The Hole You Can't See and the Problem It Causes

A hole you couldn't see. In copper tubing. Smaller than a pinhead. Sitting in a coil that had been running in that house for eleven years, leaking refrigerant slowly enough that every summer the system lost a little more of its ability to cool, and nobody found it because nobody used the right tool to look.

New evaporator coil. System holding charge for the first time in two years.

The homeowner had noticed the pattern but didn't have the vocabulary for it: every summer, the system was less effective. Every summer, the house was a little warmer on the hot days than it should be. It wasn't dramatic — it wasn't failure, it was slow decline. The kind of thing you adapt to without realizing you're adapting. You start closing the blinds earlier. You run ceiling fans more. You stop expecting the house to hit 72 in August.

He'd been told repeatedly it was just how that system ran. That older equipment loses efficiency. That it might be time to start thinking about replacement.

He wasn't wrong to be skeptical of that answer.

Formicary corrosion is a chemical process. When copper comes into prolonged contact with moisture and formic acid — which is present in the air, in building materials, in the decomposition of organic matter — the acid attacks the copper at its grain boundaries. Over years, it eats tunnels through the metal from the outside in. The resulting holes are microscopic. You can look directly at the coil surface and see nothing. You can run your finger across the copper and feel nothing.

The WNC mountain climate is exactly the kind of environment where this happens. High humidity, temperature swings, organic content in the air from the dense vegetation. The evaporator coil in a crawl-access air handler sits in exactly those conditions for the life of the equipment.

We found the leak with an electronic refrigerant detector — a probe-style instrument that reads refrigerant concentration in the parts-per-million range. You move it slowly over every inch of the coil surface. At the hairpin bend on the second pass, it alarmed.

The leak point was on the downstream side of a bend where the copper is under the most mechanical stress — exactly where formicary corrosion tends to initiate because the grain boundaries are already stressed by the bend. The hole wasn't visible. The detector found it.

We replaced the coil, evacuated the system completely, and recharged to manufacturer specification. Verified performance. The system held its charge through the remainder of the season.

Found it with electronic leak detection. Replaced the coil. That's the part that required the right tool and the willingness to use it.