What it is

The evaporator coil is the indoor heat exchanger that actually cools and dehumidifies the air. It's where the refrigeration cycle's "cold" lives: low-pressure liquid refrigerant enters through the metering device, boils off as it absorbs heat from the house air blowing across the fins, and leaves as a low-pressure vapor headed back to the compressor. The air gives up sensible heat (gets cooler) and latent heat (water condenses out), so the coil is also your dehumidifier.

A "cased coil" is just an evaporator coil that comes in its own sheet-metal box (the case) ready to bolt onto a furnace or sit in an air handler. The case routes the air through the coil and catches the condensate. Whether it's called an A-coil, slab, or cased coil, the job is identical — boil refrigerant to pull heat and moisture out of the airstream.

How it's built

  • Coil geometry. Most are an A-coil (two slabs leaned together in an A/V shape to pack more surface into a duct) or a flat slab coil (single face, common in horizontal applications and some packaged units). More face area and rows = more capacity.
  • Tubes and fins. Copper (or aluminum) tubing carries refrigerant; thin aluminum fins are pressed onto the tubes to multiply the air-contact surface. Some newer coils are all-aluminum or microchannel.
  • Metering device. A TXV (thermostatic expansion valve) or a fixed orifice/piston sits at the coil inlet and drops the high-pressure liquid to low pressure, metering it into the coil. The TXV's sensing bulb clamps to the suction line leaving the coil.
  • Distributor. On TXV coils, a distributor with several small feeder tubes splits the metered refrigerant evenly across all the coil circuits so the whole coil feeds equally.
  • Drain pan. Under (and sometimes built into) the coil, sloped to a drain connection. A cased coil's pan is part of the case; many installs add a secondary/auxiliary pan beneath the whole unit.
  • The case. Sheet-metal cabinet with the supply/return openings sized to the furnace or air handler it mates to.

How it's plumbed and where it sits

The coil is the low-pressure side indoors. The liquid line (small, warm) brings refrigerant from the outdoor unit to the metering device; the suction line (large, cold, insulated) carries vapor back out. The coil mounts downstream of the blower's discharge — on an upflow furnace it sits on top, in the supply airstream, so all the conditioned air passes through it. The condensate it wrings out drains from the pan through a trap to an approved point.

A critical install detail: on a gas furnace, the cooling coil sits above the heat exchanger in the airstream. It must be installed so its condensate can't drip down into the furnace and rot the exchanger.

In the field

Most evaporator-coil work is one of three things: it's freezing, it's leaking, or it's dirty/restricting airflow. The coil itself rarely "breaks" mechanically — it's the airflow across it, the charge feeding it, or corrosion in it that causes calls.

Normal values & targets

  • Airflow: ~350–450 CFM per ton across the coil for comfort cooling; lower CFM (toward ~350) for more dehumidification, higher (toward ~450) for max sensible. Too little airflow freezes the coil.
  • Coil/refrigerant split: the coil should run an evaporating temperature well above freezing in normal operation; a coil saturation temp at or below 32°F under load will ice up over time.
  • Superheat (fixed orifice): per the wet-bulb/dry-bulb chart, not a fixed number.
  • TXV superheat: the valve targets roughly 8–12°F superheat at its outlet; a TXV holds superheat fairly constant as load changes.
  • Subcooling: read at the outdoor unit, but a flooded or starved coil shows up here too.
  • Condensate: a coil doing real latent work produces steady water in cooling season; bone-dry in humid weather can mean low airflow icing or a coil not getting cold.

Common faults & what they mean

  • Coil freezing solid. The big one. Two root causes: low airflow (dirty filter, dirty coil, weak/slow blower, closed registers, undersized return) or low charge/restriction (the coil gets too cold for the air passing it). Diagnose which — thaw it, then check airflow and charge.
  • Refrigerant leak in the coil. Indoor coils, especially older copper-tube/aluminum-fin ones, are prone to formicary ("ant-nest") corrosion — microscopic pinhole leaks driven by VOCs in indoor air. Shows as a slow charge loss with no obvious oil spot. Find it with an electronic leak detector or bubbles after pressurizing.
  • Dirty coil / high static. Dust cakes the entering face (and gets past poor filters), choking airflow and capacity. Often hidden because it's the upstream face you can't see without pulling the coil or scoping it.
  • Water leaking / pan overflow. Plugged drain, slimed pan, cracked pan, or a low spot holding water. (Covered in depth in the condensate-system article — but the coil's pan is where it starts.)
  • Uneven coil feeding. A plugged or kinked distributor feeder tube starves part of the coil — you'll see frost on some circuits and not others, weak capacity.
  • TXV hunting or stuck. A TXV losing its bulb charge or stuck closed/open starves or floods the coil; covered in the metering-device material.

Tech tips & gotchas

  • A frozen coil is an airflow or charge problem — the coil is the victim. Don't "fix" it by adding refrigerant blind. Thaw it, then measure airflow (static, blower amps, filter, register openness) and check charge properly. Adding gas to a low-airflow freeze-up just makes it worse.
  • The dirty side is the side you can't see. The entering (upstream) face cakes with dirt that a quick look at the leaving side misses. If capacity is down and the visible side looks clean, pull or scope the coil.
  • Suspect formicary corrosion on slow, mystery charge loss in older indoor coils with no visible leak. Pinholes in the fin pack are common and hard to spot.
  • Match the metering device to the system. A TXV coil paired badly with the wrong orifice or a non-bleed TXV on a single-stage outdoor unit can cause hard-start and charging headaches. Confirm the coil and condenser are an approved match.
  • Protect the furnace below. A coil installed without proper condensate management drips into the heat exchanger and creates a rust/CO problem down the road.

Safety / code notes

  • Condensate must drain to an approved point with a proper trap per the mechanical/plumbing code condensate provisions; an auxiliary/secondary drain pan and/or float switch is required where overflow would damage the building.
  • Recover refrigerant per EPA Section 608 before cutting into a coil to replace it; never vent.
  • On a gas furnace, the cooling coil must be installed so condensate cannot drain into the furnace heat exchanger.
  • Sharp fins and refrigerant under pressure: wear eye protection and gloves; relieve pressure and purge with nitrogen before brazing on a coil.