Hydronic Heating Systems The Effect of Design on System Sensitivity

Doctoral thesis, 2002

This thesis starts from the recognition that a hydronic heating system can be optimised, but can never be totally perfect. Sooner or later, in practice, deviations - caused by one or more components having slightly different characteristics or settings than they are assumed or supposed to have - arise. The aim of this work is to show how system design affects the overall sensitivity to deviations, in terms of the effect on performance and return water temperature. The systems that have been analysed are radiator systems and air heaters controlled by valve groups, both supplied by heat from district heating. In particular, the analysis has been concentrated on differences between high-flow and low-flow systems. Based on fundamental theory in this area, as well as on physical measurements made in test rigs, models have been developed and/or applied in order to investigate system function in the desired manner. The effect of deviations have then been shown and quantified, using results from simulations. The simulations show that thermostatic radiator valves are most effective in low-flow systems. Low-flow systems, too, produce the lowest return temperatures. However, incorrect or changed radiator valve settings can result in substantial increased return temperature and differences in room temperatures in such systems. A direct connection of an air heater (that is without recirculation) presents the least risk of control instability, which means that performance tends to be more stable. In this respect, there is no difference whether the system is balanced for a high flow or a low flow. However, balancing does have a considerable effect on the control performance of valve groups with a recirculation connection and with low-flow systems running a greater risk of instability.