HVAC & Heat Recovery Glossary: 30 Terms Explained

A quick-reference dictionary for the terms on an air-to-air heat exchanger datasheet. Each links to a deeper guide where one exists.

Performance

Effectiveness (ε): the share of recoverable heat actually transferred. 80% means 80% of the theoretical maximum.
Sensible effectiveness: recovery of dry-bulb temperature only (HRV).
Enthalpy effectiveness: recovery of total heat including moisture (ERV).
NTU: number of transfer units, UA divided by the smaller stream heat capacity — the true size metric. See the ε-NTU tool.
Capacity ratio (C_r): C_min/C_max; 1.0 when both streams are equal.
U-value: overall heat-transfer coefficient, W/m²K.

CFM: cubic feet per minute (U.S. airflow). Convert with our CFM–m³/h tool.
Face velocity: air speed through the core face; sets size vs pressure drop.
Humidity ratio: kg water vapour per kg dry air — the right way to express moisture.
Dew point: temperature at which air starts condensing; key for frost.
Enthalpy: total heat content of moist air (temperature + humidity).

ERV: energy recovery ventilator — transfers heat and moisture.
HRV: heat recovery ventilator — heat only.
Rotary / thermal wheel: a spinning matrix that carries heat (and moisture) between streams.
Plate (fixed-plate): stationary plates; counterflow or crossflow.
Heat pipe: passive two-phase device, no moving parts, no cross-contamination.
Run-around coil: two coils linked by a fluid loop — used when streams are far apart or must not mix.

Pressure drop: resistance the fan must overcome; higher recovery often means more drop.
Carryover: leakage of supply into exhaust (or moisture migration) in a wheel.
Fouling: dust or film on surfaces that blocks heat transfer.
Frost: frozen condensate on the cold side in winter (see frost tool).
Bypass: air skipping the core through leaks — wastes capacity.

DOAS: dedicated outdoor air system — often paired with recovery.
AHU: air handling unit.
Free cooling: using cool outdoor air (or exhaust) to shed heat without a compressor.

Send the term and your duty — we will translate it into a size and a recommendation.