1 Influence of recirculation strategies in collective heat distribution system on the performance of dwelling heating substations Julio Efrain Vaillant Rebollar 1 , Eline Himpe 1 , Jelle Laverge 1 and Arnold Janssens 1 1 Ghent University, Department of Architecture and Urban Planning, Belgium julioefrain.vaillantrebollar@ugent.be The aim of this study is to assess the influence of different recirculation control strategies in collective heat distribution system on the performance of dwelling heating substations and network heat losses, so that decision maker can potentially identify the optimal operational conditions of this system component. To that aim, six different heating substation models are set up (using TRNsys) for investigation of the energy-efficiency and comfort issues. Regarding control strategy the effects of different recirculation methods: continuous and constant, centralized and temperature controlled and customer unit controlled are examined. Different types of substations such as storage tanks either equipped or not equipped for in-situ hot water preparation, interaction between space heating and domestic hot water circuits as well as, direct or indirect connection of dwelling space heating system are also investigated. Temperature drops in supply pipes of the distribution network during summer months due to low heat demands of consumers and heat losses at each scenario are compared. The first results indicate that the design concept of the substation in relation with the actual operational conditions has an important impact on the energy performance of the entire system. Keywords: Water recirculation in heat system, District heating substation, Network heat losses INTRODUCTION Collective heat distribution systems at the level of building or communities (district heating) is seeing more and more as an effective solution towards sustainability in the heating sector. In this context, during the last years it has been demonstrated that low-temperature district heating (supply temperatures lower than 65°C) is the next evolution in district heating systems [1]. Low- temperature district heating system aims to reduce heat losses from network as much as possible, meanwhile maintain or improve level of comfort for users. Reduction heat losses in the distribution network clearly contributes to the energy-efficiency of the entire system. While, domestic hot water comfort is considered in function of temperature of tap water delivered and the time required for domestic hot water (DHW) to reach a fixed temperature level after tapping was started, the so called waiting time. In addition to the level of DHW comfort in terms of temperature and waiting time, attention should be paid to the general guides for legionellae control in DHW-systems. The hygienic requirement for heating of DHW due to recent standards is 50°C for single-family houses and 55°C for multi-family buildings [2]. Together with comfort surveys, these requirements are based on the need to avoid legionella growth in DHW pipes and storage tanks. It should be mentioned that requirements to produce DHW with temperature higher than 50°C is especially more important for collective heat distribution system (CHDS) with old design approaches containing vertical riser, branched pipes with bigger diameter (increasing water volume of the distribution circuit), or centralized bigger storage tank. In new and/or renovated buildings, heat system are designed with reduced pipe diameter, defined by requirements for noise propagation and pressure drop, separates connection of DHW pipes between each tap and source of DHW and in general the length of the DHW-pipes in the dwellings is shorter than 5m and the volume is lower than 3L, in order to avoid that water at unsafe temperatures remain still for a long time in the distribution circuit [3;14]. Although, growth of legionella in DHW system is not in focus of this paper, the risk of increases legionella growth in the range of temperature between 20°C and 42°C was taken into account. For readers interested in this topic, more comprehensive discussion regarding DHW systems and legionella issues can be found in [4;5;6]