Hydronic Baseboard Low Temperature Derating Calculator
Calculate derated heat output (BTU/hr per foot) of finned-tube hydronic baseboards when retrofitting from traditional 180°F boilers to lower supply water temperatures (120°F to 140°F) for heat pumps and condensing boilers per I=B=R standards.
Baseboard & Water Temperature Parameters
BTU/hr-ft
Residential 3/4" copper-fin baseboard is typically 550–600 BTU/ft.
ft
°F
°F
°F
BTU/hr
Derated Output per Foot
36.8% CAPACITY FACTOR
213 BTU / hr-ft
Existing 20 ft delivers 4,265 BTU/hr (Shortfall: 3,735 BTU/hr)
Total Length Required
37.5 ft
+17.5 ft additional baseboard needed
Average Water Temp (AWT)
122.5 °F
ΔT above room: 54.5°F
Heat Pump Retrofit Alternatives
If wall space cannot accommodate 14 additional feet of baseboard, consider installing a hydronic fan-coil unit or upgrading to high-output low-temperature panel radiators.