Heat recovery can add heating energy from the extract air to the supply air which would otherwise be wasted. This drastically reduces the overall energy demand of the plant. Particularly since the EnEV09 (Energy Saving Ordinance 2009) was passed, heat recovery systems have been increasingly used and their efficiency continually improved. Since the introduction of this ordinance and the amended EnEV14, heat recovery systems have been compulsory in new air handling units with airflow rates of 4000 m³/h or higher, and must comply with at least heat recovery class H3. The heat recovery classes are staggered as follows (relative to the balanced mass flow rates of supply and extract air):

H1 >= 71 %
H2 >= 64 %
H3 >= 55 %
H4 >= 45 %
H5 >= 36 %
H6 >= no requirement

This classification corresponds to EN 13053:2012.

Heat recovery can draw on different systems which may be utilised depending on application. The following are mainly used in air handling technology:

• Run-around coil systems
  + 100 % separation of air streams
  + low space requirement
  + separate installation of supply air and extract air units possible
  + can be used with contaminated extract air, such as air with chemical contaminants or subject to ATEX protection
  - high levels of efficiency but high investment costs
  - additional energy consumption through pump operation
• Cross current or countercurrent plate heat exchangers
  + low levels of investment
  + 99.9 % separation of air streams
  + operation possible without additional energy consumption
  - generally more space required
  - high heat recovery only with lesser airflow rates or extremely high space requirement
• Thermal wheel heat exchangers
  + high levels of efficiency
  + humidity recovery possible
  - can only be used with permissible recirculation air due to leakage between supply air and extract air streams

A specific system should be determined for each application after weighing up the various pros and cons.

Naturally, all systems can also be used in combination with an adiabatic extract air humidifier. This form of humidification enables the utilisation of the heat recovery systems in summer to cool the supply air with little energy expenditure. This system utilises the adiabatic evaporation cooling of water in order to cool the extract air, thereby achieving a greater temperature differential between the outdoor air and extract air.