## 5 Conclusions  
This paper demonstrated the wider context of importance of increasing the efficiency of current space heaters connected to closed-loop heat pumps in single-family dwellings. The study was divided into two parts. The main goal of the first part of the study was to map the parameters that mainly control the thermal performance of space heaters commonly used in single-family dwellings. The second part explored in detail the potential of using water supplies of high flow rates in the considered heating systems. Both parts aimed to provide practical insights and guidelines for the design and more efficient operation of heating systems in single-family dwellings. Based on the results, the following can be concluded:  
* The capacity of a heating system to be operated with a water supply below 45 \({}^{\circ}\)C is determined to a large extent by the heating demand of a building. The size, type and efficiency of space heaters are also important factors but the heating demand is the deceive factor for efficient operation of heating systems with low-temperature water supplies. * Well-designed and well-operated low-temperature space heaters in combination with efficient heat pumps may significantly decrease the final energy use of single-family dwellings in cold countries. * The current guideline design values of 100 Pa/m for water-side pressure loss in distribution pipes and \(\leq\)0.01 kg/s for water flow per space heater or heating circuit are set too low and are not appropriate for low-temperature heating systems in single-family dwellings. * The hydraulic pressure loss of 350 Pa per meter pipeline and water flow of 0.035 kg/s per space heater or heating circuit should be used instead of traditional values (100 Pa/m and \(\leq\)0.01 kg/s) for low-temperature heating systems in single-family dwellings. The pumping power need after this increase would be about 0.2-0.6% of generated heat output for distribution pipes of 10-13 mm inner-diameter. * By increasing the water flow rate from 0.01 to 0.035 kg/s per space heater, the heat output from the considered conventional radiator system increased by 25%. This increase improved the efficiency of the considered heat pump (COP) by 6%. An improvement of up to 10% could be achieved by reducing the energy needs for the domestic hot-water and circulation pumps by half. * There is no practical justification to use water flow rates higher than 0.035 kg/s per space heater or heating circuit in heating systems supported by heat pumps in single-family dwellings. * In order to avoid unwanted noise generation, the flow coefficient (K\({}_{v}\)-value) of thermostatic valves should not be lower than 0.45 m\({}^{3}\)/h if a water flow of 0.035 kg/s per heating appliance is to be used. * The hydronic heating systems that operate with small temperature drops (\(\leq\)5 \({}^{\circ}\)C) use significantly less exergy than systems with large temperature drops. It is thus important and environmentally beneficial to create the right preconditions for heating systems to be operated with low-temperature water supplies and high water flow rates. * Based on the conclusions listed above the following steps should be implanted to maximize the potential of low-temperature heating systems. First, the building heat loss should be minimized. Second, the thermal efficiency of space/room heaters should be maximized. Third, if possible, the temperature drop across the heating system should be held constant at about 5 \({}^{\circ}\)C or lower. Fourth, the heat pump connected to the low-temperature heating system should be operated with the dynamic heating curve. conceptualization, A.P., Q.W. and S.S.; methodology, A.P. ; software, A.P. ; validation, A.P., Q.W. and S.S.; formal analysis, A.P. ; investigation, A.P., QW; data curation, A.P. ; writing--original draft preparation, A.P. ; writing--review and editing, A.P., Q.W. and S.S.; visualization, A.P., Q.W. and S.S.; project administration, A.P. ; funding acquisition, A.P.