Heating coils sit at the heart of every air handling unit operating through a UK winter. When they fail, comfort drops, complaints start, and energy bills usually climb before anyone identifies the root cause. We’ve worked on AHU heating coil issues across office blocks, industrial facilities, and 24-hour processing environments, and the diagnostic pattern is remarkably consistent.
Here’s how we approach heating coil failures and what to expect if yours is showing signs of trouble.
How AHU Heating Coils Work
Most commercial AHUs use Low Temperature Hot Water (LTHW) heating coils, where hot water from a boiler circulates through finned copper tubes while supply air passes over them. Heat transfers from water to air, warming the supply before it reaches the space.
Some applications use direct gas-fired heating — our aerospace project in Derby used gas burner heating integrated with DX cooling in a single bespoke unit. Steam heating remains common in older industrial sites. Electric heating appears in smaller units or as supplementary boost.
Whatever the heat source, the principle is the same: a heat exchanger transferring energy from a hot medium to the airstream. When this transfer drops below design performance, the symptoms appear quickly in UK winter conditions.
Common Heating Coil Failure Modes
Tube corrosion and pinhole leaks — The most common failure we see in older LTHW coils. Water chemistry, dissolved oxygen, and electrolytic corrosion gradually attack the copper tubes. Pinhole leaks let water escape into the airstream or drain pan, often appearing as damp patches downstream or unexplained water in the AHU.
Fin damage — Aluminium fins bonded to copper tubes provide the surface area for heat transfer. Physical damage from filter changes, debris impact, or improper cleaning collapses fins, restricting airflow and reducing heat transfer. Combed fins partially recover performance, but severe damage requires coil replacement.
Fouling and blockage — Dust, lint, and biological growth accumulate on fin surfaces. We’ve inspected coils completely caked with material that essentially turned the heat exchanger into a heavily insulated wall. Heat transfer drops, fan energy climbs as the system fights restricted airflow.
Frost damage — If freeze protection fails or water drains incompletely during shutdown, tubes can rupture as ice expands. This typically causes catastrophic leakage requiring immediate replacement.
Control valve and actuator failures — Sometimes the coil is fine but the control isn’t. Stuck valves, failed actuators, or incorrect mixing temperatures produce symptoms that look like coil failure until you investigate properly.
Diagnostic Process
When we attend an AHU with suspected heating coil problems, we work through a systematic check:
Visual inspection — Casing opened, coil examined for damage, corrosion, fouling, and obvious leaks. We check drain pans for unexpected water indicating internal leaks.
Pressure and temperature readings — Flow and return water temperatures across the coil compared against design figures. A coil delivering the right water-side temperature drop but inadequate air heating points to airflow or fin issues rather than waterside problems.
Airflow measurement — Supply air temperature rise across the coil at design airflow. Lower than expected indicates reduced effectiveness.
Pressure drop check — Higher than design pressure drop usually means fouling, fin damage, or partial blockage.
Control verification — Valve position confirmed, actuator response tested, BMS commands cross-checked against actual valve behaviour.
This typically takes a couple of hours on site and tells us whether we’re looking at repair, refurbishment, or full coil replacement.
Repair vs Replacement
Minor issues — surface fouling, individual fin damage, control problems — usually respond to cleaning and component-level repair. We can chemically clean coils, comb fins, and replace failed actuators without major disruption.
More serious damage tips the balance toward replacement. Multiple pinhole leaks indicate widespread corrosion that won’t stop with a single repair. Severe fouling combined with internal corrosion makes cleaning uneconomic. Frost damage almost always requires replacement.
Coil replacement is often a strong opportunity to upgrade specification at the same time. Modern coils with optimised fin geometry deliver better heat transfer per unit volume, reducing pressure drop and fan energy. If the existing unit is going to be opened anyway, upgrading the coil while you’re there usually pays back faster than waiting.
For older AHUs where multiple components are reaching end of life, full AHU refurbishment often makes more sense than repeated component-level repairs. We delivered exactly this approach at our Colmore Row project in Birmingham, upgrading fan assemblies and controls together rather than addressing issues piecemeal.
Preventing Heating Coil Failures
Most coil failures we attend could have been delayed or prevented through better maintenance:
Water treatment — Properly inhibited LTHW systems with controlled dissolved oxygen last significantly longer. Untreated systems corrode aggressively from the inside.
Filtration discipline — Filters changed before they bypass keep coils clean. We see coils caked with material that should have been caught upstream.
Annual coil inspection — Catching minor fouling or fin damage early prevents expensive failures later.
Freeze protection testing — Frost stats and pump interlocks tested annually. The cost of testing is trivial compared to a burst coil and water-damaged plant room.
How We Can Help
Heating coil failures during a cold snap are stressful. Buildings get cold quickly, complaints multiply, and quick fixes are tempting but often poor value.
We provide diagnostic visits, in-situ repairs where appropriate, and full coil replacement using our in-house manufacturing capability through EA Air Handlers. Our Electroair division handles installation and commissioning. Whether you need urgent diagnosis or a planned replacement programme, get in touch to discuss your situation.
