Radiator / Hydronic Coil Heat Output (BTU/hr)
Calculate the heat output of a hydronic radiator or baseboard heater from water inlet/outlet temperatures, flow rate, and room temperature.
Results
What is it?
Hydronic radiators and baseboard heaters transfer heat from hot water to room air by convection and radiation. The output depends on water flow rate, water temperature drop across the unit, and the temperature difference between the mean water temperature and room air. The formula Q = GPM � 500 � ?T is the standard method per ASHRAE.
How to use
Enter the supply (inlet) and return (outlet) water temperatures, room temperature, and flow rate. Results show BTU/hr output and mean water temperature vs. room temperature (excess temperature), which drives the log-mean temperature difference and is used by manufacturers to rate baseboard output.
Example scenario
A fin-tube baseboard has 180�F supply, 160�F return, in a 68�F room, at 1.5 GPM. Output = 1.5 � 500 � 20 = 15,000 BTU/hr. Mean water temp = 170�F, excess temp = 102�F. From the manufacturer's table at 170�F mean temp, a 6 ft electric baseboard provides ~15,000 BTU/hr � a good match.
Pro tip
Modern condensing boilers operate most efficiently with low return water temperatures (below 130�F) to allow flue gas condensation. When designing for condensing operation, size radiators larger or switch to radiant floor heating, which works well at 100�120�F supply. Traditional radiators require 160�180�F � incompatible with condensing mode.