High-temperature hot water systems present a specific replacement challenge that standard low-temperature hydronic coil replacements do not. The operating conditions, temperatures between 250 and 400 degrees Fahrenheit and pressures between 160 and 300 PSI, place demands on the replacement coil that exceed what standard commercial HVAC coil specifications address. And high-temperature hot water systems tend to be old. The systems installed in the 1970s and 1980s that are still operating today are operating with coils that may be 30 or 40 years old, in systems where the original engineering documentation has long since been lost or archived in formats that are no longer accessible.
The combination of demanding operating conditions and absent documentation makes high-temperature hot water coil replacement one of the most specification-sensitive replacement projects in commercial HVAC. Getting the specification right requires field measurement, system condition assessment, and material selection that goes well beyond what a standard coil replacement requires.
Why aging high-temperature hot water systems need special attention
A high-temperature hot water coil that has been in service for 30 or 40 years has experienced tens of thousands of thermal cycles between the ambient temperature when the system is cold and the operating temperature when it is at load. Each thermal cycle expands and contracts the tube-to-header joints and the tube-to-tubesheet connections. Over decades of cycling, fatigue cracks develop at these connection points, tube wall thickness decreases from corrosion and erosion, and the original material properties of the tubes and headers may have changed from long-term exposure to elevated temperature and pressure.
A coil that is still functioning in this condition is functioning on borrowed time. The failure mode for a high-temperature hot water coil that has developed fatigue cracks at the tube-to-header connections is a sudden pressure breach rather than a gradual performance decline. A coil that leaks at high temperature and pressure in a mechanical room or air handling unit creates a safety event, not just a maintenance problem.
Field measurement when documentation is unavailable
The absence of original documentation means the replacement specification must be built from field measurements of the actual coil in place. Those measurements need to be taken while the system is offline and cooled, with the coil accessible for direct measurement.
Critical dimensions include the face height and face width measured at the coil frame rather than the opening in the air handler casing, which may have been modified. The connection sizes and orientation, measured at the actual connections rather than assumed from a generic specification. The tube outer diameter and fin pitch, counted directly on the coil rather than estimated. The row count, confirmed by counting from the coil face. The header dimensions and connection locations.
For high-temperature hot water coils, the pressure and temperature rating of the replacement coil must be confirmed against the actual system operating pressure and temperature, not the nominal design pressure from a system that may have been modified since original installation. The actual operating pressure should be read from the system instrumentation at the coil location under operating conditions, not assumed from the system design documentation that may be decades old and may not reflect modifications made over the life of the system.
Material selection for high-temperature hot water replacement coils
The material specification for a high-temperature hot water replacement coil needs to address both the pressure rating and the thermal cycling requirements of the application. Standard copper tube with standard wall thickness used in low-temperature hydronic coils is not appropriate for high-temperature hot water applications where the operating pressure requires heavier wall construction and where the thermal expansion at operating temperature requires a tube-to-header joint design that accommodates movement.
Copper tube in heavier wall gauges is common in high-temperature hot water coil applications where the operating pressure is within the range that copper construction can handle. The wall thickness selection is based on the operating pressure with the safety factors required by the applicable code. Steel tube is used in higher-pressure applications where copper’s pressure capability is insufficient.
The fin material selection for a high-temperature hot water replacement coil should account for the fin temperature that will be achieved in service. At high-temperature hot water supply temperatures, the fin surface temperature is significantly higher than in a standard heating coil application. Aluminum fins are not appropriate for applications where the fin surface temperature exceeds the aluminum’s design temperature limits. Copper fins are the standard specification for high-temperature hot water coils where the operating conditions exceed what aluminum can handle.
Inspection before specification
Before a high-temperature hot water coil replacement is specified, an inspection of the existing coil and the connected system components should be performed to confirm that the coil replacement is the appropriate scope of work and that there are no system conditions that would affect the replacement coil’s service life.
The inspection should assess the tube-to-header connections for visible cracking or leakage evidence, the tube surface condition for corrosion and erosion, the casing and frame condition for structural integrity, and the connection piping for condition and compatibility with the replacement coil connections.
For systems where the inspection identifies system conditions that have contributed to the original coil’s degradation, correcting those conditions before the replacement is installed is part of the specification. A replacement coil installed in a system with aggressive water chemistry, inadequate expansion accommodation, or operating conditions that exceed the coil’s design parameters will fail on an accelerated schedule regardless of how well the replacement is specified.
For high-temperature hot water systems where the boiler plant producing the high-temperature hot water is also under review, the GP Energy Products team handles commercial and industrial high-temperature hot water boiler assessment across Pennsylvania, New Jersey, Delaware, and Maryland. Visit gpenergyproducts.com for more. For the pump systems serving high-temperature hot water distribution, the Merion Pump Company team handles pump selection and can assess whether the current pump is appropriate for the system’s operating conditions. Visit merionpump.com for more.
HX Coils fabricates custom high-temperature hot water replacement coils for commercial and industrial applications across the Mid-Atlantic region. Reach out before the replacement is specified and we will work through the field measurement, material selection, and pressure rating requirements for the specific system.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers high-temperature hot water coil design requirements, material selection, and replacement specification guidance. ashrae.org
2. ASME. Boiler and Pressure Vessel Code, Section VIII. Covers pressure vessel design requirements applicable to high-temperature hot water coil fabrication and pressure rating. asme.org
3. TEMA. Standards for Shell and Tube Heat Exchangers. Covers thermal cycling fatigue requirements and material selection for high-temperature heat exchanger applications. tema.org
4. ASTM International. Standard Specification for Seamless Copper and Copper-Alloy Tubes. Covers material and pressure rating requirements for copper tube in high-temperature hot water coil applications. astm.org
