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Evgenii Konkin
Evgenii Konkin

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Quick Tip: Why an Oversized AC Makes Your Building More Humid, Not Less

#hvac #engineering #quicktip #energy

An oversized air conditioner cools the temperature faster — but removes less moisture. The result is a space that reads 72F on the thermostat but feels clammy.

How it works

An AC does two things at the evaporator coil:

1. Sensible cooling — lowers air temperature
2. Latent cooling — condenses moisture from the air
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Moisture removal requires sustained coil contact time. The first minutes of a cycle mostly lower temperature. Dehumidification ramps up after the coil is fully cold and wet.

Short cycling kills dehumidification 

Properly sized system:
  Cycle length: 15-20 minutes
  SHR: 0.75 (25% of cooling removes moisture)
  Indoor humidity: 45-50%

Oversized system (2× design load):
  Cycle length: 5-8 minutes
  SHR: 0.90 (only 10% removes moisture)
  Indoor humidity: 60-70%
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SHR = Sensible Heat Ratio. Lower SHR = more moisture removal.

The compressor damage 

Properly sized:  4-6 starts per hour
Oversized:      10-15 starts per hour

Each start draws 5-7× running current.
More starts = more wear = shorter compressor life.
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The sizing rule 

Design cooling load (Manual J or ASHRAE): X tons
Correct equipment size: 100-110% of X

At 150%+ → short cycling begins
At 200%+ → humidity control fails completely
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Real example 

4,000 sq ft office:
  Design load:     4.8 tons
  Installed:      10.0 tons (208% of design)
  Cycle time:      5-8 minutes
  Indoor humidity: 65%

Replaced with 5-ton variable speed:
  Cycle time:     20-30 minutes
  Indoor humidity: 48%
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Bigger is not better. Match the tonnage to the load.

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