BTU per pound-Fahrenheit

This unit is the hidden constant inside the two most-used equations in North American HVAC. Water's specific heat is 1 BTU/(lb·°F) and its density is about 8.33 lb/gal, so heat carried by a water loop is $Q = 8.33 \times 60 \times \text{gpm} \times \Delta T = 500 \times \text{gpm} \times \Delta T$ BTU/h. That familiar 500 is nothing but pounds per hour per gpm times a specific heat of one. On the air side, standard air weighs about 0.075 lb/ft³ and has a specific heat near 0.24 BTU/(lb·°F), giving $0.075 \times 60 \times 0.24 = 1.08$, the constant in $Q = 1.08 \times \text{cfm} \times \Delta T$.

Both constants quietly assume standard conditions. The 500 drifts if the loop carries glycol, which has a lower specific heat and a higher density: a 30 percent propylene glycol mix runs closer to 475, and a 50 percent mix closer to 450, so a system designed on 500 and filled with glycol will move less heat than the drawings promise. The 1.08 drifts with altitude, because air density falls: in Denver it is closer to 0.89. Neither correction is difficult, and both are skipped often enough to be worth naming.