## 4 Discussion
It was shown that the pumping power need would be about 0.6% of the emitted heat output for a floor heating system with distribution pipes of 10 mm, even if the pressure loss was about 3.5 times higher than the currently recommended guideline value (100 Pa/m). For a pipe diameter of 13 mm the pumping need to heat output ratio would be approximately 0.2%, even when the water flow is 3.5 times higher than commonly used in nowadays hydronic system (0.01 kg/s per circuit or space heater). On the other hand, this increase of water flow would increase the heat output from the considered floor heating system by 38%. It was also found that a water flow rate of 0.035 kg/s per appliance would not generate unwanted noise if the K\({}_{\
u}\)-value of the thermostatic valve was set at a value higher than 0.45 m\({}^{3}\)/h. This implies two things: firstly, the water flow rates in current heating systems could be significantly increased if the distribution pipes were properly sized and thermostatic valves correctly chosen; secondly, if this measure is taken, the heat output from current heating systems could be significantly increased. However, it is important to remember that high water flow rates requires a distribution network that is well-balanced and carefully thought out. Otherwise, the result may be the opposite of that required. The results also showed that a radiator system that operates with small temperature drops requires significantly less exergy use than when it is operated with large temperature drops (low water flow rates). This implies that high-flow/low-temperature heating systems are more environmentally friendly than low-flow/high-temperature systems, since low-temperature systems use less natural resources for operation. A practical implication of this finding is that low-temperature heating systems connected to heat pumps should be operated with small temperature drops in single-family dwellings. The use of a high flow water rate in a heating system had also a positive effect on heat pump performance. With increased water flow rate, the temperature drop over space heaters come closer to the temperature drops of evaporators and condensers in heat pumps. This decreased the required temperature lift in the heat pump and increased its efficiency (COP-value). This finding is in line with previous results reported in [53]. However, although small temperature drops in heating systems would result in higher COP-values for heat pumps this is still not always engineering practice. Nevertheless, in this study it was shown that the use of high water flow rates in well-designed hydronic systems would increase efficiency of the heat pump and generate energy savings. The results presented in this study indicate that heat pump efficiency (COP) would increase by about 1.3%/\({}^{\circ}\)C for a supply temperature reduction from 55 \({}^{\circ}\)C to 30 \({}^{\circ}\)C. In the considered case domestic hot-water and circulators constituted about 20% of the total heating energy need. By reducing these two energy needs by 50%, the improvement of heat pump efficiency would increase to 2.1%/\({}^{\circ}\)C of reduced supply temperature. In that case, the efficiency of the considered heat pump would be improved by roughly 10% when increasing the water flow rate from 0.01 to 0.035 kg/s per heating appliance in a radiator system. This simple example demonstrates the importance of the reduction of these two energy needs. Therefore, in order to fully utilize the entire potential of low-temperature heating systems in single-family houses, the energy demands required by the domestic hot-water and circulation pumps must be reduced. An important subject that remains to be explored is how the energy required for the preparation of domestic hot-water could be reduced in single-family dwellings served by heat pumps in an efficient and practical way.