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Evgenii Konkin
Evgenii Konkin

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Quick Tip: Why Your Economizer Misses Free Cooling Hours — Dry-Bulb vs Enthalpy Control

#hvac #engineering #quicktip #energy

An economizer uses outdoor air instead of the compressor when conditions are favorable. But the controller decides when "favorable" means — and the wrong control strategy leaves free cooling on the table.

Two control strategies 

Dry-bulb control:
  IF outdoor temp < setpoint (55-65F) → open dampers
  Simple, cheap, works in dry climates
  FAILS in humid climates (ignores moisture)

Enthalpy control:
  IF outdoor enthalpy < return air enthalpy → open dampers
  Accurate, captures both temp + moisture
  Requires humidity sensor ($300 per location)
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The problem with dry-bulb 

Return air: 75F, 45% RH → enthalpy = 27.5 BTU/lb

Outdoor conditions:
                              Enthalpy   Dry-Bulb    Free
  Condition                   (BTU/lb)   Says...     Cooling?
  ──────────────────────────────────────────────────────────
  45F, 50% RH                  13.2      YES (< 55)   YES ✓
  55F, 60% RH                  19.8      NO (= 55)    YES ✗ missed!
  58F, 70% RH                  23.1      NO (> 55)    YES ✗ missed!
  62F, 80% RH                  27.0      NO (> 55)    YES ✗ missed!
  65F, 85% RH                  28.7      NO (> 55)    NO ✓
  70F, 70% RH                  29.4      NO (> 55)    NO ✓
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Three conditions with free cooling available — dry-bulb controller misses all three.

The dangerous case 

60F, 95% RH → enthalpy = 26.1 BTU/lb

Dry-bulb controller: "60F > 55F → no economizer" ← correct!
But barely — enthalpy is close to return air.

Now: 60F, 40% RH → enthalpy = 19.5 BTU/lb
Dry-bulb controller: "60F > 55F → no economizer" ← WRONG!
8 BTU/lb of free cooling wasted.
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Same temperature, different humidity, opposite decisions needed. Dry-bulb cannot distinguish them.

Hours captured per year 

Climate          Dry-Bulb Hours   Enthalpy Hours   Missed
──────────────────────────────────────────────────────────
Phoenix (dry)         2,800           2,900          100
Charlotte (mixed)     1,400           2,200          800
Atlanta (humid)       1,100           1,900          800
Houston (very humid)    600           1,200          600
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The math 

800 missed hours × 15 tons avg × 1.2 kW/ton × $0.12/kWh
= $1,728/year in compressor energy that was not needed

Enthalpy sensor cost: $300 × 2 = $600
Payback: 4 months
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The rule

In dry climates (Phoenix, Denver): dry-bulb control is fine — humidity rarely matters.

In humid or mixed climates (anywhere east of the Rockies): enthalpy control captures 600-800 more free cooling hours per year. The sensor pays for itself in one season.

The decision is: is outdoor air enthalpy below return air enthalpy? Temperature alone cannot answer that question.

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