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Temperature after heat recovery
Heat Recovery Efficiency
Heat recovery calculator — heat exchanger efficiency and supply air temperature
Professional calculator for analysing heat recovery in ventilation units. It computes the temperature efficiency of cross-flow, counter-flow, rotary or other heat exchanger types from three measured air temperatures, and conversely the supply-side air temperature behind the exchanger when the catalogue efficiency is known. A tool for ventilation designers, heat-recovery installers and anyone analysing the energy performance of HVAC systems.
Two calculation modes
"Compute efficiency" mode — enter three temperatures: outdoor (Tout), exhaust air leaving the building (Texh), and the supply air after the exchanger (Tsup). The calculator returns the temperature (sensible) efficiency of the heat recovery in percent.
"Compute supply temperature" mode — when the catalogue efficiency of the unit and Tout, Texh are known, the calculator returns the air temperature behind the exchanger, i.e. the temperature at which the air enters the post-heater (if any) or the rooms directly.
The results can be used to size a post-heating coil, evaluate the case for upgrading to a higher-class heat recovery unit, or estimate annual heating energy savings.
Sensible temperature efficiency formula
The temperature (sensible) efficiency of a heat recovery exchanger is described by the classic balance equation, assuming equal and steady airflows on both sides of the exchanger and negligible casing losses:
η = (Tsup − Tout) / (Texh − Tout) × 100%
where: η — temperature efficiency [%], Tsup — supply air temperature behind the exchanger [°C], Tout — outdoor air temperature drawn from the intake [°C], Texh — exhaust air temperature from the rooms (typically 20–22 °C). The formula applies only to sensible heat recovery — for enthalpy exchangers with moisture recovery, total efficiency is higher and requires additional psychrometric analysis.
Temperature efficiency varies with operating conditions. In winter, condensation of water vapour occurs on the exhaust side — the released latent heat lifts the actual recovery efficiency above the value calculated from temperatures alone. On the other hand, the need for periodic defrosting of a cross-flow exchanger (bypass system, preheater or intermittent operation) reduces seasonal efficiency.
Typical heat recovery efficiencies
The catalogue efficiency of a heat recovery unit depends strongly on exchanger design and air velocity. The values below are typical for balanced flows (Vsup ≈ Vexh):
- Cross-flow plate exchanger: 50–65% — the simplest and cheapest design, popular in smaller comfort units and compact residential heat-recovery devices.
- Cross/counter-flow plate exchanger: 75–92% — the most common choice in modern residential and office HRV units. Top models for passive-house applications exceed 90% efficiency.
- Rotary exchanger (regenerator): 75–90% — high efficiency, additionally allowing moisture recovery in the hygroscopic (enthalpy) version. Standard in large public-building units.
- Glycol run-around loop (two coils linked by a glycol circuit): 45–60% — used when supply and exhaust ducts are physically separated and a single shared exchanger cannot be installed.
- The annual (seasonal) efficiency tends to be 5–15 percentage points lower than the catalogue nominal value — mainly due to periodic defrosting and unbalanced flows. When estimating energy savings it is best to work with the seasonal value, not the peak figure.
- Eurovent / PHI certification — passive-house certification by the Passivhaus Institut requires temperature efficiency ≥ 75% with Δp ≤ 250 Pa on the air side. The H1 class (highest) requires η ≥ 80%.
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