Sensible vs Latent Heat
The difference that governs comfort, dehumidification, and equipment sizing. Get these two buckets straight and callbacks for "it's cold but clammy" stop being a mystery.
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In This Guide
The Two Buckets of Heat
Every BTU your equipment moves lands in one of two buckets. Sensible heat is heat you can measure with a thermometer — it changes the air's temperature. When a furnace warms a room from 68°F to 72°F, or a coil drops supply air from 75°F to 55°F, that is sensible heat moving. Your dry-bulb reading tracks it directly.
Latent heat is the heat tied up in changing water's state without changing temperature. It takes roughly 970 BTU to boil one pound of 212°F water into 212°F steam — same temperature, huge energy. In a comfort system you see the reverse: as humid return air passes a cold coil, water vapor condenses on the fins and releases its latent heat into the refrigerant. That heat leaves the building as condensate down the drain. A thermometer never sees it, which is exactly why techs miss it.
The one-sentence version
Sensible heat changes temperature; latent heat changes moisture. Your equipment removes both, but you only feel comfortable when both are handled.
This matters because a system can nail the thermostat and still leave a house miserable. Drop the temperature without pulling moisture and you get the classic "cold and clammy" call — 65°F air at 70% relative humidity feels like a basement. Total heat, the number that actually governs comfort, is sensible plus latent together.
Where Latent Heat Hides: Grains and Enthalpy
Latent heat rides along as water vapor, and the field unit for that vapor is the grain — 7,000 grains equal one pound of water. Humidity ratio is expressed in grains of moisture per pound of dry air. A comfortable indoor design point of 75°F at 50% RH carries about 65 grains per pound; muggy outdoor air at 90°F and 70% RH can run over 150 grains. The difference is the moisture your coil has to condense out.
To see both buckets at once, techs use enthalpy — the total heat content of air in BTU per pound, read off a psychrometric chart from dry-bulb and wet-bulb temperatures. Because enthalpy already bundles sensible and latent together, the gap between return-air enthalpy and supply-air enthalpy is your total heat removal in one number. That is why wet-bulb temperature is worth carrying a sling psychrometer for: dry-bulb alone tells you half the story.
Comfort targets to keep in your head
- Indoor RH sweet spot: 40–55% (per ASHRAE 55 thermal comfort)
- Above ~60% RH: mold, dust mites, and that sticky feeling
- Design indoor condition: 75°F, 50% RH (~65 grains/lb)
- Cooling coil delta-T target: 14–22°F dry-bulb split
The Three Field Formulas
You only need three equations to split any airside load. Each uses actual airflow (CFM) across the coil, so measuring real CFM is step zero — a fan curve guess will wreck every number downstream.
Sensible Heat
ΔT is the dry-bulb temperature split (return − supply).
Total Heat
Δh is the enthalpy difference in BTU/lb.
Latent Heat
ΔW is the humidity-ratio difference in grains/lb.
Where the constants come from
- 1.08 = 0.075 lb/ft³ air density × 60 min/hr × 0.24 BTU/lb·°F specific heat of air
- 4.5 = 0.075 lb/ft³ × 60 min/hr (enthalpy already carries the BTU/lb)
- 0.68 = 0.075 × 60 × 1,061 BTU/lb latent heat of vaporization ÷ 7,000 grains/lb
All three assume standard air at sea level. Above ~2,000 ft or in hot supply-air heating, correct the constants for density — but for most cooling work these are accurate enough.
The three tie together cleanly: Qt = Qs + Ql. Measure any two and the third falls out. In the field the easy pair is sensible (thermometer split) and total (enthalpy split), then latent is simply the leftover.
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Sensible Heat Ratio and Sizing
The sensible heat ratio (SHR) is the fraction of total capacity spent on temperature: SHR = Qs / Qt. A typical residential system runs around 0.75 — 75% of its work is sensible cooling, 25% is latent dehumidification. Equipment SHR and building SHR both matter, and mismatching them causes most humidity complaints.
| Situation | Typical SHR | What it means |
|---|---|---|
| Hot, dry climate (Phoenix) | 0.85–0.95 | Almost all sensible; little dehumidification needed |
| Mixed climate, design day | 0.75–0.80 | Standard equipment matches well |
| Hot, humid coastal (Houston) | 0.65–0.72 | Heavy latent load; needs low-SHR equipment |
| Oversized AC, mild day | 0.90+ | Short cycles, never dehumidifies — clammy house |
Why oversizing kills humidity control
Latent removal needs run time. Moisture only leaves when the coil stays wet and cold long enough to condense vapor. An oversized unit satisfies the thermostat on sensible heat in a few minutes, shuts off before the coil pulls real moisture, then re-evaporates condensate off the fins on the next fan cycle. Result: correct temperature, terrible humidity. Right-sizing per a Manual J load — not "a ton per 400 square feet" — is the real fix.
Raising fan speed pushes SHR up (more sensible, drier coil, less latent); lowering fan speed to around 350–400 CFM per ton drops SHR for better dehumidification. That single adjustment solves a surprising number of humidity callbacks.
Splitting a Load in the Field
Here is the procedure to measure exactly how a running system divides its work. You need a calibrated psychrometer (dry-bulb and wet-bulb), a way to get real airflow, and a psychrometric chart or app.
Step 1 — Measure airflow.
Get actual CFM with a flow hood, TrueFlow grid, or the fan table read against measured total external static pressure. Do not trust nameplate CFM.
Step 2 — Read return and supply air.
Take dry-bulb and wet-bulb at the return grille and again in the supply plenum, downstream of the coil.
Step 3 — Sensible heat.
Qs = 1.08 × CFM × (return DB − supply DB).
Step 4 — Enthalpy lookup.
From each dry-bulb/wet-bulb pair, read enthalpy (BTU/lb) off the chart or app.
Step 5 — Total heat.
Qt = 4.5 × CFM × (return h − supply h).
Step 6 — Latent and SHR.
Ql = Qt − Qs, then SHR = Qs / Qt. Compare against rated capacity to judge charge and airflow.
Worked Example
Problem:
A 3-ton system moves a measured 1,150 CFM. Return air reads 75°F DB / 63°F WB (enthalpy ≈ 28.6 BTU/lb). Supply air reads 55°F DB / 54°F WB (enthalpy ≈ 22.6 BTU/lb). Find sensible, total, and latent capacity plus SHR.
Step 1: Sensible heat
Qs = 1.08 × 1,150 × (75 − 55) = 1.08 × 1,150 × 20 = 24,840 BTU/hr
Step 2: Total heat
Qt = 4.5 × 1,150 × (28.6 − 22.6) = 4.5 × 1,150 × 6.0 = 31,050 BTU/hr
Step 3: Latent heat
Ql = Qt − Qs = 31,050 − 24,840 = 6,210 BTU/hr
Step 4: Sensible heat ratio
SHR = 24,840 / 31,050 = 0.80
Reading the result
About 31,000 BTU/hr total — right where a properly charged 3-ton unit (36,000 BTU/hr nominal, derated at real conditions) should land. The 0.80 SHR is healthy for a mixed climate: 80% of the work cools, 20% dehumidifies. A 20°F dry-bulb split confirms airflow and charge are in the ballpark.
If SHR had come back at 0.95
Nearly no latent removal usually means airflow is too high, the coil is not getting cold enough (low charge or restricted metering), or the space simply has little moisture that day. On a humid day a 0.95 SHR with high indoor RH points you straight at airflow or charge.
Comfort Complaints and Fixes
Once you think in two buckets, humidity callbacks sort themselves out fast:
- "Cold but clammy." Sensible fine, latent lacking. Check for oversizing, too-high fan speed, or short cycling. Drop blower to ~350 CFM/ton.
- High indoor RH, long run times. Likely low charge or dirty coil keeping the coil warmer than dew point. Verify superheat/subcooling and coil cleanliness.
- Good numbers, still humid. Latent load may exceed equipment capacity — leaky ducts in a hot attic, or a big infiltration/ventilation load. Consider a dedicated dehumidifier.
- Sweating supply registers. Supply air near dew point plus humid room air — a symptom of the space losing the latent battle.
Field takeaway
Temperature is only half the job. Carry a psychrometer, know your enthalpy values, and check SHR when a customer says a comfortable temperature still feels wrong. The thermostat measures sensible heat; the customer feels total heat.
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