In theory, the new control technology can satisfy the most demanding requirements for indoor climate and operating cost. In practice, however, not even the most sophisticated controllers always perform as promised. As a result, comfort is compromised and operational costs are higher than expected. This is often because the mechanical design of the HVAC plant does not meet some conditions necessary for stable and accurate control. Three important conditions are:

• the design flow must be available at all terminals,
• the differential pressure across the control valves must not vary too much, and
• flows must be compatible at system interfaces.

This paper discusses these conditions, problems resulting from violation of the conditions, why the problems occur, and how to prevent them at the design stage.

Six residential buildings were investigated in Hungary. The number of flats in each building was between 20 and 354. The heating one-pipe and two-pipe systems were reconditioned. This paper compares the indoor temperature and the heat consumption before and after the reconditioning. Not only was building comfort much better, but the heat consumption also decreased.

Units: SI