Why dry bulb alone undercounts a cooling coil
A thermometer measures sensible heat: the heat that changes temperature. A cooling coil below the dew point of the air also condenses water, and taking a pound of water out of the air releases somewhere near 1,054 BTU that the thermometer never sees. In humid weather that latent work can be a third of everything the coil is doing. Measure a coil with dry bulb only and you report two thirds of its output and call it the whole.
Enthalpy is the property that includes both. It is read off a psychrometric chart from dry bulb and wet bulb together, and the drop in enthalpy across a coil multiplied by the mass flow of air is the total capacity, sensible and latent in one number.
The three constants are one constant
Sixty minutes in an hour times 0.075 pounds per cubic foot is 4.5 pounds of dry air an hour for every CFM. That is the mass flow, and it is the whole of the total-heat constant: total BTU/h is 4.5 times CFM times the enthalpy drop in BTU per pound.
Multiply that same 4.5 by the specific heat of air, 0.24 BTU per pound per degree, and you get 1.08 — the sensible constant, used with a temperature difference. Multiply it instead by 1,054 BTU per pound of water and divide by the 7,000 grains in a pound, and you get 0.6776, rounded to 0.68 — the latent constant, used with a difference in grains. Three constants, one mass flow, three different properties.
All three move with density, which is why this page asks for density rather than hard-coding it. At 5,000 feet the mass flow is nearer 3.7 pounds an hour per CFM and every one of the three constants falls by about seventeen percent together.
What the defaults produce
1,200 CFM, entering at 30.2 BTU per pound and leaving at 22.9, gives an enthalpy drop of 7.3 and a total of 4.5 times 1,200 times 7.3, or 39,420 BTU/h. The dry bulb falls from 78 to 57, so the sensible half is 1.08 times 1,200 times 21, or 27,216. Latent is the remainder, 12,204 BTU/h, and the sensible heat ratio is 0.69.
That latent figure is 11.6 pounds of water an hour, or about 11.1 pints — which is why a coil in humid weather produces a steady trickle at the drain and a coil in dry weather produces nothing at all. Expressed as moisture taken out of the air it is about 15 grains per pound of dry air, on air that arrived carrying rather more than that.
| Enthalpy drop | Dry bulb drop | Total BTU/h | Sensible | SHR |
|---|---|---|---|---|
| 7.3 | 21°F | 39,420 | 27,216 | 0.69 |
| 7.3 | 25°F | 39,420 | 32,400 | 0.82 |
| 5.5 | 21°F | 29,700 | 27,216 | 0.92 |
All three rows are at 1,200 CFM. Notice that the sensible column tracks dry bulb and the total tracks enthalpy, and that they move independently — which is the reason both measurements are needed.
Wet bulb is where the error lives
Enthalpy is far more sensitive to wet bulb than to dry bulb, and wet bulb is the harder of the two to measure. A sling psychrometer with a dry wick, a digital probe that has not stabilised, or a reading taken in a spot where the air is not mixed will all shift the enthalpy by more than the dry bulb error would suggest. Two tenths of a degree of wet bulb error at typical conditions is worth roughly a tenth of a BTU per pound of enthalpy, which on 1,200 CFM is about 540 BTU/h.
The practical rule is the same as everywhere else in this trade: take the measurement twice, in two places, and be suspicious when they agree too neatly.
Related pages
For the sensible-only version of this arithmetic on a furnace, where fuel input replaces enthalpy, the temperature rise airflow calculator. For airflow read off the blower table at a measured static instead, the external static pressure calculator. The same grains arithmetic applied to ventilation air rather than a coil is the crawl space vent air calculator, and at whole-house scale the humidifier sizing calculator. Coil condensate from a given capacity and sensible heat ratio, rather than derived from measurements, is the wine cellar humidity calculator. A single pair of conditions converts in the dew point calculator. If the question is how much capacity the space needs in the first place, the heat loss calculator and the mini split sizing calculator.
Work on a sealed refrigerant system is federally regulated and needs certification. Nothing on this page is a procedure for connecting gauges, adding refrigerant, recovering it or opening a system. Refrigerant is stored under pressure and freezes skin on contact, venting it is illegal, and a sealed system is not a homeowner repair. This page does arithmetic on numbers you already have.
Questions people ask
What sensible heat ratio should a coil have?
This page will not tell you, on purpose. The ratio a coil produces depends on the entering wet bulb, the airflow, the coil design and the operating point, and equipment manufacturers publish expanded performance tables giving the split at each combination. What the page does is derive the ratio you actually have from what you measured, so you can compare it against the table for that equipment. A number derived from your own readings and compared against the maker data for your unit is worth far more than any general figure.
Why is my latent capacity coming out at or below zero?
Most often because the coil is running dry — the surface is not below the dew point of the entering air, so nothing condenses and all the work is sensible. That is entirely normal in dry weather. If the number is clearly negative rather than near zero, the usual cause is that the dry bulb drop and the enthalpy drop were not captured at the same moment or the same place, so the sensible figure belongs to one operating point and the total to another. Take both pairs of readings together and repeat.
Do I need wet bulb, or can I use relative humidity?
Either will get you to enthalpy, since a chart or app will take dry bulb plus relative humidity just as happily as dry bulb plus wet bulb. In the field wet bulb is usually the more reliable of the two to measure, because relative humidity sensors drift and because the reading changes quickly with temperature. Whichever you use, take it at the same place and the same moment as the dry bulb, and enter the barometric pressure for your altitude into the chart — enthalpy at 5,000 feet is not the same number as enthalpy at sea level for the same conditions.
Can I use this to find airflow instead of capacity?
Yes, with the mode selector at the top, and it is a genuinely useful cross-check. Enter the total capacity from the expanded performance data at your operating conditions, together with the measured enthalpies, and the page divides to get the airflow. The caveat is that the capacity has to be the figure at your conditions rather than the nominal tonnage on the box, because a nominal three-ton unit is rarely delivering exactly 36,000 BTU/h at the moment you happen to be standing in front of it.
How much does altitude change these numbers?
More than most people expect, and it changes all three constants together because they all share the same mass flow. At sea level a cubic foot of air weighs about 0.075 pounds; at 5,000 feet it is nearer 0.062, so the same volume of air carries about seventeen percent less of everything. The density field on this page exists so that the constants shown in the results are the ones for where the job actually is. If you leave it at 0.075 in Denver, every capacity figure on the page is seventeen percent high.