Coil Capacity Verification From Measured Air and Water

A coil's delivered capacity computed twice, from the air and from the water, and what the disagreement between them means. Two independent measurements are the whole value of the exercise: each uses different instruments, different quantities and different assumptions, so agreement is strong evidence that both are right, and disagreement localises the problem rather than merely flagging it. Air side high against water side points at an overstated airflow measurement, or at air bypassing the coil so the leaving temperature is not representative. Water side high usually means the flow measurement is wrong, or the temperature sensors are too close together for the delta being measured. The sensible-versus-total distinction is the trap on a cooling coil, and it is the one that produces false alarms. A wet coil is removing latent heat, and a sensible-only air-side calculation will fall well short of the water-side total for no reason except the method -- so on a wet coil the air side must be computed from ENTHALPY, which needs wet-bulb measurements on both sides. A report comparing sensible air against total water is comparing two different quantities and will condemn a coil that is working. What it does NOT license is calling any disagreement acceptable once the method is corrected: the tolerance is entered here and the verdict is computed against it, because a balance that is still outside tolerance after switching to enthalpy is a real finding, not a rounding difference. The instrument that most often fails is the water temperature difference. A modest delta measured with sensors accurate to a degree each carries a large percentage uncertainty, and on a low-delta system that uncertainty can be most of the disagreement -- which means the water-side measurement is weakest exactly where hydronic systems tend to run. This computes capacities from ENTERED measurements at a sea-level standard-air basis; it does not correct for altitude or non-standard density, derive enthalpy from dry-bulb and wet-bulb readings, assess instrument accuracy, or evaluate the coil's cleanliness, circuiting or approach. The coil manufacturer's rated capacity at the design condition and the balancing agency's own procedure govern.

air sensible Q = 1.08 x CFM x dT; air total Q = 4.5 x CFM x dh on a wet coil; water Q = fluid factor x GPM x dT with 500 for water; the heat balance difference is the gap between the air side used and the water side, as a percentage of the water side, judged against an entered tolerance.

The coil heat balance as ASHRAE and balancing practice writes it, where 1.08 is 0.075 lb/cu ft x 0.24 BTU/lb-degF x 60 min/h, 4.5 is 0.075 x 60, and 500 is 8.33 lb/gal x 60 x 1.0. The tolerance and the fluid factor are ENTERED. Sea-level standard air: it does not correct for altitude or non-standard density, derive enthalpy from dry-bulb and wet-bulb readings, assess instrument accuracy, or evaluate coil cleanliness, circuiting or approach.

Three constants and two temperature differences.

Estimate. AHJ and licensed professional govern.

Field names used by the API: airflow_cfm, entering_air_db_f, leaving_air_db_f, enthalpy_drop_btu_lb, water_gpm, entering_water_f, leaving_water_f, fluid_factor, design_capacity_btuh, tolerance_pct, air_sensible_btuh, air_total_btuh, water_btuh, balance_difference_pct, sensible_only_difference_pct, latent_btuh

• Sea-level standard air 1.08 and 4.5 carry a 0.075 lb/cu ft densityASHRAE Fundamentals
• Enthalpy is entered, not derived it needs wet-bulb readings on both sidesthe balancer's psychrometric readings
• The water delta is the weak measurement a small delta with degree-accurate sensors carries large uncertaintythe instrument calibration