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Heat recovery efficiency calculator

Enter the four air states around an air-to-air heat recovery device (outdoor, supply, room/extract and exhaust) and get the EN 308 supply-side and extract-side temperature efficiency, plus recovered heat power cross-checked from both sides.

EN 308 temperature efficiency Two-sided energy-balance cross-check Free, no sign-up

Heat recovery data

Inputs only — every result is on the right.

How do you want to work?

Airflow needed in both modes, for recovered heat power

Recovery type

Outdoor air supply in, before recovery

Supply air after recovery, into building

Extract air from room, before recovery

Exhaust air after recovery, to outdoors

Estimate annual energy & cost savings
Altitude

Heat recovery results

Air state at each point

Processes supply and extract, each as one sensible/latent transformation

Temperature efficiency EN 308

Supply-side, ηt,supply
—
Extract-side, ηt,extract
—

Recovered heat power cross-checked both sides

From supply-side gain
—
From extract-side loss
—

Condensation/frost risk inside the exchanger

Coldest surface temperature
—
Extract air dew point
—
Margin above dew point
—
Estimated condensate rate saturation-limited upper bound
—

Link copied — it reopens with these exact inputs.

How to use this calculator

  1. Pick the mode and device type.
  2. Enter supply and extract airflow.
  3. Enter temperature (and humidity, for enthalpy/latent recovery) at all four air states — outdoor, supply, room/extract and exhaust.
  4. Optional: include a savings estimate.
  5. Read the EN 308 temperature efficiency and the recovered heat.

What this calculates

Air-to-air heat recovery devices in an AHU (plate exchangers, heat pipes, rotary wheels, run-around coils) are rated by temperature efficiency — what fraction of the maximum possible temperature approach between outdoor and room air the device actually achieves. This calculator implements the standard EN 308 (Heat exchangers — Test procedures for establishing performance of air-to-air and flue gases heat recovery devices) definition, the same one used throughout the AHU industry and on manufacturer datasheets:

ηt,supply = (ts2 − ts1) / (te1 − ts1) supply-side temperature efficiency ηt,extract = (te1 − te2) / (te1 − ts1) extract-side temperature efficiency s1 = outdoor (supply in) s2 = supply air out (into building) e1 = extract air in (from room) e2 = exhaust (extract air out, to outdoors)

The two ratios are only equal when supply and extract mass flows match exactly — with unbalanced flows (a common real-world case) they diverge, which is exactly why EN 308 reports both rather than a single number, and why this calculator does too.

Recovered heat power — cross-checked both sides

Recovered heat power is calculated independently from each side — how much heat the supply air gained, and how much heat the extract air gave up — and shown side by side on purpose. For a leak-tight device with no bypass air, simple energy balance says these two numbers should match; a large mismatch is flagged, and usually means casing leakage, bypass air, or that the four temperatures weren't all measured at the same steady operating point. This is the same "compute it two independent ways and compare" approach already used elsewhere on this site (e.g. the splitter silencer designer's VDI-vs-general-model cross-check) — not a fitted correction, just arithmetic that has to agree if the inputs are self-consistent.

Total-energy (enthalpy) efficiency

A purely sensible device (plate, heat pipe, most run-around coils) only exchanges temperature. A rotary enthalpy wheel also transfers moisture between the air streams, so its real performance needs the same EN 308 ratio structure applied to specific enthalpy instead of temperature — selecting "Total energy" asks for the relative humidity at all four points and computes this the same way, reusing the exact psychrometric engine (Arden Buck + barometric formula) already used and verified by this site's Mollier (h-x) diagram calculator and condensation risk calculator — no new psychrometric physics here, only the EN 308 ratio structure applied to it.

Annual savings estimate

An optional, simple estimate: the average of the two recovered-power cross-check figures, multiplied by your entered operating hours and energy price. It assumes constant recovered power for every operating hour, which a real system won't have (outdoor temperature varies through the year) — treat it as an order-of-magnitude figure for comparing options, not a substitute for a proper bin/degree-day energy simulation.

Temperature and enthalpy efficiency use the EN 308 ratio definitions, standard throughout the AHU/heat-recovery industry. Recovered heat power uses a simple volumetric-flow × density × specific heat (or enthalpy) calculation, using combined moist-air density for the mass-flow conversion on both sensible and total-energy paths — a minor simplification versus separating out dry-air mass flow specifically. It does not model frost formation/defrost cycling (a real limiting factor for air-to-air recovery at low outdoor temperatures), device-specific correction factors, or condensation within the exchanger itself. The annual savings estimate assumes constant operating conditions, not a real annual load profile. Provided for engineering guidance — verify against manufacturer-tested performance data (per EN 308 or EN 13141-7) for critical designs.