A hot water coil is a heating heat exchanger that warms air by circulating hot water (typically 140°F to 180°F) from a boiler or water heater through its tubing. Hot water coils are extensively used in commercial and industrial HVAC systems for primary space heating, supplemental heating, and make-up air unit applications.
Technical Details
Hot water coils operate with supply temperatures between 140°F and 180°F and return water 10°F to 20°F cooler depending on system load. Heating capacity ranges from 50,000 to 500,000 BTU/h based on water temperature, flow rate, and air volume. Water-side pressure drop typically ranges from 1 to 4 psi. The coil’s leaving air temperature is controlled by modulating water flow through a three-way or two-way valve.
Applications and Significance
Hot water coils are preferred in large buildings because a single boiler supplies multiple heating zones, improving efficiency and control. They maintain consistent performance across a wide range of outdoor temperatures. Common sizing standards include ASHRAE guidance for proper flow rates and pressure drops to maximize heat transfer while maintaining system efficiency.