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)
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 ✓
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.
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
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
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.
