The HVAC heat transfer constants

500 rule1.08 rule0.68 rule4.5 rulesensible and latent heatBTU formulas

The 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.

Hydronic Heat Transfer (Water)

\dot{Q} = \rho_w c_w \dot{V} \, \Delta T

Heat carried by a water loop from flow rate and supply-to-return ΔT — the SI form of the trade rule BTU/hr = 500 × GPM × ΔT.

Glycol Loop Heat Transfer (Capacity Derate)

\dot{Q} = \rho c \dot{V} \, \Delta T

Heat carried by a glycol loop using the actual mix density and specific heat, which is how the 500 constant derates for antifreeze.

Air Sensible Heat (1.08 Rule)

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

Sensible heat carried by an airstream from CFM and dry-bulb ΔT, assuming standard air — the trade rule BTU/hr = 1.08 × CFM × ΔT.

Air Latent Heat (0.68 Rule)

\dot{Q}_l = \rho_a \dot{V} h_{fg} \, \Delta W

Latent heat from dehumidifying an airstream, set by airflow and the change in humidity ratio — the trade rule BTU/hr = 0.68 × CFM × Δgrains.

Air Total Heat (4.5 Rule)

\dot{Q}_t = \rho_a \dot{V} \, \Delta h

Total (sensible plus latent) heat carried by an airstream from airflow and enthalpy change — the trade rule BTU/hr = 4.5 × CFM × Δh.

Sensible Heat Ratio (SHR)

\mathrm{SHR} = \frac{\dot{Q}_s}{\dot{Q}_s + \dot{Q}_l}

The fraction of a cooling coil's total load that is sensible, the number that decides whether a room ends up cool or merely cold and clammy.

How they fit together

All four constants are the same equation — ṁcΔT — with the unit conversions pre-multiplied and hidden. For water, 60 min/hr × 8.33 lb/gal × 1.00 BTU/(lb·°F) = 499.8, which the trade rounds to 500. For air, 60 min/hr × 0.075 lb/ft³ × 0.240 BTU/(lb·°F) = 1.08. The latent constant 0.68 is the same 60 × 0.075 carrying 1060 BTU/lb of vaporisation with humidity ratio in grains, and 4.5 is just 60 × 0.075 applied to enthalpy in BTU/lb, which is why the 4.5 rule needs a psych chart and the other two do not.

Pick by what changed. Temperature only, on either fluid: 500 for water, 1.08 for air. Moisture removed at a cooling coil: 0.68, using grains of moisture, not relative humidity. Both at once: 4.5 with entering and leaving enthalpy, and the sensible heat ratio to see how the load splits. The mistake that costs money is treating the constants as physics rather than as bookkeeping at one condition: 0.075 lb/ft³ is sea-level air at about 70 °F, so in Denver 1.08 is really closer to 0.89 and every airflow you back-calculate is roughly 20% optimistic. Glycol is the same trap on the water side — a 40% propylene mix drops the 500 to roughly 450, so use the glycol page rather than derating in your head.