The trend toward low-temperature hydronics is relatively new in North America, but it’s been happening in other places for several decades. Europeans accept low water temperature distribution systems as the norm, while some American hydronic systems are still being designed around water temperatures exceeding 180°F because the heat distribution hardware is cheaper.
Standard fin-tube baseboard heaters are a good example. Originally designed to replace cast-iron radiators, most haven’t changed much over the last several decades. When fin-tube baseboard heaters first entered the market, fuel was cheap so nearly all boilers operated at 180°F or higher. Some manufacturers’ literature still lists water temperature as high as 240°F. The economics are simple—the higher the water temperature, the greater the heat output. The greater the heat output, the shorter the required fin-tube length. The shorter the length, the lower the equipment cost. But, unfortunately, the higher the temperature, the higher the energy and fuel costs.