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Evgenii Konkin
Evgenii Konkin

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Enthalpy vs Temperature: Why Sensible Load Alone Undersizes Cooling Coils

#hvac #engineering #psychrometrics #coildesign

Temperature tells you half the cooling story. Enthalpy tells you the whole thing.

What enthalpy captures that temperature misses 

Sensible heat = heat from air temperature (what a thermometer reads)
Latent heat   = heat from moisture content (invisible, but real energy)
Enthalpy      = sensible + latent combined (BTU/lb of dry air)
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The formula:

h = 0.24 × T + W × (1061 + 0.444 × T)
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Where:

h = enthalpy, BTU/lb
T = dry-bulb temperature, °F
W = humidity ratio, lb moisture / lb dry air
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Same temperature, different enthalpy

Two air conditions at 80°F:

Condition A: 80°F, 40% RH
  Humidity ratio: 62 gr/lb
  Enthalpy: 28.5 BTU/lb

Condition B: 80°F, 70% RH
  Humidity ratio: 110 gr/lb
  Enthalpy: 37.8 BTU/lb
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Same temperature. 33% more total heat in Condition B. A coil sized for A cannot handle B.

The coil load comparison

5,000 CFM outdoor air cooled from 95°F/75°F-wb to 55°F/54°F-wb:

Sensible-only calculation:

Q_sensible = 1.08 × CFM × ΔT
Q_sensible = 1.08 × 5,000 × (95 − 55)
Q_sensible = 216,000 BTU/hr = 18.0 tons
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Enthalpy-based calculation (total load):

Q_total = 4.5 × CFM × (h_in − h_out)
Q_total = 4.5 × 5,000 × (38.5 − 22.7)
Q_total = 355,500 BTU/hr = 29.6 tons
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The gap:

Sensible:  18.0 tons (61%)
Latent:    11.6 tons (39%)
Total:     29.6 tons (100%)
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Sizing by temperature alone: 18 tons. Actual need: 29.6 tons. The coil is 39% undersized.

When latent load dominates 

Climate              Latent %    Enthalpy matters?
──────────────────────────────────────────────────
Phoenix (hot-dry)    5–15%       minimal
Denver (moderate)    10–20%      moderate
Atlanta (humid)      25–35%      yes
Houston (very humid) 35–50%      critical
Miami (tropical)     40–50%      critical
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In Phoenix, sensible-only sizing is close enough. In Houston, it undersizes by 35–50%.

The practical shortcut

If you know the entering and leaving wet-bulb temperatures, you can look up enthalpy from psychrometric tables or a calculator:

Entering: 95°F db / 75°F wb → h = 38.5 BTU/lb
Leaving:  55°F db / 54°F wb → h = 22.7 BTU/lb

Total load = 4.5 × CFM × (38.5 − 22.7)
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No need to separate sensible and latent. Enthalpy captures both in one subtraction.

For quick psychrometric property lookups including enthalpy from any two known air properties, use the psychrometric calculator on CalcEngineer.

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