Abstract

Condensing gas boilers achieve high efficiency by recovering latent heat from flue gases. However, in residential buildings with multi-zone radiant floor heating systems, intermittent operation of individual zones can create partial-load conditions that affect boiler performance. This study experimentally investigated the efficiency of a condensing gas boiler in a residential building equipped with a multi-zone radiant floor heating system. Two control methods were analysed: constant water temperature (CWT) control and outdoor reset (OR) control. The results showed that OR control generally improved boiler efficiency compared with CWT control. However, under low-heating-load conditions, when only a few heating zones were operating, the heating demand frequently fell below the minimum boiler capacity leading to short cycling. During these periods, frequent on–off operation considerably reduced boiler efficiency. When short cycling did not occur, the average boiler efficiency increased from approximately 88% under CWT control to approximately 92% under OR control. However, when short cycling occurred under low-heating-load conditions, frequent on–off operation reduced the average boiler efficiency to approximately 79%. Owing to variations in supply water temperature and the minimum firing capacity of the boiler, short cycling may occur depending on operating conditions, leading to reduced boiler efficiency. Therefore, further study on control strategies or system configurations is required to mitigate this effect.