SectionsHeat-recovery air handling units
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Heat-recovery air handling units

Recovery transfers energy between extract and incoming air. Choose the transfer method together with air quality, weather, pressure losses and operating modes.

Principle and scope

Heat recovery uses a temperature or moisture difference between extracted and incoming air to reduce treatment demand. A plate device exchanges energy across separating surfaces; a rotor transfers it through a rotating matrix; a run-around system uses two coils and a pumped liquid loop. Moisture transfer depends on the selected technology.

Compare performance at the same airflows and conditions, plus leakage or carry-over (extract air retained in the rotor and transported into the supply path), pressure losses, pump/fan energy, bypass and frost strategy. A high recovery percentage is not the same as annual energy savings or adequate supply temperature.

Compare recovery technologies

Plate recovery keeps nominal air passages separate; verify leakage and whether the selected material transfers moisture. Rotary recovery needs assessment of seal leakage, carry-over and any purge sector (a rotor segment flushed with clean air to reduce retained extract air carried into the supply path). Run-around coils separate the air paths spatially but add pump energy and liquid-side maintenance. None of these names alone fixes efficiency or cross-contamination performance.

Related components: Liquid-to-air coil

Calculate a temperature-change duty for one air stream. Recovery efficiency, leakage, frost protection and the second stream remain outside this calculation.

Calculate sensible air load

Sensible air load calculation

Calculate the heat rate associated with a specified temperature change in one air stream. Enter all five values for your stated operating conditions.

Mass flow ṁ = ρ × V̇ / 3600; temperature change ΔT = Tout − Tin; sensible heat rate Q̇ = ṁ × cp × ΔT. With the units below, the result is in kW. Positive Q̇ means heating; negative Q̇ means cooling. Zero flow or zero temperature difference gives zero sensible load.

For a steady, single-phase air stream with constant user-supplied properties. The balance excludes latent heat, moisture changes, phase change, shaft work and kinetic/potential energy changes. It does not select a fan, coil or complete AHU.

Topic for the local draft

Opening a page does not select anything. A topic choice records context only; no product or configuration is selected.

Operating inputs

Use density at the same state and volume basis as the entered airflow. Enter cp in kJ/(kg·K), not J/(kg·K). Density and cp are editable assumptions; no universal values are prefilled. Check that your temperatures and properties describe gas-phase air. This tool does not check phase boundaries or property-table consistency.

Interactive calculation requires JavaScript. The formula, units, scope and reference information remain available here for manual use.

Frost, bypass and summer operation

Frost protection may use preheating, bypass or a controlled defrost sequence; assess its effect on delivered airflow and energy. A bypass routes air around recovery when that improves the required supply state. In summer, cooler extract air can precool hotter outdoor air. Compare annual modes rather than applying one winter efficiency to every hour.

Extract-air conditions

The recovery choice begins with what the extract air contains. Avoid assuming that an energy benefit permits any contact or leakage between streams. For separated extract and supply equipment, evaluate a run-around circuit together with pump energy, access and coil contamination.

Related components: Liquid-to-air coil

Supply and extract

A two-stream recovery unit is selected at both airflows, both inlet conditions and the required supply state. Unequal flows and leakage can change the reported temperature performance. Confirm the performance definition, fan location and control sequence before comparing units.

Related components: Fan

Compact

For compact recovery equipment, compare complete installed dimensions and service clearance as well as rated efficiency. Ceiling installations need usable access and drainage. Verify that the selected mounting orientation preserves condensate collection and the approved frost strategy.

Related components: Condensate drainage

Commissioning and service

Trend both inlet temperatures, flows and recovery control state. A low measured temperature gain may result from bypass, frost protection, imbalance, dirty surfaces or sensor placement. Identify the active mode before diagnosing a failed exchanger. Retain the selected efficiency definition and test conditions.

Back to section: Heat recovery

A guide to equipment and selection; the final configuration and performance require project-specific confirmation.