Evaporator Coil

For homeowners

The evaporator coil is the indoor heat exchanger where refrigerant boils and absorbs heat from your home’s air. The A-frame shape — two slabs of finned tubing meeting at the top — is the most common residential configuration. Cold refrigerant inside the tubes; warm humid return air passing across the aluminum fins outside; heat flows from the air into the refrigerant, and water vapor in the air condenses on the cold fins and runs down into the drain pan below.

The TXV (thermal expansion valve) sits at the top of the coil. It meters refrigerant flow into a distributor, which splits the liquid into several parallel circuits running through the coil. This parallel arrangement is what makes the coil work — a single serial tube run wouldn’t cool the whole coil evenly.

The drain pan beneath catches the condensed water. A 3-ton system in coastal Florida can pull 20+ gallons of water out of the indoor air per day during peak humidity. That water has to drain reliably, every cycle, or the pan backs up and the float switch shuts the system off.

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For technicians

The A-frame geometry is two slab coils joined at the top peak. Refrigerant enters as warm high-pressure liquid through the TXV, gets metered into the distributor at the peak, and the distributor sprays liquid into multiple parallel circuits — typically four to eight tube runs in parallel. The liquid sprays down through both slabs, boils into gas as it absorbs heat from the air, and exits the bottom of each slab into the suction header. Both headers join into the single insulated suction line that heads back outside to the compressor.

Why parallel circuits matter. A single tube fed into a 24-inch slab would have the first foot flooded with liquid and the rest starved of refrigerant — capacity falls off badly. Parallel circuits ensure every section of the coil sees fresh liquid simultaneously. The distributor at the top is essentially a manifold that splits the metered flow into equal portions. Some distributors are static (just a header with multiple outlets); others have internal spray nozzles that ensure even distribution even at low flow rates.

The TXV is the metering device. Bulb senses suction line temperature, equalizer line senses suction pressure, both forces act on a diaphragm against a spring. The diaphragm modulates the valve pin position to maintain a constant superheat at the coil outlet — typically 8–12°F superheat. Superheat too low means the coil is overfed and liquid is reaching the suction line (bad for the compressor). Superheat too high means the coil is underfed and capacity is dropping (bad for the system).

Fin density. Twelve to fifteen fins per inch is standard for residential. The fins are thin aluminum sheets press-fit onto the copper tubes. The whole assembly is a heat exchanger that turns refrigerant phase change inside the tubes into temperature drop in the air outside.

Coil cleanliness is the single most impactful maintenance item on a residential system. A coil packed with biofilm, dust, fiberglass from filter media, pet hair, or any other accumulation has reduced airflow and reduced heat transfer. Capacity drops, the system runs longer to meet the load, energy bills climb, the compressor spends more hours running. Annual coil cleaning with a coil-safe cleaner (no acid-based cleaners on aluminum — they’ll etch the fins) and a gentle rinse from the airflow exit side toward the entry side is the right protocol.

Condensate. The fins run at approximately 35–45°F when the system is cooling, which is well below the dew point of indoor air at typical conditions. Water vapor condenses on the fin surfaces, runs down by gravity, and drips into the drain pan below the coil. The drain pan is sloped toward an outlet on one side, and from there to a P-trap and primary drain line.

A 3-ton (36,000 BTU) system at typical Florida summer conditions removes about 1.5 gallons of water per hour of operation. Over a 12-hour cooling day, that’s 18 gallons — enough to fill a small bathtub. All of that has to drain reliably or you have a problem.

Drain pan design. Primary pan beneath the coil itself is part of the air handler casting. Secondary pan beneath the entire air handler is required by code for attic installations and is a separate flat pan with its own float switch and drain. The float switches on both pans wire in series with the 24V control circuit — either one trips, the whole system shuts off.

Failure modes:

Coil leaks. The most common refrigerant-side failure. The aluminum fins are press-fit onto copper tubes, and over years of thermal cycling plus formicary corrosion (a form of pinhole corrosion caused by VOCs in indoor air reacting with copper tubing), pinhole leaks develop. Customer complaint: “AC blowing warm.” Tech finds low refrigerant charge. Leak search with electronic detector or UV dye traces it to the coil. Repair is rarely worthwhile — you replace the coil assembly.

Coil freeze-ups. Most often caused by low airflow (dirty filter, restricted ducts, blower problem) combined with normal operation. The evaporator drops below 32°F, the moisture on the fins freezes solid, ice blocks airflow further, runs progressively colder, eventually iced over. Customer complaint: “AC blowing warm” — at the moment they call, the coil is frozen and no air is moving through it. Diagnostic: feel for ice on the suction line near the air handler. Fix: thaw the coil (turn unit off, run fan only for 30+ minutes), find and fix the airflow restriction.

Distributor problems. Less common but happens. Internal blockage in one of the spray nozzles starves one circuit, the other circuits get overfed. Coil has hot spots and cold spots. Performance is uneven. Replacement is a coil assembly job because the distributor is brazed into the coil.

Refrigerant line vibration cracks. The lines coming off the coil sometimes crack at brazed joints from vibration over the years. Slow refrigerant leak. Detectable with leak search, repairable in place if accessible.

Florida-specific corrosion. Coastal homes see significant aluminum corrosion on the fins from salt air. Black/grey crystalline deposits between fins, fins crumbling at the edges, eventual fin loss. Coil-saver coatings during initial install help significantly. Coils less than 10 years old that look heavily corroded are probably in coastal homes.

The metering device variants. Not all coils use TXVs. Cheaper systems use a fixed orifice (a piston with a fixed-diameter hole) and live with whatever superheat they get at varying conditions. Fixed-orifice systems are less expensive, less precise, and slightly less efficient. They also benefit more from hard-start kits because the head-to-suction pressure doesn’t equalize as cleanly during the off cycle. TXV systems are the modern standard for anything above entry-level equipment.