Beyond a reported 55/45 °C regime: Conditional radiator capacity and flow limits for lower-temperature operation

A design-temperature entry in an energy report does not establish whether existing radiators can deliver room loads at a still lower temperature. This study develops a conditional emitter-level screening analysis from an anonymised nonresidential consultancy report that specifies radiators and a 55/45 °C circuit regime. A logarithmic-mean-temperature-difference power law is coupled with the steady-state water energy balance. The reference indoor temperature is assumed to be 20 °C, with a radiator exponent of 1.3 and sensitivity values of 1.1 and 1.5. At a fixed 10 K water-temperature drop, the calculated output fractions at 50/40, 45/35 and 40/30 °C are 0.7847, 0.5811 and 0.3908 relative to 55/45/20. Solving for the return temperature at a prescribed output reveals a different limitation: at 45 °C supply and 75% of reference output, the required flow is approximately 3.23 times the reference, while full reference output has no finite-flow solution. At 50 °C supply, full reference output is mathematically attainable only with a flow ratio of approximately 17.92, illustrating how a formal solution can be practically unusable. Room-temperature and emitter-size sensitivities are provided. Source plant capacities and annual energy reductions are deliberately not substituted for radiator capacity or room peak loads. The contribution is a reproducible feasibility map and evidence checklist, not a certification that the building can operate at the tested temperatures.