Heat Exchanger Glossary of Terms

Reading a heat-exchanger datasheet is easier once the jargon is fixed. This glossary covers the terms that decide whether a recovery core meets your project, with the numbers a specifier actually uses. For the physics behind these terms see our efficiency deep-dive.

Effectiveness (ε)

Effectiveness is the fraction of the maximum possible heat that is actually transferred: ε = (T_supply_out - T_supply_in) / (T_exhaust_in - T_supply_in) for sensible recovery in winter. A 65% plate core returns 65% of the available temperature difference to the incoming air. It is the single most-quoted rating and the basis of code compliance, but it is only valid at the face velocity it was tested at, so always read it on the datasheet at your operating point, not the catalogue headline.

NTU (Number of Transfer Units)

NTU = UA / (m_dot x c_p), where U is the overall heat-transfer coefficient, A the area, m_dot the mass flow and c_p the specific heat of air (about 1.005 kJ/kg-K). NTU bundles the core size and performance into one dimensionless number; for a pure counterflow core ε = NTU / (NTU + 1), so NTU of 4 already gives 80% effectiveness, while NTU of 1 gives 50%. Designers use NTU to compare cores of different size on equal terms without re-deriving the heat balance each time.

Enthalpy

Enthalpy is the total heat content of moist air, combining the dry-air sensible heat and the water-vapour latent heat. A sensible-only core transfers temperature but not moisture; an enthalpy (desiccant or wheel) core transfers both, expressed as total effectiveness on enthalpy rather than temperature alone. In humid climates the latent term often dominates the plant energy, so "effectiveness" without the sensible/total qualifier is meaningless.

Face velocity

Face velocity is the volumetric flow divided by the core frontal area, typically 1.5 to 3.0 m/s for plates. Effectiveness falls and pressure drop rises as velocity climbs, so a core sized at 2.0 m/s may test at 65% but deliver only 58% at 2.8 m/s. It is the knob that trades recovery against fan power, and the figure you must hold constant when comparing two cores.

Carryover and bypass factor

Carryover is the fraction of exhaust air that leaks into the supply side. A rotary wheel carries 0.5% to 5% of exhaust into supply by purge design; a plate core has effectively zero, which is why separation duties forbid wheels. The bypass factor is the share of air that skips the core through leaks or an open damper, eroding both recovery and the commissioning test.

Frost point

The frost point is the exhaust temperature at which condensation on the plate freezes. For typical indoor exhaust at 22 degrees C and 40% RH leaving into -10 degrees C outdoor air, the exhaust-side surface drops below 0 degrees C and ice forms, blocking the core. Frost control (preheat, exhaust recirculation or automatic bypass) keeps the surface above freezing and is mandatory in cold climates. Pair this glossary with the basics article and the materials guide when you shortlist a core.

Related reading