If we know the heat exchanger efficiency, we can calculate the supply air temperature after the heat exchanger.
Ext. temp - outside temperature
Exh. temp - temperature of air exhausted from the building (room)

The application allows calculating the heat recovery efficiency in the mechanical ventilation heat exchanger. Enter the appropriate temperatures and the program will calculate the percentage efficiency of the heat exchanger
Ext. temp - outside temperature
Exh. temp - temperature of air exhausted from the building (room)
Sup. temp - Required temperature after the heat exchanger

According to the Regulation of the Minister of Infrastructure of April 12, 2002 on technical conditions that buildings and their location should meet (Dz.U. 2002 No. 75 item 690):

• In general supply and exhaust mechanical ventilation systems or comfort air conditioning with a capacity of 500 m3/h or more, heat recovery devices with a temperature efficiency of at least 50% or recirculation should be used when permissible.
• The EP index of the building cannot exceed 70 kWh/(m²year) (for buildings for which the building permit was submitted after January 1, 2021).

Example efficiencies of different types of heat exchangers:

• Cross-flow heat exchanger: 50-65%
• Counter-flow heat exchanger: ~85% (temporarily up to 90%)
• Rotary heat exchanger: 80-85%