Split-system AC and heat pump install field guide

What a split system is, and the three connections that join it

A split system is a central air conditioner or heat pump split into two pieces: an outdoor condensing unit and an indoor evaporator coil or air handler. Three things join them. A refrigerant lineset carries the charge between them, the control wiring tells the equipment when to run, and on a cooling or heat pump system the indoor coil drains condensate. Get all three right and the system runs the way the data plate promised. Miss one and you have a callback waiting.

This is the most common residential and light-commercial setup in the country, which is exactly why it gets installed wrong so often. The equipment is forgiving enough that a sloppy install still cools, for a while. The compressor that fails at year four instead of year fifteen, the coil that ices, the metering device that plugs: those trace back to the install, not the equipment.

The work below follows the order a real install runs. Set and level the condenser, run and size the lineset, braze it under nitrogen, pressure test, evacuate deep, weigh in and trim the charge, wire the disconnect and controls, then commission against the data plate. Each step protects the one after it.

Why does the condenser and indoor coil have to be a matched pair?

The condenser and the indoor coil have to be a matched, AHRI-rated pair because the published efficiency and capacity only exist for the specific combinations the manufacturer tested together. AHRI, the Air Conditioning, Heating, and Refrigeration Institute, certifies those combinations and issues a reference number. Bolt a new condenser onto whatever coil is already in the closet and you have a system with no certified rating at all.

The losses are real, not paperwork. A high-efficiency SEER2 condenser on an old, mismatched coil can net out closer to the efficiency of equipment two tiers down, because the coil surface and the metering device were never matched to the new compressor. You paid for capacity you will not get.

Then there is the warranty. Most manufacturers tie the ten-year parts warranty on a new condenser to a matched indoor coil. Install it mismatched and the coverage commonly drops to five years, one year, or nothing, and the utility rebates and tax credits tied to the rated system go with it. Verify the AHRI reference for the exact indoor and outdoor pair in the AHRI directory before the equipment goes on the truck, not after the inspector asks for it.

Setting the condenser: pad, level, clearance, and tie-down

The condenser sits on a level, stable pad, with clearance on every side for airflow and service, and it is anchored against wind. Set it out of level and the compressor oil return and the fan bearings both suffer over time. A composite or concrete pad on a compacted base keeps it level after the ground settles.

Clearance is set by the manufacturer, and the install manual is the authority, but the common figures are worth carrying. Roughly 12 to 24 inches on the sides where air enters, more on the service side so a tech can pull the panel and the fan, and several feet of open space above the fan discharge. Crowd the coil against a wall or a fence and the unit recirculates its own hot discharge air, head pressure climbs, and capacity drops on the hottest day, which is the day it matters.

Raise it off the ground enough to clear standing water, and in snow country to clear the seasonal snow line, which matters most on a heat pump that runs through winter. Keep it away from bedroom windows for noise. In high-wind and hurricane zones, anchor the unit to the pad with the manufacturer's tie-down or a code-approved hurricane strap, because a condenser that walks off its pad in a storm takes the lineset with it.

Sizing the lineset: length, lift, and oil return

The lineset is two copper lines: an insulated suction (vapor) line and a smaller bare liquid line. Size both per the manufacturer's tables, by the total equivalent length and the vertical lift, not by whatever diameter matches the stub-outs. Equivalent length is the straight run plus the added loss of every elbow and bend, so a tidy lineset with a lot of turns is longer than the tape says.

Every system has a maximum lineset length and a maximum vertical separation between the indoor and outdoor units. Common limits land around 80 feet of total line length for standard equipment, with the vertical separation usually capped lower, often around 50 to 60 feet, and long-line kits and specific models rated higher; the numbers that count are the ones in the install manual for that model. Exceed it and you starve the coil or fail to return oil to the compressor.

Oil return is the part that bites on tall risers. Refrigerant carries the compressor's oil around the loop, and on a long vertical rise the gas can lose the speed it needs to drag oil up the suction line. Manufacturers handle this with line sizing, suction traps at intervals on tall lifts, and in many long-line applications a required TXV. When the lift runs past what the manual allows without a trap, you add the trap. Skip it and the compressor slowly runs itself dry.

Why braze the lineset with nitrogen flowing?

You flow dry nitrogen through the lines while you braze to keep the inside of the copper from oxidizing. Heat a copper joint with air inside and the oxygen reacts with the hot copper to form a black scale, cupric oxide, that flakes off the wall. Those flakes travel with the refrigerant and oil straight into the metering device, the TXV or the piston, and into the compressor, where they plug orifices and score parts.

A trickle is all it takes. Set the regulator for a low flow, on the order of 2 to 5 cubic feet per hour, just enough that you feel a whisper of gas at the open end of the line. Start the nitrogen before the torch and keep it flowing until the joint cools, so no air gets pulled back across the hot copper. Too much flow cools the joint and fights the braze, so it is a whisper, not a blast.

This got more important after the industry moved to R-410A and the POE oils that came with it. POE has a solvent action that lifts loose scale the old mineral oil left alone, so the flakes that nitrogen prevents are the same flakes that now end up jammed in the metering device. The leak detection and recovery guide covers the brazing technique and the joint prep in more detail, since the same skill closes a repair.

How do you pressure test the lineset before evacuation?

After the brazing cools, pressurize the lineset and the coil with dry nitrogen and watch it hold before you ever pull a vacuum. A vacuum can hide a leak that pressure finds, so the pressure test comes first, always. Step the pressure up rather than slamming it to full. Bring it up in stages, hold, then climb, so a gross leak shows at low pressure before you put the full nitrogen charge behind a bad joint.

Test pressure is set by the weakest component you have isolated, and you never exceed it. For R-410A and the newer high-pressure refrigerants, shops commonly test in the 300 to 500 psi range, with manufacturers calling out their own figures and some stepping through low, medium, and high holds. Hold it long enough to trust it, commonly 30 to 60 minutes on a residential system and longer on larger work, and soap every joint and flare while it sits.

Read the gauge against a thermometer, because temperature moves the pressure on its own. A cool evening after a hot afternoon pulls the reading down with no leak at all, roughly a pound for every degree or two of swing, so a temperature-compensated gauge or a noted ambient keeps you from chasing a leak that physics caused. The leak guide walks the standing pressure test and how to localize a real drop.

How deep a vacuum does an AC install need?

Pull the system to about 500 microns with a micron gauge, then prove it holds. Microns are the unit that matters here, not inches of mercury, because the last bit of moisture and air lives below what a compound gauge can even read. Five hundred microns is the common target, and many techs pull to 300 on a system that sat open or carries POE oil, since that oil grabs moisture out of the air.

Two things make or break the time. Pull through the service ports with the valve cores removed using a core removal tool, because the Schrader core is a restriction that slows the pull and lies to the gauge. Use large-diameter, vacuum-rated hoses, not the quarter-inch charging hoses that choke the pump. Put the micron gauge as far from the pump as you can, ideally at the unit, so it reads the system and not the pump.

Then run the decay test. Valve off the pump and watch the gauge. A system that is tight and dry holds below roughly 500 microns for 10 to 15 minutes. A reading that climbs fast and keeps climbing is a leak. A reading that climbs slow and levels off is usually moisture still boiling off, so you keep pulling. The evacuation, the core tools, and the standing decay test are covered in depth in the leak and recovery guide.

Do you add refrigerant for a long lineset?

Yes. The condenser ships with a factory charge sized for a set lineset length, commonly the first 15 feet of standard liquid line, and any line beyond that needs refrigerant added per foot. The install manual gives the adder for the liquid line diameter. For a 3/8 in liquid line it runs in the neighborhood of half an ounce to six tenths of an ounce of R-410A per foot, but you use the number on the plate for the actual line size and refrigerant.

Weigh it in. Put the cylinder on a scale and add the calculated amount by weight, factory charge plus the per-foot adder for the line beyond the rated length. Weighing is the accurate way to set a new install, because it does not depend on the indoor and outdoor conditions the way a gauge reading does.

Then verify, and trim if needed. With the system running and the indoor airflow correct, confirm the charge by subcooling on a TXV system and by superheat on a fixed-orifice system, against the data plate target. Many plates call for subcooling around 8 to 12 degrees, or a stated value, whichever the plate gives. The full method, both numbers, and how to read them together is in the refrigerant charging guide. Do not set the charge by pressure alone.

Insulating the lineset and sealing the penetration

The suction line gets insulated end to end. The liquid line normally does not. The suction line runs cold, and the insulation keeps it from sweating and dripping inside walls and ceilings and from giving up capacity to the air it passes through. Slide the insulation on before you braze where you can, because threading split insulation over a finished line and taping the seam never seals as well.

Outside, the insulation has to survive sun. Bare foam chalks and crumbles in a couple of seasons of UV, so the outdoor run gets a UV-rated jacket, a paint made for the foam, or a line cover. The most common neglected spot is the few feet at the condenser where the line leaves the wall and the foam bakes.

Where the lineset passes through the wall, sleeve it and seal both sides against air, water, and pests. An open penetration is a path for humid air into the wall cavity and for water down the line into the structure. Seal it with the right sealant, pitch the sleeve slightly to the outside, and the wall stays dry.

Condensate: the drain, the trap, and the float switch

The indoor coil pulls water out of the air, and that water has to drain away cleanly or it finds the ceiling. The drain pan ties to a condensate line with a trap sized to the unit's static pressure, because without the trap a draw-through air handler either sucks air back up the drain or holds water in the pan that never leaves.

The trap shape matters. On a draw-through unit, where the blower pulls air across the coil, the pan sits under negative pressure, and the trap depth has to beat that negative pressure or the water will not flow. Get the trap too shallow and the pan stays wet, the coil grows biofilm, and the line clogs. Prime the trap at startup so it seals.

The float switch is cheap insurance, and it is the step that gets skipped. A secondary float switch in the pan or the secondary drain shuts the system off when the primary drain backs up, before the water reaches the ceiling. On an attic or above-ceiling air handler it is the difference between a clogged drain and a destroyed ceiling. Wire it into the control circuit so a blocked drain stops the call and the homeowner phones you before the drywall is ruined. The same drain, trap, and float logic applies to fan coil units and rooftop units by topic.

The disconnect, the whip, and reading MCA and MOCP

The condenser needs a disconnecting means within sight of it and not more than 50 feet away, so a tech can kill power standing at the unit. From the disconnect to the unit runs a flexible whip, usually liquid-tight, into the unit's connection. HVAC equipment falls under NEC Article 440, and the manufacturer sets the electrical limits on the nameplate.

Two numbers off the nameplate size the circuit. The MCA, minimum circuit ampacity, sizes the conductors: you size the wire to the MCA, not to the breaker. The MOCP, maximum overcurrent protection, is the largest breaker or fuse the unit allows, so you pick a device at or below the MOCP, never above it. Oversize the breaker past the MOCP and you have defeated the unit's protection, which is a violation and a hazard. The MCA already builds in the 125 percent on the compressor, so do not add it twice.

On a three-phase scroll compressor, rotation matters. A scroll only pumps one direction, and backward rotation is loud, moves no refrigerant, and damages the compressor fast. Check phase rotation at startup and swap two legs if it is wrong, before the compressor runs long enough to hurt itself.

Line voltage and the low-voltage control wiring

Two electrical systems live in the same equipment. The line-voltage side powers the compressor and the fans. The low-voltage side, commonly 24 volts from a transformer, is the control wiring that runs between the thermostat, the indoor unit, and the outdoor unit and tells everything when to run.

On a straight cooling system the low-voltage runs are simple: a wire for the call, the fan, the compressor contactor, and the common. A heat pump adds wires for the reversing valve and for the defrost and auxiliary heat, and the color and terminal conventions are a guide, not a guarantee, so you land wires by the wiring diagram on the unit, not by the color you expect. Mislabel the reversing valve wire and the system heats when it should cool.

Communicating systems change the picture. Instead of a wire per function, a communicating system runs a data bus between matched indoor and outdoor equipment and a communicating thermostat, and the boards talk to each other. That buys better staging and diagnostics, but it works only as a matched, same-brand set, and it is less forgiving of a miswire or a marginal connection. Follow the manufacturer's wiring exactly, because a communicating bus does not tolerate the field improvisation a 24-volt system shrugs off.

Heat pump specifics: reversing valve, defrost, and aux heat

A heat pump is the same split system that runs its refrigerant cycle backward in winter to pull heat from outdoor air into the house. A reversing valve does the flip, and the indoor coil that was the evaporator in cooling becomes the condenser in heating. Everything about the install holds the same, the matched pair, the lineset, the vacuum, the charge. The heat pump just adds controls.

Defrost is the first one. Running in heat, the outdoor coil drops below freezing and frost builds, so the control periodically reverses to cooling for a few minutes to melt it, with the outdoor fan off and often the auxiliary heat on so the house does not blow cold. A heat pump that ices over and never clears has a defrost control, sensor, or charge problem, and it shows up first in cold, damp weather.

Auxiliary and emergency heat back it up. Electric strip heat or a furnace covers the load when it is too cold for the heat pump to keep up and during defrost, and emergency heat locks out the compressor entirely if the heat pump fails. In cold-climate work the balance point, the outdoor temperature where the heat pump alone stops keeping up, drives how the aux heat is staged. Set it up so the expensive strip heat does not run when the heat pump could have carried the load.

Startup and commissioning the system

Commissioning is where the install gets proven, and most of the callbacks that look like bad equipment are commissioning that nobody finished. Before you ever judge the charge, set the airflow. The blower has to move the rated air against the actual external static pressure of the duct, because a system choked on undersized or dirty-filtered duct reads like a charge problem and is not. Measuring and correcting duct static and airflow is its own job, covered in air balancing by topic.

With airflow right, work the checks in order. Confirm the charge by subcooling or superheat against the plate. Take the supply and return temperatures and confirm a reasonable temperature split across the coil, commonly in the high teens to low twenties of degrees for cooling, depending on indoor humidity. Clamp the compressor and fan amps and compare to the nameplate. On a heat pump, run it through cooling, heating, defrost, and the auxiliary and emergency heat modes and watch each one actually do its job.

Then write it down. A commissioning record with the airflow, the subcooling and superheat, the split, the amps, and the vacuum you achieved is what proves the install and what the next tech reads when something drifts. An install with no numbers is an install nobody can stand behind.

A2L refrigerants on new equipment

New split systems now ship with A2L refrigerants like R-32 and R-454B, which are mildly flammable, and that changes a few things on the install. Equipment built and sold after the 2025 transition uses these refrigerants instead of R-410A, so most new installs are A2L now whether the homeowner knows it or not.

Mildly flammable does not mean it ignites easily. A2Ls burn slowly, need a high concentration, and take a strong ignition source, far more than a fuel gas. The real install considerations are charge limits and leak handling. The listing standards cap the refrigerant charge for a given room size so a worst-case leak cannot reach a flammable concentration, and larger systems in occupied spaces now commonly require a refrigerant detection system that shuts the unit down and runs the indoor fan to dilute a leak.

The handling rules carry over with additions. You still need EPA Section 608 certification to handle the refrigerant, and the leak and recovery practices apply, with attention to ignition sources around an open system. Flow nitrogen and keep open flame away from a charged A2L circuit, and follow the manufacturer's A2L-specific install instructions, which are not optional on this equipment. The leak guide covers A2L detection and handling in more detail.

What the owner has to maintain

An install is the start of a maintenance relationship, and the system the owner inherits has four things that need attention or it fails early. The filter is first and most ignored. A dirty filter chokes airflow, drops capacity, and mimics a charge problem, and it is the one thing the owner can do. Set them up with the right filter size and tell them the interval.

The outdoor coil has to stay clean. Grass clippings, cottonwood, and dryer lint mat the coil fins, head pressure climbs, and efficiency falls, so the coil gets rinsed gently from the inside out, not blasted with a pressure washer that folds the fins. The condensate drain has to stay clear, which on a humid-climate system means flushing the trap and line before each cooling season so the float switch never has to do its job.

The charge should not need touching. A sealed system that was installed tight and evacuated deep does not consume refrigerant. If it is low next year, it leaked, and the answer is to find and fix the leak, not to top it off. A tech who adds refrigerant to a leaking system every spring is selling the customer a slow vent and a bigger repair later.

What to document

The commissioning record is the proof the system was installed right and the baseline the next tech measures against. Capture the readings and the install details together, because a number with no context cannot be reproduced.

Common mistakes

• Installing a mismatched coil and condenser, losing the rating, the efficiency, and the warranty.
• Brazing without nitrogen flowing, so cupric oxide scale plugs the metering device later.
• A weak or skipped evacuation, leaving moisture and air that acid up the oil and ice the metering device.
• Charge off because the per-foot lineset adder was never added, or the charge was set by pressure instead of subcooling and superheat.
• Setting the condenser with no service clearance or out of level, starving airflow and the oil return.
• Wiring a three-phase scroll backward and running it on reversed rotation.
• Skipping the condensate float switch on an attic or above-ceiling air handler.
• Sizing the conductors to the breaker instead of the MCA, or fusing above the MOCP.

Field checklist

Standards and references

The manufacturer's installation instructions govern this work, and that is not a hedge. They set the lineset sizing and limits, the clearances, the charge and the per-foot adder, the test pressure, and the required vacuum, and where general guidance and the install manual disagree, the manual wins because the equipment is listed to it.

AHRI certifies the matched indoor and outdoor combination and the rating it carries, and the AHRI directory is where you confirm the pair. Refrigerant handling, recovery, and the ban on venting fall under EPA Section 608, covered in the leak and recovery guide. The electrical side follows the adopted electrical code, with HVAC equipment under NEC Article 440, the disconnect within sight, conductors sized to the MCA, and overcurrent at or below the MOCP. The mechanical code, the IMC in many jurisdictions, covers the condensate, the clearances, and the equipment access.

The energy code and the efficiency rating, SEER2 on current equipment, depend on the matched, properly charged system actually being installed the way it was rated. A2L equipment follows the equipment listing and the manufacturer's A2L instructions for charge limits and leak detection. Article and section numbers shift between code cycles and the refrigerant rules keep moving, so confirm the adopted editions and the current refrigerant regulations against the jurisdiction and the manufacturer before you rely on a specific number.

Units and terms

Split-system work carries its own vocabulary, and the same part goes by a few names across a data plate, a manual, and a spec.

The condenser is the outdoor condensing unit. The indoor coil is the evaporator, paired with a furnace or an air handler. The suction line is the larger vapor line; the liquid line is the smaller one. Vacuum is read in microns, where 25,400 microns is one inch of mercury and 760,000 microns is atmospheric pressure. Charge is added by weight in ounces and pounds. Electrical limits read off the plate as MCA and MOCP.

AHRI

The body that certifies the matched indoor and outdoor combination and its rating

Lineset

The insulated suction (vapor) line and the bare liquid line joining the units

Equivalent length

Straight lineset length plus the added loss of every elbow and bend

Micron

Vacuum unit; 500 microns is the common evacuation target, 25,400 microns is 1 inch Hg

MCA / MOCP

Minimum circuit ampacity sizes the wire; maximum overcurrent protection caps the breaker or fuse

Subcooling / superheat

Liquid below and vapor above saturation; the readings that verify charge

A2L

The mildly flammable low-GWP refrigerant class, such as R-32 and R-454B

Reversing valve

The valve that flips a heat pump between cooling and heating

Related tools

Calculators and readiness checks for this work

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FAQ

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