Most HVAC coils in commercial buildings handle chilled water between 42 and 55 degrees Fahrenheit or hot water between 120 and 180 degrees. Standard temperatures, standard pressures, standard material specifications. The selection process is well understood and the equipment is widely available.
High temperature hot water is a different world. HTHW systems typically operate between 250 and 400 degrees Fahrenheit at pressures between 160 and 300 PSI. The coil that handles a standard hydronic heating circuit cannot handle an HTHW circuit. The design requirements, the material selection, and the pressure ratings are in a different category entirely, and specifying a coil without accounting for those differences produces equipment that either fails in service or never gets approved by the authority having jurisdiction.
Why the temperature and pressure change everything
At standard hydronic temperatures, copper tube and aluminum fin construction handles the job reliably. At HTHW temperatures, the thermal expansion characteristics of the coil become a primary design concern. A coil that expands and contracts normally at 180 degrees will experience significantly greater dimensional changes at 350 degrees. If the coil is not designed to accommodate that movement, the stress concentrates at the tube-to-header joints and the tube-to-tubesheet connections, which is where fatigue cracks develop over time.
The operating pressure is the other constraint. Most standard commercial HVAC coils are rated for working pressures in the 200 to 300 PSI range, which covers standard hydronic applications comfortably. HTHW systems operating at the upper end of the temperature range require higher saturation pressures to keep the water liquid. The coil pressure rating needs to match the system operating pressure with appropriate safety margin, not just the nominal working pressure of the distribution system.
Material selection shifts accordingly. Copper tube remains common in HTHW applications but the wall thickness increases relative to standard coils to handle the elevated pressure. Steel tube is used in higher-pressure applications where copper’s pressure limitations become a constraint. The fin material and coil casing need to be rated for continuous exposure to the operating temperature without degradation.
What the specification needs to include
An HTHW coil specification needs to go beyond the standard entering and leaving temperatures, flow rate, and face dimensions. The maximum working pressure of the system needs to be stated explicitly, not just the operating pressure. The thermal expansion requirements need to be addressed, either through a coil design that accommodates movement or through the installation configuration that allows the coil to expand and contract without stressing the connections. The tube material and wall thickness need to be specified for the actual operating conditions, not defaulted to standard commercial coil construction.
HX Coils reviews every HTHW application before fabrication begins. The review confirms the pressure rating against the system operating conditions, the material selection against the fluid temperature and chemistry, and the coil configuration against the thermal expansion requirements of the installation. For HTHW applications where the heat source is a high-temperature hot water boiler, the GP Energy Products team handles the boiler side of that conversation. Visit gpenergyproducts.com for more on GP Energy’s commercial boiler capabilities including the Unilux Evolv HTHW line.
HX Coils manufactures custom HTHW coils for commercial and industrial applications across the Mid-Atlantic region. If you have an HTHW application in development, reach out before the coil is specified and we will make sure the design is right for the operating conditions.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers coil types, material selection, and pressure ratings for high-temperature applications. ashrae.org
2. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and certification. ahrinet.org
3. ASME. Boiler and Pressure Vessel Code, Section VIII. Covers pressure vessel design requirements applicable to high-pressure coil applications. asme.org
