Every air heating system has its strengths and weaknesses. In order to ensure it works as efficiently as possible, it is crucial that the system be designed to meet the users own specific needs. Directly heated Indirectly heated Warm air heater (circulating air systems) Directly heated fresh air units PWW Air Heater Installation efficiency ++ ++ 0 Investment/ Installation costs ++ + 0 Maintenance effort + 0 0 System dynamics ++ ++ - Comfort - + + Danger of freezing ++ ++ - Ventilation options + ++ ++ Regulation options - + ++ Noise development - 0 0 Use in explosion hazardous rooms - ++ 0   ++ very advantageous   + advantageous   o average   - not so good E-Mail, address, route Heat technology Process technology Directly-heated fresh air unit  Warm air heaters with condensation technology Control systems Selection criteria