Drawing No. EH–TH–017 // Thermal Engineering & HVAC
Heat Recovery Effectiveness Calculator
Reviewed August 2026
Find the sensible effectiveness of an air-to-air heat recovery device (heat wheel, plate exchanger, run-around coil) from measured supply and exhaust temperatures, and the resulting heat recovered.
What problem does this solve?
Air-to-air heat recovery devices transfer heat from outgoing exhaust air to incoming fresh outdoor air (or vice versa in summer), reducing the heating or cooling load that would otherwise fall entirely on the HVAC system. This calculator estimates sensible effectiveness from measured temperatures and both airstream flow rates, accounting for unequal heat-capacity rates instead of assuming they are identical.
Inputs
Results
Background
For unequal airstreams, sensible effectiveness is best written as ε = Q̇actual/Q̇max, where Q̇max = Cmin(Thot,in − Tcold,in) and C = ṁcp. The actual recovered heat is calculated from the supply-side temperature rise. This reduces to the simple temperature-ratio form only when the relevant heat-capacity rates are equal.
Q̇actual = Csupply(Tsupply,out − Toutdoor,in). With constant air properties, C is proportional to airflow. The exhaust outlet temperature is then estimated from the same heat balance, Q̇actual = Cexhaust(Texhaust,in − Texhaust,out).
Fixed-plate heat exchangers commonly achieve 50–70% sensible effectiveness; rotary heat wheels often reach 70–85%, and some can exceed this with enthalpy wheels recovering latent heat too; run-around coil loops (two coils connected by a pumped glycol loop, useful when airstreams can't be adjacent) typically show lower effectiveness, often 45–65%, due to the extra heat-transfer step involved.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
No — some approach temperature is always needed to drive heat transfer, and no real heat exchanger reaches the theoretical maximum. Effectiveness in the 60–85% range is typical for good commercial heat recovery devices; anything reported near 100% should be checked carefully for a measurement or definition error.
Sensible effectiveness uses temperature alone, which fully captures dry heat exchange but not moisture transfer. Enthalpy (energy) recovery ventilators also transfer moisture between airstreams, requiring humidity ratio measurements on both sides to compute total effectiveness — this calculator covers the sensible-only case, common for simpler heat-recovery ventilators (HRVs) without moisture transfer.
Field-measuring the three temperatures and computing effectiveness is a practical way to verify a heat recovery device is actually performing to its rated effectiveness after installation, or to check for degraded performance (fouling, bypass damper stuck open, wheel not rotating) during commissioning or troubleshooting.