Modelling and optimization of different hydraulic schemes of a heating appliance for domestic hot water (DHW) comfort and efficiency

In this study, dual function heating appliances, commonly known as combi boiler type heating appliances, have been investigated. Combi boilers are used for both space heating and cold usage water heating and generally use natural gas as their energy source. The hot water requested by the users is called as domestic hot water (DHW). Throughout this study, only DHW supply function of a combi boiler type heating appliance has been investigated from the comfort and efficiency points of view. Firstly, water heating appliances have been categorized generally. Gas-fired integrated space/water heating appliances which are one branch of this classification have been divided into different groups by considering the differences in their DHW supply function. From these groups, standard combi boilers and primary DHW concepts have been chosen for the efficiency and comfort comparison. Later on, a mathematical model of the standard combi boilers has been employed by taking all system parameters into consideration in terms of their DHW supply function. 1D energy equations of the heat exchangers in a standard combi boiler have been established using thermodynamic laws and solved simultaneously in Matlab. The numerical results obtained from the theoretical analyses have been compared with the experimental results and the constructed mathematical model has been verified for a standard combi boiler. After experimental verification, a similar mathematical model has been established for the primary DHW concepts. Through the constructed mathematical models, efficiency values and comfort levels of the appliance concepts have been compared theoretically. Besides, the effects of some common parameters on the system behavior have been investigated and optimized for each appliance concept. Finally, in the primary DHW concepts, DHW outlet temperature reaches the steady-state temperature more rapidly and they have higher efficiency in addition to lower wasted energy/useful energy ratio when compared to the standard combi boilers. Keywords: Water heating appliances, combi boiler type heating appliances, domestic hot water (DHW), DHW comfort, DHW efficiency, mathematical modelling