Short Answer

Heat recovery efficiency, more precisely called effectiveness, is the percentage of the maximum possible heat that the exchanger transfers from the outgoing airstream to the incoming airstream.

Why a Percentage

A exchanger can never reach 100 percent, because the supply air would have to leave at the exact temperature of the exhaust air, which breaks the thermodynamic gradient. Real devices land between 50 and 85 percent depending on type and size.

Two Numbers to Know

• Sensible effectiveness - dry-bulb temperature recovery only.
• Latent (enthalpy) effectiveness - temperature plus moisture recovery.

What It Means for You

A 70 percent effective unit removes 70 percent of the heating (or cooling) load that ventilation would otherwise impose. That is the single number that drives your energy saving and payback.