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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 please see this page.

- 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.

 


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