Latent vs Sensible Capacity and Sensible Heat Ratio

The short version

An air conditioner removes two kinds of heat, and the total capacity everyone quotes is only their sum. Sensible capacity is the part that lowers air temperature. Latent capacity is the part that condenses water vapor out of the air and sends it down the drain. Sensible heat ratio is the sensible share of the total, and it moves with the air entering the coil and with how much air the blower is moving, so the same unit delivers a different split in August than it does in June. The trap is a humidity complaint on a system where the charge is correct and the temperature split looks normal, because a dry bulb temperature reading cannot show the half of the job that is failing. The customer pays for that twice: the house is still damp, so the complaint that brought you out is unresolved, and the reflex of moving more air to fix a comfort problem cuts latent capacity further, which turns a service call into a dehumidifier quote or an equipment replacement that was never the fault.

Where the manufacturer publishes expanded performance data, the split is already in it, so read it instead of estimating it from tonnage. That table gives cooling performance across a grid of entering wet bulb temperatures, outdoor temperatures and airflows. Some manufacturers print total capacity and sensible capacity in the cells, and others print total capacity with a sensible to total ratio you multiply out. Either way, latent capacity is total minus sensible, so one subtraction gets you there.

What sensible and latent capacity are

Trane’s glossary states both definitions in one line each. Sensible heat, it says, “is the heat you can feel or sense that can be measured by a thermometer or thermostat.” Latent heat “is the heat energy required to cause a substance to change state.” On the air side of a cooling coil the substance changing state is water, and the change runs from vapor to liquid. Refrigerant is changing state inside the same coil at the same time. That is a second latent transfer, and it is not the one that dries the house.

Dew point is the temperature at which air becomes saturated and water vapor starts condensing out of it. That temperature is what determines whether a coil does any latent work at all. Bryan Orr at HVAC School puts the condition on the coil: “When the coil’s temperature is below the dewpoint of the air moving over it, there is also a transfer of latent energy from the air as some of the water vapor in the air condenses to liquid water (condensate) on the evaporator coil.” He then states the part technicians miss. “This latent heat transfer does not result in colder air but rather a lower moisture content in the air.”

Capacity is a rate, measured in British thermal units per hour, usually written Btu per hour. Sensible capacity is the rate at which the coil lowers the temperature of the air passing through it. Latent capacity is the rate at which the same coil condenses moisture out of that air. Both are counted in the same unit, and they add up to total capacity. That sum is what an equipment rating reports at its rating condition. What the nameplate gives you is the nominal tonnage encoded in the model number, which is not the capacity the unit is making today.

That accounting is why a temperature split is only half a measurement. Delta T is return air dry bulb temperature minus supply air dry bulb temperature, and dry bulb temperature is the temperature a plain thermometer reads. JD Kelly, writing at HVAC School, states what the latent side does to it: “The process of latent heat removal affects the sensible heat exchange over the coil, causing the temperature split to be lower than that of dry air.” He gives the field version in the same article, saying that “air high in humidity will produce a lower temperature split.”

Read that in the right direction. A narrow split on a humid day can be a coil doing more latent work rather than a coil in trouble. A split alone will not tell you which, because it reports the sensible half and shows nothing of the rest.

Sensible heat ratio and where the number lives

Sensible heat ratio, usually shortened to SHR, is the sensible share of total capacity. Orr defines it as “the percentage of the capacity that goes toward sensible cooling at a given set of conditions for a given piece of equipment or space.” The arithmetic is a division and a subtraction, and he gives both: “You can easily calculate the SHR by dividing the sensible capacity by the total capacity, and the latent is simply the sensible subtracted from the total.”

Orr works an example. A system with an SHR of 0.70 and 30,000 Btu per hour of total capacity produces 21,000 Btu per hour of sensible cooling and 9,000 Btu per hour of latent removal. Higher SHR means more temperature change and less moisture removed. Lower SHR means the opposite.

Rob Falke, writing in Contracting Business, publishes a rule of thumb for what to expect: “sensible Btus typically are 70% of total Btus or about 8,400 sensible Btu’s per ton.” Treat that as a sanity check on a measurement rather than as a target. A unit does not have one ratio. Its performance data carries a figure for each set of conditions, given either as a ratio or as sensible and total capacity you divide yourself, and a rule of thumb carries no such figure.

Two documents bear on this, and only one of them has the split. The equipment rating fixes total capacity at a single condition. The manufacturer’s expanded performance table is where the sensible and latent halves actually are.

The rating is one condition. AHRI is the Air-Conditioning, Heating, and Refrigeration Institute, the trade body that runs the certification program those ratings come from. The Building America Solution Center, a technical resource published by the Department of Energy, gives the conditions behind an AHRI rating: “AHRI uses a specific set of conditions (95°F outdoor, 80°F indoor, and 67°F wet bulb) when determining the equipment performance data, such as heating and cooling capacity and SEER2 and EER2 cooling efficiencies; these performance data are then listed on the AHRI certificate.” That list is capacity and efficiency. No sensible or latent figure appears on it, which is why the answer to a humidity question is not on the certificate. One condition produces one total capacity, and the house you are standing in is almost never at that condition.

The expanded performance table is the grid. ACCA is the Air Conditioning Contractors of America, and Manual S is its procedure for selecting equipment against a load calculation. The same Building America Solution Center guide says the more accurate method, and the one ACCA Manual S requires, is to use the manufacturer’s expanded performance table to obtain performance data at design conditions. Wes Davis, writing in Contracting Business, describes what Manual S is matching: “The sensible load is the heat that is measured by a thermometer or a thermostat,” while “Latent load is the heat associated with airborne moisture (produces relative humidity as measured by a hygrometer or humidistat).” The table you need is indexed by entering wet bulb temperature, outdoor temperature and airflow, and by entering dry bulb temperature as well on many models.

What sits in the cells varies by manufacturer. Some tables give total capacity and sensible capacity outright. Others give total capacity and a ratio column, commonly headed S/T and sometimes SHR, which you multiply by total capacity to get sensible. ACCA’s own worked example spells that column out as the “sensible temperature ratio,” and the percentage in it is sensible capacity divided by total capacity, which is the same quantity Orr calls sensible heat ratio.

Nominal tonnage is not a reliable input to any of this. Orr states it directly: “The nominal or listed tonnage on a piece of equipment is often NOT what the equipment produces at current load conditions.” Falke makes the same point about the rating itself, writing that under AHRI conditions “the typical system is only rated for around 11,200 BTU per ton” rather than the nominal 12,000.

What airflow does to the split

Airflow is the adjustment most likely to be in front of you, and it moves the split more than it moves the total. Airflow is measured in cubic feet per minute, shortened to CFM. It is usually described against equipment size as CFM per ton, which is the airflow divided by the nominal tons of cooling.

ACCA published a worked example that shows the size of the effect. Matt Akins, writing on the ACCA blog, takes a 2.5 ton air conditioner and reads its performance table at three airflows. He states the conditions: a 75°F indoor dry bulb temperature, “a 75°F outdoor ambient temperature and a 63°F entering indoor wet-bulb temperature.”

Airflow	Airflow per ton	Total capacity	S/T	Sensible capacity	Latent capacity
875 CFM	350 CFM per ton	29,500 Btu/h	52%	15,340 Btu/h	14,160 Btu/h
1,000 CFM	400 CFM per ton	29,800 Btu/h	58%	17,284 Btu/h	12,516 Btu/h
1,125 CFM	450 CFM per ton	30,300 Btu/h	61%	18,483 Btu/h	11,817 Btu/h

ACCA publishes the total capacity, the S/T percentage and the sensible capacity for all three airflows, and the latent capacity for the 875 CFM row only. The latent figures in the other two rows are total capacity minus sensible capacity.

The arithmetic across the table is the finding. Going from 875 CFM to 1,125 CFM raises total capacity by 800 Btu per hour, which is 2.7 percent. Over the same change, sensible capacity rises 3,143 Btu per hour, or 20.5 percent, and latent capacity falls 2,343 Btu per hour, or 16.5 percent. A blower speed change that barely registers as a capacity change cuts moisture removal by roughly a sixth. Airflow is a humidity adjustment as much as it is a capacity adjustment. These figures belong to one unit at one set of conditions, so read your own table rather than reusing these.

Orr traces the mechanism for reducing airflow. Slower air spends more time against the fins and tubes, and two entries on his list of what happens next are the ones that matter here. “Coil temperature decreases because less overall heat is being picked from the air.” “Coil drops further below the dewpoint, causing more moisture to be removed from the air, increasing dehumidification.” The cost arrives with the benefit: he names “lower overall sensible capacity and, therefore, a drop in the efficiency rating.”

Both published sources point the same way on where to set it. ACCA gives the humid climate setting inside the same worked example: “In hot, humid climates, lower airflow like 350 CFM per ton can ensure the customer is comfortable.” It gives the dry climate setting in the same place: “In warmer climates, that have less humidity, 450 CFM per ton lets the same system deliver more sensible capacity.” Orr’s guidance for a wet house is the same figure: “when humidity is a challenge, setting up the equipment for 350 CFM per ton is generally a good practice.”

There is a floor under this, and Orr states it: “We all know that if you have far too little airflow, a system can freeze up when the coil temperature drops below 32°F.” A frozen coil stops the cooling, so the customer calls back with a no-cooling complaint instead of a humidity one. Stay inside the airflow range the equipment manufacturer publishes for the unit, and follow that unit’s own procedure for changing airflow, because some blowers change speed at a motor tap and others change it at a switch, a dip setting or a control menu.

Where the change means touching wiring, disconnect every electrical source feeding the equipment rather than only the one you can see, lock and tag each disconnecting means, release any stored energy, and then test with a meter at the exact parts your hands will touch to confirm they are dead. Move one step at a time, restore power, and take the readings again after each step.

Altitude changes what a given airflow means. Orr explains that the coil responds to the mass of air crossing it rather than the volume, so “more airflow in CFM is required in high altitudes where the air density is lower.” CFM per ton describes typical air at sea level.

How to measure the split on the job

Measure the split when the complaint is about humidity rather than temperature, when a house feels damp at a setpoint the system is holding, or when you want to know whether equipment is delivering the latent capacity the selection assumed. Do not run it as a routine check. It takes an airflow measurement and two psychrometric readings, and a wrong airflow number scales sensible, total and latent capacity by whatever the error is. The one figure it leaves alone is the ratio, because the same airflow sits above and below the line in that division and cancels out.

Establish what you are holding first, because the instrument determines the method. Most digital psychrometers sold for this work read dry bulb temperature and relative humidity and compute wet bulb temperature and dew point from those two. Wet bulb temperature is the temperature a thermometer reads with a wet wick in moving air, and it responds to both heat and moisture. You need one reading in the return and one in the supply, plus a way to convert each wet bulb temperature to enthalpy. Enthalpy is the total heat content of the air, and Orr defines it as being “represented in BTUs per lb of air.” He also gives the reason wet bulb is the usable input: on a psychrometric chart “wet-bulb temperature and enthalpy are slanted lines descending from left to right, and they are equivalent.”

1. Establish the airflow. Falke names the two ways to get it: “CFM = the fan CFM plotted on the fan tables, or measured at several points in the system.” External static pressure is the resistance the blower works against outside the equipment cabinet. A blower table gives airflow for a stated blower speed at a stated external static pressure, so you need to know which speed the blower is actually on and to measure the pressure rather than assume either one.

2. Take return and supply readings during the same run. Both readings describe one moment on a system that is still responding to the house and the weather, so a return reading taken before you walked to the supply plenum belongs to a different system state. Let the system run until the readings stop moving before you record anything.

3. Calculate sensible capacity from the dry bulb temperatures. Falke writes the formula as “cfm x The system temperature change (Delta T) x 1.08 = Sensible Btu.”

4. Calculate total capacity from the enthalpies. Falke writes that one as “Cooling BTUs = CFM x ∆h x 4.5,” where the enthalpy change is return enthalpy minus supply enthalpy. His worked example is “905 CFM x 4.87 BTU per lb. x 4.5 = 19,833 Total System BTUs.”

5. Subtract to get latent capacity, then divide to get the ratio. Latent capacity is total minus sensible, and sensible heat ratio is sensible divided by total, which is the pair of operations Orr describes.

Carry Falke’s 905 CFM and 4.87 Btu per pound through to see why step 3 cannot be skipped. Total capacity is 19,833 Btu per hour. Suppose the dry bulb split measured 17°F on the same system. Sensible capacity is 905 multiplied by 17 multiplied by 1.08, which is 16,616 Btu per hour, so latent capacity is 3,217 Btu per hour and the sensible heat ratio is 0.84. Nothing in the temperature split by itself produced that answer, and nothing in the total capacity by itself produced it either. You need both measurements.

Three things limit how precise the result can be. The constants 1.08 and 4.5 are derived for air at sea level, so they need adjusting at altitude for the same reason CFM per ton does. An airflow measurement that is off by ten percent moves sensible capacity, total capacity and latent capacity by ten percent each. Whether that or the psychrometric readings dominate the error depends on how you got the airflow and on how large the enthalpy change is. A small error in either wet bulb reading also moves the enthalpy change, which is a difference between two numbers that sit close together.

The humidity complaint with normal readings

This is the call the rest of the post is for. The house is holding its setpoint, the charge checks out, the temperature split sits inside the range you expect, and the customer says it feels clammy. Nothing you normally measure is wrong, because the measurement that would show the problem is one you have not taken.

Work through the causes in the order they cost you the least.

Check the thermostat fan setting before anything else. Leaving the blower on continuously blows air over a wet coil after the compressor stops, and the water that condensed during the cycle goes back where it came from. Allison Bailes at Energy Vanguard describes the mechanism from a test in his own house: “the air passing over the coil as it warmed up would evaporate that water and put it right back into the house.” He measured what running the blower continuously did to the humidity. Indoor relative humidity “spiked and averaged 70% during the time the fan was running continuously,” against an average of “58-59% with the fan set to auto.” HVAC School reaches the same conclusion for humid climates: “In a humid climate, having the fan stay off except when there is a call for cooling is typically the best strategy.”

Check airflow next, and check it against the table rather than against 400 CFM per ton. The ACCA example above shows a 250 CFM change moving latent capacity by 2,343 Btu per hour. A blower left on a high tap after a duct repair, or a variable speed blower commissioned for a dry climate, is a common cause of the complaint you were called out for.

Then look at runtime. Equipment that satisfies the thermostat quickly gets fewer minutes of cold coil per hour. Condensation starts as soon as the coil surface sits below the dew point of the air crossing it, so runtime determines how much water leaves the air rather than whether any leaves at all. Manual S caps how far cooling equipment may be oversized against the load, and Davis gives the arithmetic in his introduction to it: for a 30,000 Btu per hour total cooling load, the selected air conditioner “can have no more than 34,500 Btu/h (115% x 30,000 = 34,500).” That 115 percent belongs to his single speed example rather than to every selection. ACCA’s guidance on variable capacity equipment allows a cooling size factor as high as 1.30, so look up the limit for the equipment type in front of you. Equipment above its own limit has a selection problem, which does not rule out a service problem alongside it, and saying so plainly at the end of a service call is better than adjusting around it.

Ask what the equipment does at part load. Variable capacity equipment does not always help here. HVAC School reports that inverter driven systems mostly “ramp up or down based on sensible BTUs, not latent,” and that “low compression ratios are great for efficiency, but the ways inverter-driven systems achieve it also reduce the equipment’s dehumidification capabilities.” Check the manufacturer’s commissioning settings for the humidity control features on the specific model before you conclude the equipment cannot do it.

Be ready to say the load needs more than the air conditioner has. Bailes states the size of the gap: “In humid climates, the air conditioner can handle only 50 to 70 percent of the dehumidification needed.” He also names the trend behind it: “Modern air conditioning equipment isn’t as good at dehumidifying as older equipment was.” Where that is the case, no adjustment to a correctly operating system closes the gap, and the honest answer is dedicated dehumidification instead of another airflow change.

Common mistakes

Pull the manufacturer’s expanded performance table for the model in front of you and read total capacity and either the sensible column or the ratio column at the actual entering wet bulb temperature, outdoor temperature and airflow.
Treat a narrow temperature split on a humid day as a possible sign of latent work rather than as an automatic fault, since JD Kelly at HVAC School says humid air produces a lower temperature split.
Measure both the dry bulb split and the enthalpy change when the complaint is about humidity, because sensible capacity and total capacity are two separate measurements.
Check the thermostat fan setting on every humidity call, since Allison Bailes at Energy Vanguard measured indoor relative humidity averaging 70 percent with the blower running continuously against 58 to 59 percent on auto.
Set airflow to the climate the equipment is in, which both ACCA and HVAC School put near 350 CFM per ton where humidity is the problem.
Record the airflow you used and how you got it, because every capacity number you report rests on it together with the temperature and humidity readings that go with it.

Don’t

Read nominal capacity off the nameplate and assume the sensible and latent halves, since even the certified rating behind it carries no sensible or latent figure, and that rating’s capacity comes from one set of conditions that the Building America Solution Center gives as 95°F outdoor, 80°F indoor and 67°F wet bulb.
Raise blower speed to answer a humidity complaint, which in the ACCA example moved latent capacity down 2,343 Btu per hour while total capacity barely moved.
Drop airflow below what the equipment manufacturer publishes for the unit, because Bryan Orr at HVAC School notes that far too little airflow lets a system freeze up once coil temperature falls below 32°F.
Use the 70 percent sensible rule of thumb Rob Falke publishes as a target when the equipment has published performance for the conditions you are working in.
Carry the capacity numbers in the ACCA table above to another unit, since they belong to one 2.5 ton system at one indoor dry bulb temperature, one outdoor temperature and one entering wet bulb temperature.
Apply the 1.08 and 4.5 constants at altitude without adjusting them, since they are derived for air at sea level.

Frequently asked questions

What is sensible heat ratio?

Sensible heat ratio, shortened to SHR, is sensible capacity divided by total capacity. Bryan Orr at HVAC School defines it as “the percentage of the capacity that goes toward sensible cooling at a given set of conditions for a given piece of equipment or space.” A system with an SHR of 0.70 and 30,000 Btu per hour of total capacity delivers 21,000 Btu per hour of sensible cooling and 9,000 Btu per hour of latent removal. The ratio is not a fixed property of the equipment, because it moves with entering air conditions, outdoor temperature and airflow.

What is the difference between sensible and latent capacity?

Sensible capacity is the rate at which a coil lowers air temperature, and latent capacity is the rate at which the same coil condenses water vapor out of the air. Trane’s glossary defines sensible heat as “the heat you can feel or sense that can be measured by a thermometer or thermostat,” and latent heat as “the heat energy required to cause a substance to change state.” The two add up to total capacity, which is what an equipment rating reports at its rating condition, while a nameplate model number gives nominal tonnage instead. Orr notes that the latent transfer “does not result in colder air but rather a lower moisture content in the air,” which is why a thermometer cannot see it.

Does lowering airflow improve dehumidification?

Yes, within limits. Bryan Orr at HVAC School lists what lower airflow does: “Coil drops further below the dewpoint, causing more moisture to be removed from the air, increasing dehumidification.” ACCA’s worked example on a 2.5 ton unit puts numbers on it. ACCA states 14,160 Btu per hour of latent capacity at 875 CFM, and at 1,125 CFM its 30,300 Btu per hour total and 18,483 Btu per hour sensible subtract to 11,817 Btu per hour. The limits are the airflow range the equipment manufacturer publishes and the point Orr names where far too little airflow lets the coil freeze below 32°F.

Why is my delta T low when the house is humid?

Because part of the coil’s capacity is going to moisture rather than to temperature. JD Kelly at HVAC School writes: “The process of latent heat removal affects the sensible heat exchange over the coil, causing the temperature split to be lower than that of dry air.” He adds that “air high in humidity will produce a lower temperature split.” Delta T is a dry bulb measurement, so it reports the sensible half of the work and shows nothing of the latent half. Confirm with an enthalpy measurement before treating a narrow split on a humid day as a fault.

Where do I find the sensible and latent capacity for a unit?

In the manufacturer’s expanded performance table for that model, which gives performance for combinations of entering wet bulb temperature, outdoor temperature and airflow. Some tables list total and sensible capacity outright, and others list total capacity with a ratio you multiply out, commonly headed S/T. The Building America Solution Center calls that table a more accurate method than the rating, and the one ACCA Manual S requires. Latent capacity is total capacity minus sensible capacity. The capacity rating on the AHRI certificate is one condition, given by the Building America Solution Center as 95°F outdoor, 80°F indoor and 67°F wet bulb.

Does running the blower on ON help with humidity?

No, and in a humid climate it usually makes the humidity worse. Allison Bailes at Energy Vanguard describes air moving over a warming coil after the compressor stops, where it will “evaporate that water and put it right back into the house.” In his own house the indoor relative humidity “spiked and averaged 70% during the time the fan was running continuously,” against “58-59% with the fan set to auto.” HVAC School gives the same guidance, saying that in a humid climate having the fan stay off except on a call for cooling is typically the best strategy.

Glossary

ACCA: Air Conditioning Contractors of America, the trade association that publishes the Manual J load calculation and Manual S equipment selection procedures.
AHRI: Air-Conditioning, Heating, and Refrigeration Institute, the trade body whose certification program produces the equipment ratings quoted here.
Btu: British thermal unit, the unit heat is counted in. Capacity is stated as Btu per hour, a rate.
Building America Solution Center: a technical resource published by the Department of Energy, cited here on rating conditions and expanded performance tables.
CFM: cubic feet per minute, the volume of air a blower moves.
CFM per ton: airflow divided by nominal tons of cooling, used to describe how a blower is set relative to the equipment size.
Delta T: return air dry bulb temperature minus supply air dry bulb temperature, also called the temperature split.
Dew point: the temperature at which air becomes saturated and water vapor begins condensing out of it.
Dry bulb temperature: the air temperature a plain thermometer reads.
Enthalpy: the total heat content of the air, stated in Btu per pound of air.
Expanded performance table: the manufacturer’s grid of cooling performance at combinations of entering wet bulb temperature, outdoor temperature and airflow, giving total capacity with either sensible capacity or a sensible to total ratio.
External static pressure: the resistance the blower works against outside the equipment cabinet, which a blower table uses as its input.
Latent capacity: the rate, in Btu per hour, at which a coil removes latent heat from the air passing through it by condensing moisture out of it.
Latent heat: the heat involved in a change of state, which in a cooling coil is water vapor becoming liquid.
Manual S: the ACCA procedure for selecting equipment against a calculated load, including the sensible and latent parts of that load.
Psychrometer: an instrument that reports wet bulb temperature, usually by measuring dry bulb temperature and relative humidity and computing the rest.
Sensible capacity: the rate at which a coil lowers the temperature of the air passing through it.
Sensible heat: heat that changes air temperature and can be read with a thermometer.
SHR: sensible heat ratio, sensible capacity divided by total capacity.
S/T: the column heading manufacturers commonly use for the sensible share of total capacity.
Total capacity: sensible capacity plus latent capacity, which is the capacity an equipment rating reports.
Wet bulb temperature: the temperature a thermometer reads with a wet wick in moving air, which responds to both heat and moisture.

Drafted with AI assistance and reviewed by the author.

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