Since most gas water heaters on the market still use diffusion or partially premixed combustion. Fully premixed combustion, with its advantages of complete combustion and low CO and NOx emissions, offers a promising alternative. In response to this, a slit-type fully premixed gas water heater with a rated power of 20 kW was designed. The combustion and heat transfer performance, as well as the combustion temperature and burnout characteristics, were analyzed under ten different load conditions ranging from 100% to 8% using theoretical modelling. The study identified the variations in physical parameters and used numerical simulations to partially validate the theoretical results. Subsequently, by individually modifying structural variables, the optimal structure under single-variable conditions was determined. This optimal structure was then combined in various configurations to form a multi-variable structural design. Further theoretical analysis was conducted to assess whether the changes in the multi-variable structure, as well as the original design, met economic efficiency requirements. The results showed that the slit-type fully premixed burner achieved optimal performance when the number of burner plates was 120, with a length of 99 mm and a height of 10 mm.