Paritian

Heat & Fluids

Heat Recovery Ventilation Calculator (Supply Temperature and Energy Saved)

Temperature of the fresh air leaving a heat-recovery ventilation unit, the heat it recovers and the heating still needed, and the efficiency from a measured temperature.

Results

Supply air temperature after the unit 16.8 °C
Heat recovered 1.69 kW
Heating needed without recovery 2.11 kW
Heating still needed with recovery 0.42 kW
Efficiency from the measured temperature 80.0 %
How it changes
Supply air temperature after the unitTemperature efficiency of the unit 0 – 100. Supply air temperature after the unit: 0 → 21Supply air temperature after the unit0510152025020406080100Temperature efficiency of the unit (%)

The dashed line marks the value you entered.

What this tool does

A heat-recovery unit hands the warmth of the stale air leaving the house to the fresh air coming in, without mixing them. Its temperature efficiency is the share of the temperature gap it closes, so at 80 % with 21 °C indoors and 0 °C outside the fresh air arrives at about 17 °C. Air carries 0.335 W per m³/h for each degree, so 300 m³/h of fresh air at 21 °C above the outside temperature needs about 2.1 kW to warm. This is sensible heat only: units that also recover moisture, and condensation or frost on the exchanger, change the picture.

Formula

η = (T_supply − T_outdoor) ÷ (T_extract − T_outdoor) · P = ρ c_p × V̇ × ΔT = 0.335 × V̇(m³/h) × ΔT (W)

Variables

SymbolMeaningUnit
qVentilation airflowm³/h
tqOutdoor air temperature°C
tiExtract air temperature (room)°C
efTemperature efficiency of the unit%
tmMeasured supply air temperature (to check the efficiency)°C
TSSupply air temperature after the unit°C
PRHeat recoveredkW
PWHeating needed without recoverykW
PLHeating still needed with recoverykW
EMEfficiency from the measured temperature%

Worked example

  • Ventilation airflow300 m³/h
  • Outdoor air temperature0 °C
  • Extract air temperature (room)21 °C
  • Temperature efficiency of the unit80 %
  • Measured supply air temperature (to check the efficiency)16.8 °C
  • Supply air temperature after the unit16.8 °C
  • Heat recovered1.69 kW
  • Heating needed without recovery2.11 kW
  • Heating still needed with recovery0.42 kW
  • Efficiency from the measured temperature80.0 %

Limitations

  • The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
  • The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.