Introduces the control difficulty (CD) method, a new, practical-theoretical approach for controller choice and dynamic controller tuning. Stresses the importance of control system knowledge, the dominating time-constant, time-delay and non-linear system gain. Demonstrates that these values may be found by fairly simple field measurements using the Ziegler and Nicols step-response approach. Gives typical values for air handler heating coils, humidifiers and domestic hot water systems. Introduces the relative control difficulty (D) as a new characteristic parameter. States that by using D, the system to be controlled will be characterised as 'simple', 'medium-difficult' or 'difficult', so that the choice may be made between P, PI or PID control strategies. States that the approach has been applied successfully to several air conditioning installations and some district heating plants in Norway in the last five to ten years. Presents it as a systematic design and tuning method.

KEYWORDS: Controls, calculating, air conditioning, district heating, Norway.