Step 1 – Known airflow
Start with the supply/exhaust airflow Q (m3/h). For balanced AHUs, Q_supply approx Q_exhaust.
Step 2 – Temperature difference
Find the outdoor design temperature T_out and the exhaust (room return) temperature T_exp. The available temperature lift is DeltaT = T_exp – T_out.
Step 3 – Effectiveness
Choose an effectiveness epsilon (typical plate 0.60-0.75, rotary 0.75-0.85). Recovered temperature into supply air: T_rec = epsilon x DeltaT.
Step 4 – Heat recovered
Q_rec [W] = rho x c_p x Q x T_rec, where rho=1.2 kg/m3 and c_p=1005 J/(kg.K). Convert Q from m3/h to m3/s (divide by 3600).
Step 5 – Pressure drop & fan cost
Every Pascal of recovery-device pressure drop raises fan power: P_fan [W] = Q[Pa] x Q[m3/s] / eta_fan. Balance effectiveness against fan energy using our pressure-drop guide.
Step 6 – Payback
Annual energy saved = Q_rec x operating hours x (1 – penalty) x energy price. Compare against first cost and the payback method. Pick the device type from selection by application.
Worked example
Q=10000 m3/h, T_out=-10C, T_exp=22C, epsilon=0.70 -> T_rec=22.4C, Q_rec approx 75 kW. At 4000 h/yr and 0.10 EUR/kWh that is about 30,000 EUR/yr saved.
Effectiveness versus pressure drop
The calculation above maximizes recovered heat, but a higher-effectiveness core usually costs more pressure drop, and that pressure drop is paid continuously by the fan. The true optimum is not the highest epsilon but the point where the value of extra recovered heat equals the extra fan energy it demands. Run the fan-power term in Step 5 for two or three candidate cores before choosing.
Part-load and real operation
Design-day calculations flatter a device; most hours are at part load. Check how effectiveness and pressure drop behave as airflow falls, and whether the control modulates the core or simply bypasses it. A unit that performs well only at full flow may under-deliver across the annual load profile, hurting the payback estimate.
Verifying the installed result
After commissioning, measure the actual supply-temperature rise and pressure drop and compare them to the calculation. Differences reveal fouling, bypass leakage, or a mismatch between assumed and real airflow. Re-running the steps with measured values closes the loop between design intent and delivered performance, and supports any performance guarantee.
Frequently Asked Questions
How do I calculate heat recovered in an AHU?
Use Q_rec equals rho times c_p times airflow times effectiveness times the temperature difference, with rho 1.2 and c_p 1005, converting cubic metres per hour to cubic metres per second.
Should I maximize effectiveness?
Not always. Higher effectiveness often means more pressure drop and fan energy, so optimize the trade-off rather than chase the highest number.
What fan-power formula applies?
Fan power equals pressure drop times airflow divided by fan efficiency. Balance this against the heat recovered when choosing a core.
How do I verify installed performance?
Measure the actual supply temperature rise and pressure drop and compare them to the calculation. Trend for fouling or bypass leakage.