Attic Heat Harvester
attic heat harvester system showing heat in hot air


Heat in hot air

using heat in hot air - buy now The heat, and indeed cooling energy transferred by forced air heating and cooling systems is given by using an equation from Thermodynamics that gives the thermal energy transferred by fluid flow:

P = q * ρ * c * Δ T / 3600

Where in S.I. units:

P - power [kW]
q - flow rate [m³/h]
ρ - density of fluid [kg/m³]
c - specific heat of fluid[kJ/kgK]
Δ T - temperature difference[K]

For dry air

ρ, the density of dry air at STP (1 atm, 273 K) is 1.293 kg/m³

c, the specific heat of air at constant pressure is 1.010kJ/kg-K

∴ P [kW] = K * q * Temp diff /3600

Where K = 1.293 * 1.01 /3600 = 0.000362

∴ P [kW] = K (0.000362 * volume of air moved in m³/h * Temp difference in °C

For air volume in ft³/min (cfm) and temperatures in degrees Fahrenheit

K = 0.000362 * 1.698 * 5/9

∴ P [kW] = 0.000341 * volume of air in ft³/min * Temp difference in °F


The formula shows that the energy transferred by hot air is directly proportional to:

a) the 'heat source - heat destination' temperature difference, (in our case by attic temperature - room temperature) and

b) the air flow rate.