Useful HVAC Formulas

– Total Heat (BTU/hr) = 4.5 x cfm x Δh (std. air)

– Sensible Heat (BTU/hr) = 1.1 x cfm x Δt (std. air)

– Latent Heat (BTU/hr) = 0.69 x cfm x Δgr. (std. air)

NOTE: For conditions other than standard air only

– Total Heat (BTU/hr) = 500 x gpm x Δt (water)

– TONS = 24 x gpm x Δt (water)

– GPM cooler = (24 x TONS) / Δt (water)

– Fluid Mixture Tm = (Xt1 + Yt2) / X + Y (this works for air or water)

– BTU/hr = 3.413 x watts = HP x 2546 = Kg Cal x 3.97

– Lb. = 453.6 grams = 7000 grains

– psi = ft. water/2.31 = in. hg/2.03 = in. water/27.7 = 0.145 x kPa

– Ton = 12,000 BTU/hr = 0.2843 x KW

– HP (air) = cfm x Δp (in.H2O)/6350 x Eff.

– HP (water) = gpm x Δp (ft.)/3960 x Eff.

– Gal. = FT³/7.48 = 3.785 Liters = 8.33 lb. (water) = 231 in. ³

– gpm = 15.85 x L/S

– cfm = 2.119 x L/S

– Liter = 3.785 x gal = 0.946 x quart = 28.32 x ft³

– Therm = 100,000 BTU = MJ/105.5

– Watt/sq. ft. = 0.0926 x W/M²

– yd. = 1.094 x M

– ft. = 3.281 x M

– ft² = 10.76 x M²

– ft³ = 35.31 x M³

– ft/min = 196.9 x M/S

– PPM (by mass) = mg/kg

NOTE: Liter/sec is the proper SI term for liquid flow. M3/sec is the proper SI term for airflow. Due to the awkward nature of using M3/S at low air flow rates (lots of decimal points), L/S is commonly used to express air flow for HVAC applications.