What is a simple way to decrease the output of a cast iron radiator?

• A. Paint it
• B. Replace with a larger radiator
• C. Increase water temperature
• D. Remove insulation around the pipe

Answer: (A)

Radiator output can be trimmed by changing how its surface emits heat. The amount of heat a radiator sends into the room comes from both convection and radiation, and the radiative part depends on the surface’s emissivity. Bare cast iron has a relatively high emissivity, so it radiates a good amount of heat. Painting the surface adds a coating that lowers emissivity, which reduces the amount of radiant heat emitted at the same surface temperature. That makes the radiator feel cooler and deliver less heat to the room, and it’s a simple, reversible way to dial down output without changing the system settings.