A mechanical contractor standing in front of an air handling unit that needs a replacement coil has two options. They can open the unit, measure the coil, photograph the connections, and give HX Coils the information needed to quote an exact-fit replacement. Or they can pull a nameplate, call a supplier, and hope the stock replacement is close enough. The first approach produces a coil that fits and performs correctly. The second approach produces uncertainty that typically resolves itself at the installation, with the coil either in place or on its way back to the supplier.
This article covers exactly what HX Coils needs to quote a replacement coil from field measurements, and how to take those measurements correctly so the replacement arrives ready to install.
What to bring to the job
A replacement coil measurement visit requires a tape measure, a flashlight, a camera or phone, and about 20 minutes with the air handler open and the system locked out. Access to the coil section of the air handler is required, which means the access panel on the coil section needs to be removable without major disassembly. For air handlers where the coil section access is difficult or requires special tools, note that during the measurement visit so the replacement coil installation scope can be planned accordingly.
A notepad or a simple form for recording the measurements is worth having on site rather than relying on photos alone. Photos are useful for reference and for confirming connection details, but a recorded set of measurements with clear labels is faster to work from when building the quote.
Face dimensions
The face dimensions are the height and width of the coil as seen from the approaching airstream. Measure the coil frame height and width, not the casing opening. The casing opening in an older air handler may have been modified or may have accumulated gasket material and sealant that makes it larger or smaller than the coil frame that fits inside it.
Measure the face height at both sides and the face width at both the top and bottom. Older coils that have been in service for decades sometimes show slight dimensional variation from one side to the other from casing deformation or from installation in a casing that was not perfectly square. Recording all four measurements rather than assuming the coil is square prevents the replacement from arriving at a dimension that matches one measurement but not the other.
Depth
The coil depth is the dimension in the direction of airflow, from the entering air face to the leaving air face. It is determined primarily by the number of tube rows. Count the rows directly by looking at the coil from the side where the supply and return headers are located. Each row is a tube header connection running across the coil at that depth position.
Measure the overall depth from the entering face to the leaving face as a confirmation of the row count. For a standard coil with standard row spacing, the measured depth should correspond to the row count within a small tolerance. A measured depth that does not correspond to the expected row count for the number of connections visible at the headers indicates a non-standard row spacing that needs to be noted.
Fin count and fin pitch
The number of fins per inch determines the airside heat transfer surface area for a given face area and depth. Count the fins over a one-inch span at a location on the coil that has not been damaged or deformed. Use the flashlight to see the fin edges clearly and count carefully, as the difference between 8 and 10 fins per inch represents a significant change in heat transfer performance and pressure drop.
If the fin pack is damaged or corroded in a way that makes counting difficult, count over a two-inch span and divide by two. For heavily damaged coils where fin counting is not possible at any accessible location, note that in the measurement record and HX Coils will work from the available information and confirm the fin pitch from any documentation available for the original equipment.
Tube diameter and circuit configuration
The tube outer diameter is typically 3/8 inch, 1/2 inch, or 5/8 inch for commercial HVAC coils. It can be measured directly with calipers at an exposed tube end or estimated from the fin collar diameter visible at the fin pack face. Note the tube diameter measurement method so HX Coils knows whether it is a direct measurement or an estimate.
The circuit configuration determines how the fluid flows through the coil from the supply header to the return header. For a replacement coil, the circuit configuration needs to match the original to ensure the supply and return connections are on the correct side and at the correct elevation. Photograph both headers, noting which is supply and which is return, and which side of the coil each header is on when viewed from the entering air face. Note whether the connections are on the left side or the right side and whether they are at the top, bottom, or both.
Connection sizes and type
Measure or confirm the connection sizes at the supply and return headers. For older coils, the connection type may be MPT, FPT, sweat, or flanged. Note the connection type along with the size. If the original coil has sweat connections and the contractor wants to change to MPT or grooved connections for the replacement, note that preference in the measurement record and HX Coils will accommodate it in the replacement design.
What to send to HX Coils
A complete field measurement set for a replacement coil quote includes face height and width at multiple points, coil depth with row count, fins per inch, tube outer diameter, circuit configuration with supply and return header locations and connection sizes, and photographs of both headers and the full coil face. With that information HX Coils can produce a quote for an exact-fit replacement coil.
For contractors who prefer a standardized measurement form rather than a notepad, contact HX Coils for the field measurement form. It covers every dimension in a structured format and includes a space for photos and notes. Text or email the completed form and photos and HX Coils will turn around a quote.
For boiler plant applications where the replacement coil serves a heating system supplied by GP Energy Products equipment, the GP Energy team handles the boiler side of the application. Visit gpenergyproducts.com for more. For hydronic system pump conditions that may be contributing to coil performance issues, the Merion Pump Company team handles pump assessment. Visit merionpump.com for more.
HX Coils fabricates custom replacement coils for commercial and industrial HVAC applications across the Mid-Atlantic region. Reach out with field measurements and photos and we will quote an exact-fit replacement.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers coil dimensional standards, circuit configuration terminology, and fin pitch specifications for commercial HVAC coils. ashrae.org
2. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and dimensional standards for commercial HVAC replacement coils. ahrinet.org
3. Sheet Metal and Air Conditioning Contractors National Association. HVAC Installation Standards. Covers coil replacement measurement and installation requirements for commercial air handling units. smacna.org
4. Air Conditioning Contractors of America. HVAC Quality Installation Specification. Covers coil replacement procedures and measurement requirements for commercial HVAC applications. acca.org
