## 4 Discussion  
The literature review conducted in this study has revealed that the classification of hydronic systems according to their operational temperatures is arbitrary, and has been constantly changing over time. It seems, however, that since around the year 2000, there has been a general agreement that heating systems that are able to cover heat loss with water supplies of 45 \({}^{\circ}\)C and below should be regarded as low-temperature heating systems. It is, however, still unclear which outdoor temperature should be used when considering the heat loss; that is, whether the heat load should be considered at the design, at the mean winter, or at the mean annual outdoor temperature. A common perception among today's heating, ventilation and air conditioning (HVAC) engineers is that thermal benefits of using high water flow rates (higher than 0.01 kg/s per appliance/circuit) in heating systems may be outweighed by the increased power needed for the circulation pumps and the cost increase for the distribution pipes. In this study, it was proved that this common conviction  
Figure 9: The pressure loss over a thermostatic valve and a conventional radiator at different water flow rates for four K\({}_{\rm v}\)-values. does not hold for well-designed hydronic systems adapted for operation with high water flow rates. Figure 10 shows variation of optimum pipe diameter with water flow rates between 0.01 kg/s and 0.035 kg/s. As seen, economic pipe diameter for 0.035 kg/s is approximately 8.5 mm. This value is within \(\pm\) 1.5 mm interval of pipe diameters used in this study (7.7 mm and 10 mm). This proves that pipe diameters commonly used for water distribution in current hydronic systems can also be used for high-flow systems. Figure 11 demonstrates the change of COP of the analyzed GSHP with increased water flow rate and supply water temperature in a hydronic system. Not surprisingly, COP is mostly affected by the level of supply water temperature of the heating system. A reduction of the supply temperature from 50 \({}^{\circ}\)C to 30 \({}^{\circ}\)C would improve the COP by a factor of 1.4. Moreover, COP would increase by 14% by reducing the temperature drop from 30 \({}^{\circ}\)C to 5 \({}^{\circ}\)C in a heating system with a supply temperature of 50 \({}^{\circ}\)C. The corresponding increase would be 13% in a heating system with a supply temperature of 40 \({}^{\circ}\)C if the temperature drop would be decreased from 20 \({}^{\circ}\)C to 3 \({}^{\circ}\)C. This shows that heating systems supported by domestic heat pumps should be operated with low-temperature water supplies and high water flow rates. Figure 11: Variation of COP with temperature drop across a heating system in a single-family dwelling. Maximum water flow rate 0.035 kg/s per heating appliance. Figure 10: Change of economic optimum pipe diameter with water flow rate according to [51; 52]. The evidence presented in this study indicates that well-designed low-temperature heating systems may significantly decrease the energy use in a single-family dwelling. It showed that the use of floor heating instead of conventional radiators in a typical Scandinavian single-family dwelling would increase the efficiency of a closed-loop heat pump by about 17%, and that the use of baseboard radiators instead of conventional radiators would increase the heat pump efficiency by about 7%. It was shown that the capacity of a heating system to be operated with water supply lower than 45 \({}^{\circ}\)C is determined to a large extent by the heating demand of a building. The thermal insulation of a building and the efficiency of heat recovery from the used ventilation air are thus key factors for a hydronic heating system to be operated with low-temperature supply. It was revealed that the commonly used design guideline values of 100 Pa/m for water-side pressure loss in distribution pipes and 0.01 kg/s for water flow rate per heating circuit are not suitable for low-temperature heating systems in single-family dwellings.