ARI Standard 1060 — 2000, Rating Air-to-Air Energy Recovery Ventilation Equipment, provides the following equation to describe the effectiveness of an air-to-air heat exchanger used to recover energy:

where:

e = sensible, latent, or total effectiveness

x1 = dry bulb (°F), humidity ratio (gr/lb), or enthalpy (Btu/lb) of entering supply air

x2 = dry bulb (°F), humidity ratio (gr/lb), or enthalpy (Btu/lb) of leaving supply air

x3 = dry bulb (°F), humidity ratio (gr/lb), or enthalpy (Btu/lb) of entering exhaust air

x4 = dry bulb (°F), humidity ratio (gr/lb), or enthalpy (Btu/lb) of leaving exhaust air

ws = mass flow rate of supply air

we = mass flow rate of exhaust air

wmin = the smaller mass flow rate (supply or exhaust air)

For a given heat-exchanger geometry, can be determined at test conditions for various flow rates. Generally, as the airflow rate decreases, increases but the overall energy transfer decreases.

How much heat will a total-energy recovery device transfer? The answer lies in this equation:

where:

Qt = total heat flow, Btu/h

Vmin = the smaller airflow (supply or exhaust), cfm

h1 = entering supply-air enthalpy, Btu/lb

h3 = entering exhaust-air enthalpy, Btu/lb

For sensible heat transfer,

where:
Q_s = sensible heat flow, Btu/h
t₁ = entering supply-air dry bulb, °F
t₃ = entering exhaust-air dry bulb, °F 

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