivers 1.08 × 1,000 × 20 = 21,600 BTU/hr, which this page returns as 6.33 kW.

Two traps. First, "standard air" means roughly 70 °F at sea level — at Denver's 5,300 ft the density drops about 18 %, so the constant falls to about 0.89 and a rooftop unit sized on 1.08 quietly under-delivers. Second, this is sensible heat only: it says nothing about the moisture the coil pulls out, which on a humid August afternoon can be a third of the total load. Balancing technicians run the formula backwards constantly — measure supply and return temperature at a known airflow, and you have the delivered capacity without opening a single panel.

Air Sensible Heat (1.08 Rule) formula

\dot{Q}_s = \rho_a c_a \dot{V} \, \Delta T

Where

$\dot{Q}_s$ = Sensible heat rate (W)
$\dot{V}$ = Airflow (L/min)
$\Delta T$ = Dry-bulb ΔT (C°)

Missing one of these? Work it out first, then come back

Sensible heat rate — Air Total Heat (4.5 Rule), Hydronic Heat Transfer (Water)
Airflow — Air Latent Heat (0.68 Rule), Air Total Heat (4.5 Rule)
Dry-bulb ΔT — Hydronic Heat Transfer (Water), Glycol Loop Heat Transfer (Capacity Derate)

Part of this formula set

The HVAC heat transfer constants

6 formulasThe 500, 1.08, 0.68 and 4.5 rules for water and air heat transfer — where each constant comes from and when it stops being true.