Reading Wet Bulb in the Field, and Why It Sets Your Target

The short version

Indoor wet bulb, read together with dry bulb, shows that the coil has moisture to remove as well as temperature, which dry bulb alone cannot show. The reason is that two houses at 75°F can give the same coil very different jobs. When you charge a fixed orifice system by the superheat method, indoor wet bulb is one of the two inputs that set your target superheat. Working that method without it means charging to a target superheat you guessed at.

A manufacturer’s weigh-in procedure needs no wet bulb, but the weigh-in is its own conditional method. It uses a calculated total charge or charge addition from the nameplate and line set, starting from a known state such as an evacuated system, not a guess added to an unknown charge. If you are working from a charging chart, you need the wet bulb.

The customer pays for a wrong charge twice. The house does not cool or dehumidify the way it should, and a badly overcharged or undercharged system can flood liquid back or starve the evaporator, which in a bad enough case costs a compressor. Those symptoms have other causes too, airflow and the metering device and the coil among them, so never diagnose the charge from those symptoms alone.

On a wetted-wick instrument, the wick is the error source you control. A wick that is not fully wet, or that dries out while you are watching it, drives the wet bulb back up toward dry bulb; airflow, radiation, and the sensor’s own accuracy are the other places the measurement goes wrong. Take the wet bulb at the plateau and read it before the wick dries, not after.

What wet bulb measures

Dry bulb is the air temperature an ordinary thermometer reads. Wet bulb is what a thermometer reads when its sensing tip is covered in a wet wick with air moving across the wick. Water evaporates off the wick, evaporation takes heat, and in air that is not saturated the sensor settles below air temperature.

How far below depends on how much moisture the air is already carrying. Dry air pulls a lot of water off the wick and the wet bulb drops a long way. Air near saturation pulls almost none and the wet bulb barely moves. At 100% relative humidity the two readings are the same, which is the definition of the saturation line on a psychrometric chart.

That gap is the useful part. Dry bulb reads air temperature, which is the sensible side: sensible heat is the heat that changes the air’s temperature. Wet bulb responds to moisture as well, so it moves with the total heat in the air fed to the coil, sensible and latent together.

Wet bulb is an indicator, not the load itself; what the coil actually does with that air depends on airflow, dry bulb, and outdoor conditions too. ACHR News states the point plainly in its charging guidance: wet bulb gives an indication of both the latent and the sensible load on the coil.

This is why two calls at the same thermostat setting are not the same call. A house at 75°F and 40% relative humidity and a house at 75°F and 60% relative humidity read identically on a dry bulb thermometer. The coil sees two different jobs.

Where to take it

Take the wet bulb in the return air, upstream of the coil, as close to the evaporator inlet as access allows.

AC Service Tech specifies the return duct a few feet before the coil. California’s Title 24 refrigerant charge protocol (the reference appendices to the 2025 Energy Code, RA3.2), which is code-enforced procedure rather than advice, requires the return plenum ahead of the blower at a dedicated access hole the installer has to provide.

Two placement mistakes are worth spelling out. The first is reading at the return grille when the protocol specifies the plenum. Under Title 24 the measurement belongs at the labeled return plenum access hole, and a grille reading is allowed only when the unit’s return sits entirely inside conditioned space. The protocol names two examples of that arrangement, a pedestal air handler in a closet and a grille integral to the air handler.

The second is reading downstream of the coil, which gives you supply air. That is a real measurement used for other purposes, and it is not this one.

HVAC School adds one more placement rule: keep the sensor out of line of sight of the evaporator coil, because radiant exchange with the cold surface pulls the wet bulb down. HVAC School’s general temperature guidance says the same thing about any probe that can see a cold or hot surface, so treat the placement as part of the method rather than a nicety.

How to take it

Let the system stabilize first. Title 24 requires fifteen minutes, before any reading and again after the final charge adjustment. Fifteen minutes is a California rule rather than a universal one:

Daikin’s installation instructions vary by model, with one manual giving 10 to 15 minutes or until the refrigerant pressures stabilize and another giving 15 to 20 minutes.
A TruTech Tools charging procedure allows ten to fifteen.

Run the manufacturer’s clock, or its until-stable test, when you have one. Either way, readings taken before the system settles describe a system that is still moving.

Put the panels back. Title 24 requires all cabinet panels that affect airflow to be in place while you measure; on other work, put the cabinet in the configuration the manufacturer says the equipment runs in. A blower door left off changes the air path and therefore the wet bulb.

If your instrument has a wick, use distilled water and saturate it. This is the wetted-wick method; Title 24’s version specifies a clean cotton wick wetted with distilled water and fully saturated. Title 24 equally accepts a digital hygrometer that reads humidity and temperature and calculates wet bulb, and that instrument has no wick to tend; it needs calibration and good placement instead.

The National Park Service conservation guidance gives the reason: tap water carries salts and other contaminants that prevent uniform evaporation. The same guidance notes that a new wick often carries sizing, a starch left from manufacture, and that you should wash the wick in distilled water before first use.

Get air moving across the wick. A sling psychrometer does this when you swing it. Give yourself clear room to swing it, well away from energized equipment, moving machinery, and other people. Mind the water too: droplets fly off the wick, and they should not land on anyone or anything. If there is no safe room to swing, use an aspirated or electronic instrument instead.

Whatever the instrument, move air across the wick the way the instrument’s manufacturer says to:

Swing the sling.
Run the aspirator’s fan.
Follow the probe’s own instructions.

Title 24 accepts a wetted-wick sensor in the return plenum. That acceptance comes with instrument qualifications on accuracy and response time. It is not a promise that any probe reads true at whatever air velocity the return happens to supply. A digital psychrometer that calculates wet bulb from temperature and relative humidity has no wick to aspirate at all.

ACHR News notes that even a thermocouple with moist cotton wrapped around it, placed through a hole drilled in the duct, will suffice. All the same rules apply to that improvisation as to a proper wick: a clean, saturated wrap, adequate air across it, and time to settle.

Drilling a hole carries its own rules:

Cut power first, and remember that power off does not depressurize the refrigerant circuit.
Use an existing access hole when there is one.
Keep the bit shallow, and know what is behind the metal. The DOE warns that a bit between the blower and the coil can puncture the coil and release refrigerant.
Seal the hole when you are done.

Watch for the plateau, and read it before the wick dries. The reading falls as water evaporates, holds, then climbs again as the wick dries out. The plateau is the wet bulb. Keep going until consecutive readings agree at the lowest value. If the wet bulb starts rising, stop and check the wick; if it has dried, re-wet it fully and start over.

Reading before equilibrium is a documented error, and with the usual procedure the wet bulb comes out high. Too little water on the wick and the wet bulb goes up rather than down.

The National Park Service guidance says that an unequilibrated wick gives a wet bulb that is too high, and that over-slinging dries the wick and drives the wet bulb back up from its lowest point. Published work on psychrometer error puts the same point more bluntly: an insufficiently wetted sensor dries out and its readings approach the dry bulb temperature.

Read wet bulb first if you are using a sling. The National Park Service guidance is explicit that the wet bulb starts rising the moment you stop swinging, so wet bulb is the one to take before the dry bulb rather than after.

Record dry bulb at the same place, back to back with the wet bulb. You want both. The two readings together give you your place on the psychrometric chart, and wet bulb alone does not give you the split between sensible and latent load.

Condition	Why it matters	Source
15 minutes runtime before reading	Title 24’s stabilization time; manufacturers set their own clocks	Title 24 RA3.2
Return air, upstream of the coil	Downstream is supply air, a different measurement	AC Service Tech, Title 24 RA3.2
All cabinet panels that affect airflow in place	Missing panels change the air path across the coil	Title 24 RA3.2
Distilled water, wick fully saturated (wetted-wick method)	Salts and contaminants in tap water can prevent uniform evaporation	Title 24 RA3.2, National Park Service
Read at the plateau, before the wick dries	A drying wick sends the wet bulb back toward dry bulb	National Park Service
Return dry bulb at or above 70°F	Below that the Title 24 verification is not valid; manufacturers state their own operating conditions	Title 24 RA3.2
Airflow verified first. Title 24’s standard test uses 350 cfm per ton for new systems and 300 for altered ones, with small-duct high-velocity systems at 250 and an alternative compliance path in RA3.3.3.1.5	Low airflow can invalidate or confound the charge test	Title 24 RA3.2, RA3.3

What the number is for

On a fixed orifice system charged by the superheat method, indoor wet bulb sets your target superheat. That method is how Title 24’s standard procedure verifies fixed metering devices, and manufacturers’ charging charts work the same way inside their stated conditions. Outside those conditions the instructions point elsewhere. The fixed orifice chart in one set of Lennox instructions, for example, points to weigh-in when the outdoor temperature is 39°F or below.

Indoor wet bulb and outdoor dry bulb are the two inputs to the chart. They are not the whole method, since you have to verify airflow first, stabilize the system, and respect the manufacturer’s operating limits. They are still the two numbers the target comes from.

Title 24 states the instruction directly: determine target superheat from the manufacturer’s chart using return air wet bulb and condenser air dry bulb. AC Service Tech gives the same two inputs and has you put them into a superheat chart, calculation, app, or digital manifold set.

Two consequences follow.

The first is that target superheat is not a property of the system. It moves with conditions. AC Service Tech points out that the target changes as the building’s wet bulb comes down, and when the system is pulling a warm, humid house down, that is happening while you stand there charging. If the wet bulb has moved since you looked the target up, look it up again before the final adjustment.

The second is that superheat charging has an operating envelope. Outside the envelope that the chart or procedure states, that procedure is not valid and the manufacturer’s alternate method takes over.

Title 24 notes that when outdoor conditions are hot and indoor conditions are cool, the target computes below 5°F and the test is not valid. In that window a severely undercharged unit still shows over 9°F of superheat, but the superheat method cannot show an overcharge at all. The table marks those conditions with a dash. That dash rule belongs to Title 24’s Table RA3.2-2, and Title 24 gives you two ways forward:

Warm the house up, by running the heat or opening windows, until a valid target appears.
Retest when conditions are different.

The standard procedure has a cold boundary too. Below 55°F outdoor temperature, Title 24 moves you to its modified winter charge setup rather than the standard setup.

A manufacturer’s chart comes with its own notes and symbols, and Lennox’s note is blunt: do not attempt to charge where a dash appears, because the system could be overcharged. Never assume a blank or undefined cell authorizes charging; find that chart’s instructions, or use the manufacturer’s alternate procedure.

On a thermostatic expansion valve (TXV) system you normally set the charge by subcooling instead, though the details belong to the manufacturer. Lennox’s instructions for one single-stage line point to weigh-in when the outdoor temperature is 64°F or below. Lennox’s current variable-capacity instructions call for weighing in the initial charge and then verifying by subcooling, in heating mode when it is cold out.

The wet bulb reading does different work there. Paired with dry bulb and the airflow, it characterizes the job in front of the coil, and it is one of the inputs you need when you read an expanded performance table.

Why the rated capacity assumes a wet bulb you never checked

The rated capacity everyone quotes is one point on a table, a rating produced at defined test conditions rather than a measurement of what the equipment is delivering today. Check the plate in front of you rather than assume what it carries.

ACHR News describes a manufacturer charging curve built on 400 cfm per ton at 50% relative humidity across the evaporator. That is a stated design condition, and away from that condition the curve may simply not apply to the unit in front of you.

The same logic runs through rated capacity. The full-load cooling capacity everyone quotes is reported at one standard rating point. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) tests at 95°F outdoor temperature with 80°F dry bulb, 67°F wet bulb air entering the indoor unit. The seasonal ratings are built from a set of additional test conditions. Either way, the quoted total is what that equipment produced on the test stand at a stated condition, not what it is doing in this house.

The expanded performance tables in the engineering data show how that total divides into sensible and latent at any other condition, indexed by indoor wet bulb alongside indoor dry bulb, airflow, and outdoor temperature.

Some damp-house complaints can start in this gap. The Air Conditioning Contractors of America (ACCA) calls them the humidity problems that result from incorrectly applied or improperly sized systems. The table cannot determine that by itself.

The expanded data describes what the equipment removes while it is running at those conditions, and at part load it may not run long. ACCA’s guidance on humidity control says bluntly that oversized fixed-capacity equipment satisfies the thermostat and cycles off long before the moisture is dealt with.

And a damp house is not proof that nothing is broken. ENERGY STAR lists overly damp air among the results of poor system airflow. Duct leaks belong on the list of causes in both directions: supply leaks lose conditioned air before it reaches the rooms, and return leaks pull unconditioned, often moist air into the system. So airflow and the ducts have to check out along with the charge and the coil’s own condition, and sizing is its own question.

The house side has to check out too. The same ACCA guidance points at envelope leaks, infiltration, ventilation air, plumbing problems, and indoor moisture sources like pools and spas as latent load the original design may never have counted. Thermostat setpoints and how the system is being operated belong on that list too.

When all of that is true and the equipment is doing exactly what its table shows it does at that operating point, the honest conversation with the customer usually turns to equipment selection and dehumidification rather than a repair. Those are the same options ACCA says to consider when selected equipment cannot carry the latent load.

You cannot get to that answer without the wet bulb, because wet bulb is one of the columns you use to look up the table.

Common mistakes

Take the wet bulb in the return, upstream of the coil, as close to the evaporator inlet as access allows.
Let the system run until it is stable, fifteen minutes under Title 24 or the manufacturer’s stated time otherwise, with all cabinet panels that affect airflow in place before you read anything.
If you are reading from a wick, use distilled water and make sure it is fully saturated before you take the wet bulb.
Wait for the wet bulb to plateau instead of recording the first one that looks settled.
Re-check the target superheat as you charge, because indoor wet bulb moves while the house dries out.
Confirm airflow before you evaluate charge, because airflow below the required minimum can invalidate or confound the charge test.

Don’t

Do not charge a fixed orifice system to a remembered superheat number, because the target depends on conditions that change between calls and during the call.
Do not read at the return grille by default. Title 24 puts the measurement at the return plenum access hole and allows the grille only when the unit’s return sits entirely inside conditioned space.
Do not read from a wick that is dry or partly dry, because a drying wick drives the wet bulb back up toward the dry bulb temperature.
Do not stop at the first settled-looking number and do not sling forever. The endpoint is consecutive readings agreeing at the lowest value, and a rising number means stop and check the wick; if it has dried, re-wet it and start over.
Do not charge into a dash or a blank cell; read that chart’s instructions. On the Title 24 table a dash means the target is under 5°F, the verification is not valid, and the superheat method cannot show an overcharge. Title 24’s own fix is to warm the indoor air or retest later. On a manufacturer’s chart, Lennox’s note reads: do not attempt to charge where the dash appears, and treat a cell the instructions do not explain as a reason to stop rather than an authorization.
Do not treat the rated capacity as what the system is delivering today, because that capacity was produced at one set of conditions and the house in front of you may not be at them.

Frequently asked questions

Where exactly should indoor wet bulb be measured?

Indoor wet bulb should be measured in the return air upstream of the evaporator coil, as close to the coil inlet as access allows. AC Service Tech specifies the return duct a few feet before the coil, and California’s Title 24 charge verification protocol requires the return plenum ahead of the blower. Title 24 accepts a grille reading only when the unit’s return sits entirely inside conditioned space, its stated examples being a pedestal air handler in a closet and a grille integral to the air handler. Otherwise take the wet bulb at the return plenum access hole.

Why does target superheat need wet bulb instead of just dry bulb?

Target superheat needs wet bulb because the load on the evaporator is both temperature and moisture, and dry bulb accounts only for temperature. Two houses at the same thermostat setting with different humidity put different loads on the same coil, and superheat responds to that difference. Dry bulb alone cannot tell them apart.

Does wet bulb matter on a TXV system?

Wet bulb matters on a TXV system, though normally not for setting the charge. TXV systems are commonly charged by weigh-in, by the manufacturer’s subcooling target, or both in sequence, and Title 24’s subcooling procedure and the Lennox procedures cited here take no wet bulb input. The method, the operating mode, and the allowed conditions are the manufacturer’s call, so follow the instructions for the unit in front of you. Read together with dry bulb and airflow, wet bulb is still what characterizes the load on the coil, and it is one of the columns you use to look up capacity in the manufacturer’s expanded performance data.

What happens if the wick is not fully wet?

A wick that is dry or partly dry reads high rather than low, because less evaporation means less cooling and the sensor drifts back toward the dry bulb temperature. The National Park Service conservation guidance states that an unequilibrated wick produces a wet bulb that is too high, and that over-slinging dries the wick and pushes the wet bulb back up. Published work on psychrometer error describes the same failure, an insufficiently wetted sensor drying out and approaching the dry bulb value.

How long does the system need to run before I take the wet bulb?

The system needs to run long enough to stabilize, by whichever clock governs the job. Title 24 requires fifteen minutes of runtime with all cabinet panels that affect airflow in place, and fifteen more after the final charge adjustment. Manufacturers set their own times: Daikin manuals variously call for 10 to 15 or 15 to 20 minutes, often adding or until the refrigerant pressures stabilize, and a TruTech Tools procedure allows ten to fifteen. Before the system settles, pressures and temperatures are still moving and the wet bulb describes a system in transition.

Can I use a digital psychrometer instead of a sling?

You can use a qualifying digital psychrometer in place of a sling. Title 24 accepts two kinds of instrument: a digital temperature sensor under a wetted wick, and a digital hygrometer that reads temperature and humidity and calculates wet bulb with no wick at all. On a wetted-wick sensor the wick still has to be saturated with distilled water. You swing a sling to move air across the wick, so for a wetted-wick instrument, follow its instructions for airflow and aspiration. Title 24’s acceptance of a wetted-wick sensor carries accuracy and response-time qualifications; it does not establish that every probe reads accurately at every return-air velocity.

What if the charging chart gives me a blank cell?

On Title 24’s chart, Table RA3.2-2, a dash means the target superheat at those conditions is below 5°F and that verification method is not valid, usually because outdoor conditions are hot and indoor conditions are cool. Under those conditions a severely undercharged system still reads over 9°F of superheat, but the superheat method cannot detect an overcharge.

Title 24 gives two ways forward: warm the indoor air by running the heating system or opening windows until the table produces a valid target, or retest another time when conditions are different. On a manufacturer’s chart, read that chart’s own instructions. Lennox’s note reads: do not attempt to charge where a dash appears, and an unexplained cell is a reason to find that chart’s instructions, not to charge.

Glossary

ACCA: Air Conditioning Contractors of America, a trade association cited here on sizing and humidity.
AC Service Tech: a practitioner training organization whose placement guidance is cited here.
AHRI: Air-Conditioning, Heating, and Refrigeration Institute, which defines the standard rating point.
Dry bulb: the air temperature an ordinary thermometer reads.
Equilibrium: the point at which the wet bulb stops falling and holds steady, also called the plateau.
Evaporator coil: the indoor coil where refrigerant absorbs heat from the air.
HVAC School: a practitioner training and reference organization cited here.
National Park Service: the source of the conservation guidance on sling technique cited here.
Psychrometer: the instrument that reads wet bulb and dry bulb together, slung or aspirated.
Rated capacity: the cooling output a unit produced at one defined set of test conditions.
Return plenum: the duct section carrying air back to the equipment, upstream of the coil.
Sling psychrometer: a wet bulb instrument swung through the air to move air across the wick.
Subcooling: the temperature drop of liquid refrigerant below its condensing saturation temperature.
Superheat: the temperature rise of refrigerant vapor above its evaporating saturation temperature.
Title 24: California’s building energy code, whose charge verification procedure is cited here.
TXV: thermostatic expansion valve, a metering device that holds superheat steady.
Wet bulb: the temperature a wetted, ventilated sensor settles at, which falls as the air gets drier.
Wick: the wetted sleeve over a wet bulb sensor, which must stay fully saturated to read correctly.

Drafted with AI assistance and reviewed by the author.

← All posts