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Calculator · Cooling diagnostics
AC temperature split calculator
The split across a cooling coil is set by two things you can name, the sensible heat ratio and the airflow per ton. At sea level a coil with an SHR of 0.75 moving 400 CFM per ton should show about 20.8 °F. Work out the number for your system here, or enter return and supply readings to see the total, sensible and latent capacity the coil is delivering. To size airflow for a new design, use the HVAC CFM calculator instead.
- Choose the expected split to know what a coil should show, or delivered capacity to see what it is doing now.
- Expected mode takes SHR and CFM per ton. Delivered mode takes total CFM and dry-bulb and wet-bulb at the return and supply.
- Enter elevation above sea level, then read the split, or the total, sensible and latent capacity with the SHR.
Example result for the default inputs
SHR 0.75, 400 CFM per ton, sea level. Cooling mode.
Expected temperature split
20.8 °F
| Sensible capacity per ton (12,000 x SHR) | 9,000 BTU/h |
|---|---|
| Sensible heat constant at this elevation | 1.080 |
| Return minus supply dry-bulb at the coil | 20.8 °F |
For estimation purposes only. Consult a licensed engineer for final designs.
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How the split and the capacity are calculated
Expected mode works from the coil's sensible heat ratio and the airflow you plan to move. Delivered mode works the other way, from four readings and the airflow, and returns what the coil is putting out right now, sensible and latent.
- Expected split
dT = 12,000 x SHR / (1.08 x altitude factor x CFM per ton)Each ton is 12,000 BTU/h, and SHR is the share of it that lowers the dry-bulb. The sensible heat equation turns that share into a temperature drop for the airflow you set.- Altitude factor
p(z) / 14.696, p(z) = 14.696 x (1 - 6.8754e-6 x z)^5.25591.08 is a sea-level number. Above sea level the air is thinner, so this page scales it down with the ASHRAE standard atmosphere, the same correction BuildSolver's load engine uses.- Enthalpy of each reading
h = 0.240 x t + W x (1061 + 0.444 x t)BTU per pound of dry air, from the ASHRAE psychrometric equations. W, the humidity ratio, comes from the dry-bulb and wet-bulb pair.- Delivered total capacity
Q_total = 60 x CFM / v x (h_return - h_supply)v is the specific volume of the air you measured the CFM in, so 60 x CFM / v is pounds of dry air per hour. Tons are Q_total / 12,000.- Sensible part and SHR
Q_sens = 60 x CFM / v x (0.240 + 0.444 x W_supply) x (t_return - t_supply), SHR = Q_sens / Q_totalThe rest of the total is latent, the moisture the coil pulled out. If the readings put sensible above total, the calculator says so and does not print an SHR above 1.
Expected splits and measured coils
Expected split at sea level for three sensible heat ratios and three airflows, from the same function the form runs.
| SHR | 350 CFM per ton | 400 CFM per ton | 450 CFM per ton |
|---|---|---|---|
| 0.70 | 22.2 °F | 19.4 °F | 17.3 °F |
| 0.75 | 23.8 °F | 20.8 °F | 18.5 °F |
| 0.80 | 25.4 °F | 22.2 °F | 19.8 °F |
At an SHR of 0.75, slowing the airflow from 450 to 350 CFM per ton widens the split from 18.5 °F to 23.8 °F. At 5,280 ft the 0.75 and 400 case reads 25.3 °F instead of 20.8 °F, because thinner air carries less heat per cubic foot. The form adds a note to any split above 22 °F at sea level, as in the 350 CFM per ton column. A real coil at that airflow usually runs a lower SHR than it does at 400, which pulls the split back, so read the SHR for that airflow from the expanded data.
Delivered capacity for four sets of readings, airflow measured in the air entering the coil.
| Readings | Split | BTU/h | Tons | SHR |
|---|---|---|---|---|
| Worked example, 1,200 CFM, 80/67 to 58/56 | 22.0 °F | 39,724 | 3.31 | 0.70 |
| Humid return, 1,000 CFM, 76/66 to 56/55 | 20.0 °F | 32,713 | 2.73 | 0.65 |
| Dry return, 1,200 CFM, 80/61 to 60/52 | 20.0 °F | 29,292 | 2.44 | 0.87 |
| Same as the dry return, at 5,280 ft | 20.0 °F | 27,393 | 2.28 | 0.76 |
The humid and the dry return both show a 20.0 °F split, yet the SHR is 0.65 on the first and 0.87 on the second, and the humid coil moves 11,416 BTU/h of latent heat. A dry-bulb split alone would miss all of that. At 5,280 ft the same four readings mean different air. Each CFM carries about 18 percent fewer pounds, while a given wet-bulb holds more moisture in thinner air, so the enthalpy drop grows by about 14 percent. The net is about 6 percent less capacity and an SHR of 0.76, which is why the elevation field matters.
Standards and data sources
- ASHRAE Handbook, Fundamentals, Psychrometrics chapter - moist air enthalpy, humidity ratio and specific volume, and the cooling and dehumidifying coil balance. The test suite reruns the chapter's worked cooling coil example through this calculator.
- ASHRAE standard atmosphere (same chapter) - atmospheric pressure at elevation, which scales the sensible constant and the air density.
- NIST Special Publication 811, Appendix B.8 - 1 ton of refrigeration = 12,000 BTU/h.
- Manufacturer's expanded performance data - the sensible heat ratio for expected mode, at your airflow, entering wet-bulb and outdoor temperature. This page does not carry equipment data.
Limitations of this calculator
- Cooling only. Heat pump heating mode is not calculated, because the heating rise depends on outdoor temperature and the manufacturer's heating data.
- Delivered capacity is only as good as the airflow. The result scales one for one with CFM, so an airflow 10 percent high reads 10 percent more capacity.
- Readings must be taken at steady operation, close to the equipment. Supply readings at a far register pick up duct heat gain and read low on capacity.
- Expected mode takes 12,000 BTU/h for every nominal ton. A real unit at your airflow and temperatures delivers more or less than its nominal rating, and the manufacturer's expanded data gives the actual figure.
- Expected mode uses the standard-air constant 1.08. Air at room temperature is a little lighter than standard air, so a real coil at the same CFM shows a slightly wider split.
- Delivered mode leaves out the heat carried off in the condensate, which is under 1 percent for comfort cooling.
- The page does not compare the result with a specific model's rating and does not judge refrigerant charge. Charge is set by the manufacturer's procedure.
- Saturation pressure comes from the Magnus-Tetens fit, within about 0.5 °F of the full ASHRAE formulation in the HVAC range.
- Delivered mode flags readings that work out below 250 CFM per ton, a sensible heat ratio below 0.3, or supply air below 40 °F. Those limits are this calculator's assumptions for a comfort cooling coil, not code limits. Readings inside them can still be wrong.
- Expected mode flags a split outside 16 to 22 °F, the sea-level range typical for cooling. At elevation the split is scaled back to sea level first, so the thinner air alone does not trip it. A split outside the range is not wrong by itself, so the result still shows.
Common mistakes when reading a split
- Reading the split as a charge test. Airflow, indoor humidity, a dirty coil, the load and the charge all move the split. The split alone does not tell you which one is at work. Check charge the way the manufacturer specifies, with superheat or subcooling.
- Assuming 400 CFM per ton. The split moves with airflow. If the system is moving 320 CFM per ton because of a loaded filter or tight ducts, a wide split is the airflow talking, not the coil. Measure static pressure before judging the split.
- Taking the supply reading in view of the coil. A probe that can see the coil reads radiant cold and wet air in streaks. Read the supply in the trunk past the first turn and the return before the filter.
- Using a dry wick. A wet-bulb with a dry or dirty wick reads high, close to the dry-bulb. That inflates enthalpy and can push the sensible share above the total. Wet the wick with clean water and let the reading settle.
- Judging the split at startup. The coil needs several minutes at full load to reach steady state. Readings in the first few minutes show a split that is still changing.
Common questions about temperature split
What is a good temperature split for an AC?
It depends on the coil's sensible heat ratio and the airflow. At sea level, an SHR of 0.75 at 400 CFM per ton gives about 20.8 °F, an SHR of 0.70 at 350 CFM per ton gives about 22.2 °F, and an SHR of 0.80 at 450 CFM per ton gives about 19.8 °F. Work out the expected number for the system in front of you, then compare your reading with it.
How do I calculate AC delta T?
Multiply 12,000 by the sensible heat ratio and divide by 1.08 times the CFM per ton. For SHR 0.75 and 400 CFM per ton that is 9,000 / 432, about 20.8 °F. At elevation the 1.08 gets smaller and the split gets wider.
Why is my AC temperature split only 10 degrees?
A narrow split can mean high airflow, a humid house where much of the capacity goes into removing moisture, a weak coil, or a probe in the wrong place. The dry-bulb split does not show the latent part, so run delivered mode with wet-bulb readings to see total capacity before deciding anything is wrong.
What does a high temperature split mean?
For the same coil the split goes up as airflow goes down. At SHR 0.75 the expected split is 20.8 °F at 400 CFM per ton and 27.8 °F at 300. A wide split often points to low airflow from a dirty filter, a closed damper or restrictive ductwork, so check static pressure first.
What is a good temp split for a heat pump?
In cooling mode a heat pump is an air conditioner, so the same expected-split formula applies. In heating mode the rise across the coil depends on outdoor temperature and the manufacturer's heating data, and this calculator does not compute it.
How do I measure the actual capacity of an AC?
Take dry-bulb and wet-bulb at the return and the supply at steady operation, and get the airflow from a flow grid, a flow hood or the blower table at the measured static pressure. Delivered mode turns those into total, sensible and latent BTU/h with the ASHRAE enthalpy equations.
Is the 4.5 x CFM x enthalpy rule accurate?
The 4.5 is 60 minutes x 0.075 lb per cubic foot, standard air at sea level. Air entering a coil at 80 °F weighs less than that, so on the worked example (1,200 CFM, 80/67 to 58/56) the shortcut reads 41,249 BTU/h against 39,724 BTU/h from the actual air density, about 4 percent high. At elevation the gap grows.
Why does the calculator ask where the airflow was measured?
CFM is a volume, and capacity depends on pounds of air. Cold supply air is denser than warm return air, so the same pounds per hour show up as fewer CFM on the supply side. The calculator uses the density of the air you measured in.
Used on the same job
Check the load behind the split in chat
A coil can deliver its rating and still lose to the house. In chat the assistant runs the free whole-house Manual J cooling load for the job, and it can solve the air state of any reading you give it. Compare the delivered tons from this page with that load.
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