Replace an Evaporator Coil (Air Handler)

Before you start

•Replacement coil matched to the condensing unit/refrigerant and capacity (a mismatched coil ruins capacity and efficiency), and the correct cased vs uncased / upflow-downflow orientation.
•EPA 608 certified; recover the charge before opening the system.
•Correct metering device for this coil + outdoor unit (factory TXV/check-valve, or the correct fixed orifice/piston size).

Tools & materials

Recovery machine + DOT cylinder + scale · Brazing torch + rod, nitrogen + regulator (flow purge AND pressure test) · Tube cutter, sheet-metal tools, new filter drier · Vacuum pump + micron gauge, charging scale + refrigerant, digital manifold + temp probes · Condensate fittings, P-trap, primer/cement, pan tablets/treatment

0% complete

Recover & remove the old coil

Recover the charge and open the cabinet
Recover the full charge per EPA 608, then remove the coil-access panel. Disconnect and cap the condensate, and disconnect the TXV equalizer/bulb if applicable.
Tools: Recovery machine + cylinder
⚠ Recover before brazing — never open or torch a pressurized system. Venting is illegal.
Cut the coil free and slide it out
Cut the suction and liquid lines (cutter, not unbraze in place) and slide the cased/uncased coil out of the cabinet.
Tools: Tube cutter

Set the new coil & metering device

Set the matched coil level in the airflow
Slide the new coil in, correct orientation (the condensate must drain to the pan — an upside-down or wrong-airflow coil floods). Seat it square on the cabinet so air can't bypass it.
Target: Coil oriented for drainage + correct airflow direction; no air bypass around the slab
Install the correct metering device
Fit the matched metering device: a factory TXV (with its check valve on a heat pump) or the correct fixed orifice/piston for this coil/outdoor combination. Wrong orifice size = wrong feed and bad superheat.
Target: Correct TXV or piston/orifice per the AHRI-matched combination
Install a new liquid-line filter drier
Always install a fresh liquid-line drier when the system is open; arrow in the direction of flow.
Tools: New filter drier
Target: New drier, flow arrow correct

Braze, pressure-test & evacuate

Braze the connections with flowing nitrogen
Flow a trickle of dry nitrogen while brazing the suction and liquid connections (and heat-sink the TXV body/drier) so no oxide scale forms inside.
Tools: Brazing torch, Nitrogen + regulator
Target: Low flowing-N2 purge during brazing; heat-sink the TXV/drier
⚠ Never trap N2 between two closed points while heating. Fire watch.
Pressure-test with nitrogen, then deep-evacuate
Pressurize with dry nitrogen and confirm no pressure drop (leak test) BEFORE pulling a vacuum — never use the vacuum to find leaks. Then evacuate to a deep vacuum with a decay hold.
Tools: Nitrogen + regulator, Vacuum pump, Micron gauge
Target: N2 pressure test holds; then evacuate ≤ 500 microns and hold

Charge, drain & seal

Recharge and verify superheat / subcool
Weigh in the nameplate charge, run the system, and verify subcooling (TXV) or superheat (fixed orifice) to the target window.
Tools: Charging scale, Digital manifold + temp probes
Target: SH/SC to target after the system stabilizes
Rebuild the condensate pan, P-trap & secondary
Reconnect the primary drain with a correctly-sized P-trap (a draw-through system needs a trap or it won't drain / will pull air), confirm the pan slopes to the drain, and connect the secondary/overflow with its float switch. Pour water in and confirm it drains.
Target: Trapped primary drain that flows; secondary + float switch wired; verified with a water test
Seal the cabinet / plenum
Seal the coil access panel and plenum joints with mastic/tape so you don't lose conditioned air or pull attic air across the coil.
Target: Cabinet + plenum sealed (no air bypass/leakage)