The ideal time to inspect an HVAC coil is during the pre-season window when the system is offline and accessible. That window has closed for most facilities. The heating season is underway, the coils are serving occupied spaces, and taking a system offline for a coil inspection during heating season creates an occupant comfort problem and in cold weather a freeze risk that makes the inspection impractical.
The practical alternative is a set of inspection and assessment techniques that provide meaningful information about coil condition without requiring the system to be taken offline. None of these techniques replace a direct physical inspection of the coil with the air handler open and the system shut down. All of them provide useful information about whether a direct inspection needs to be prioritized and scheduled despite the operational disruption.
Performance monitoring as a diagnostic tool
The most accessible coil condition assessment during heating season is performance monitoring: comparing the system’s actual leaving air temperature against what the system should be producing at the current entering conditions, water supply temperature, and flow rate.
A heating coil that is delivering leaving air at 90 degrees Fahrenheit when the system should be producing 100 degrees at the current conditions is not performing to specification. The performance gap reflects either a change in the operating conditions, a controls or valve problem, or a coil condition problem. Ruling out the operating condition and controls explanations leaves coil condition as the most likely cause.
For facility managers without detailed system documentation, the current performance baseline can be established by recording the leaving air temperature, the entering air temperature, the supply water temperature, and the fan speed or static pressure at the same time each week. A trend of declining performance at consistent conditions is a reliable indicator of developing coil fouling or degradation even without a historical baseline to compare against.
Airside pressure drop monitoring
Airside pressure drop across the coil is one of the most sensitive and reliable indicators of airside fouling. As particulate accumulates on the fin surfaces, the resistance to airflow increases and the pressure drop across the coil rises. Measuring and trending pressure drop over time provides a direct indicator of airside fouling accumulation without requiring the air handler to be opened.
A magnehelic gauge installed with taps on either side of the coil provides a continuous pressure drop reading that maintenance staff can record during routine rounds. A digital differential pressure gauge can be used for periodic measurements without permanent installation. The baseline reading for a clean coil at design airflow is the reference point. A pressure drop that has increased significantly from baseline indicates fouling that warrants investigation.
For variable air volume systems where the fan speed varies with load, pressure drop measurements need to be taken at comparable fan operating points to be meaningful. A pressure drop that looks normal at reduced fan speed may indicate significant fouling that is masked by the reduced airflow.
Infrared thermography for surface condition assessment
Infrared thermography can identify coil problems that are not visible from outside the air handler without requiring the unit to be opened. A thermal camera aimed at the coil through an access panel opening, or in some installations through an inspection port, can identify hot spots and cold spots on the coil face that indicate uneven flow distribution, blocked tubes, or sections of the coil that are not performing.
A coil with a uniform temperature distribution across its face is distributing fluid and transferring heat consistently. A coil with distinct hot or cold zones has a flow distribution problem that may be caused by tube fouling, a partially closed control valve, air entrainment in the coil circuit, or a failed balancing valve. The thermal image does not identify the cause, but it identifies the location and extent of the problem and tells the facility team whether the issue is localized or widespread.
Infrared thermography requires access to the coil face from outside the air handler or through an access panel opening large enough to aim the camera at the coil surface. For tightly configured air handling units where the coil is not accessible from any external opening, thermography is not practical without opening the unit.
Water side assessment without draining
For waterside coil condition assessment during heating season, water chemistry analysis provides useful information about what the system water is doing to the internal tube surfaces without requiring the coil to be drained or opened.
A water sample taken from a sample valve in the coil circuit provides iron, copper, and pH data that indicates whether active corrosion is occurring in the system. Elevated iron in a heating hot water circuit indicates active corrosion of ferrous components somewhere in the system. Elevated copper indicates active corrosion of copper components. Either reading warrants investigation of the system water treatment program and consideration of whether a borescope inspection of accessible tube ends should be prioritized before the end of the heating season.
For facilities where the coil circuit does not have a sample valve, a sample can sometimes be taken from a drain valve or an air vent connection with appropriate precautions for the system pressure and temperature at the sample point.
When in-season assessment points toward an offline inspection
The in-season assessment techniques described here are screening tools. They identify whether a problem is likely present and whether it is likely to be significant. They do not replace the direct physical inspection that confirms the problem and characterizes its severity.
A coil that is showing performance decline, elevated airside pressure drop, thermal non-uniformity on infrared assessment, or elevated corrosion indicators in the system water has given sufficient evidence to justify scheduling an offline inspection despite the operational disruption. The risk of operating a coil with a developing problem through the rest of the heating season needs to be weighed against the disruption cost of an offline inspection.
For facilities where an offline inspection during heating season would affect occupied critical spaces, the risk assessment needs to include the consequence of a coil failure during peak heating season compared to the consequence of a planned inspection outage. In most cases the planned inspection outage, even during heating season, is significantly less disruptive than an unplanned failure.
For boiler plant applications where the coil condition assessment is part of a broader system evaluation, the GP Energy Products team handles commercial boiler assessment and service across Pennsylvania, New Jersey, Delaware, and Maryland. Visit gpenergyproducts.com for more. For hydronic system pump conditions that may be contributing to coil performance issues, the Merion Pump Company team handles commercial pump service and system assessment. Visit merionpump.com for more.
HX Coils manufactures custom replacement coils for commercial and industrial HVAC applications across the Mid-Atlantic region. If your in-season assessment has identified a coil with a likely developing problem, reach out and we will help determine whether replacement should be planned for the end of heating season or prioritized sooner.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers coil performance monitoring, fouling assessment, and inspection techniques for commercial HVAC applications. ashrae.org
2. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Covers coil performance testing methodology applicable to in-season performance comparison. ahrinet.org
3. ASHRAE Guideline 4. Preparation of Operating and Maintenance Documentation for Building Systems. Covers performance baseline documentation and trending methodology for mechanical systems. ashrae.org
4. American Society for Nondestructive Testing. Infrared Thermography Standards. Covers thermographic inspection methodology applicable to HVAC coil condition assessment. asnt.org
