Residential HVAC Install Walk-through

A new construction changeout or replacement install is the single most common type of work a second-year HVAC tech will run. A clean install takes a day. A dirty install takes three days, one of which is the comeback to find the slow leak you left in a flared connection that was never torqued. The difference is not speed - it is the sequence and the skipped steps.

This walk-through covers a residential split system changeout from pre-install load calculations through final start-up. Every step matters. Skipping pressure test, skipping nitrogen purge during brazing, or skipping the static pressure reading at commissioning builds a time bomb into the install.

Pre-Install: Manual J, Manual S, Manual D

Before a tech touches a wrench, the install needs three ACCA (Air Conditioning Contractors of America) calculations. In many jurisdictions these are required by code; in every case they are required to size equipment correctly.

Manual J - Load Calculation

Manual J is a room-by-room heat loss and heat gain calculation for the house. Inputs:

• Square footage of each room.
• Ceiling height.
• Window area, type (single / double / low-e), and orientation.
• Wall insulation R-value.
• Ceiling / attic insulation R-value.
• Floor construction (slab / crawl / basement).
• Air leakage (ACH50 or estimate).
• Design day outdoor temperatures for the location (ASHRAE 99 percent winter, 1 percent summer).
• Internal heat gains (people, appliances, lights).

Output is total BTU/hr heat gain (cooling load) and total BTU/hr heat loss (heating load) at design conditions. Wrightsoft Right-J, Elite Software RHVAC, and CoolCalc are the industry tools.

Do not size by square footage alone. The old "500 sq ft per ton" rule of thumb oversizes modern tighter homes by 30-50 percent. An oversized system short-cycles, does not dehumidify, and eats compressor life.

Manual S - Equipment Selection

With the Manual J load in hand, pick equipment whose AHRI rating matches the load at design conditions. Rules:

• Cooling equipment should not exceed 115 percent of the Manual J sensible load. Oversize = poor dehumidification.
• Heating equipment (heat pump or furnace) can be oversized moderately, 125-140 percent of heating load on a heat pump with strip backup.
• Match indoor coil (evaporator) to outdoor condenser per AHRI certified pairing. Mismatched coils lose 15-20 percent capacity and void warranty.
• Refrigerant must match: R-410A coil for R-410A condenser, R-454B for R-454B, etc.

Manual D - Duct Design

Manual D sizes every duct run for the CFM it must carry, at a target friction rate (typically 0.08 inches water column per 100 feet) and a total external static pressure budget of about 0.5 inch wc for the complete air handler-plus-duct system.

Inputs:

• CFM per room (400 CFM per ton cooling / room square footage share).
• Total trunk size based on air handler CFM.
• Branch duct size from a friction rate chart.
• Return duct sized equal to or larger than supply, same velocity budget.

Bad duct design kills good equipment. A high-SEER system on undersized ducts sees high static pressure, reduced airflow, lower capacity, and premature failure.

Condenser Pad and Placement

The outdoor unit needs:

• Level pad - concrete (pre-cast 3 inch pad is standard) or a plastic composite pad (DiversiTech). Level left-to-right and front-to-back. An unlevel compressor wears bearings and oil-starves on slope.
• 18-24 inch service clearance on the control side for panel access. 4 ft above for airflow (6 ft minimum in some manufacturer specs).
• Not under a dryer vent, kitchen exhaust, or pool chemical storage - lint, grease, and chlorine eat coils.
• Not in a roof valley or under a roof drip line without a drip shield.
• Secured against uplift - hurricane straps in coastal zones (Florida, Gulf Coast), snow stands in heavy snow zones.
• Condenser disconnect within sight of the unit per NEC 440.14. Fused or non-fused per the outdoor unit MOCP.

Line Set Selection and Prep

Line sets connect indoor and outdoor. Two copper refrigerant lines plus the thermostat cable.

Typical sizes by tonnage (standard split):

Tonnage	Liquid Line	Suction Line
1.5 ton	1/4 inch	5/8 inch
2 ton	3/8 inch	3/4 inch
3 ton	3/8 inch	7/8 inch
4 ton	3/8 inch	1-1/8 inch
5 ton	3/8 inch	1-1/8 inch

Always confirm against the manufacturer spec for the specific model. Variable-speed and heat pump units may call for different sizes.

Insulation: suction line always, with minimum 1/2 inch closed-cell foam (Armaflex or similar). Liquid line typically bare, though 3/8 inch insulation is recommended in hot attics to prevent flash gas.

Line Set Prep - Cutting, Deburring, Flaring

1. Cut square. Use a tubing cutter, not a hacksaw. Hacksaw burrs contaminate the system.
2. Deburr inside and outside with a deburring tool. A burr creates turbulence and pressure drop, and can flake off into the compressor.
3. Flaring. Use a 45 degree SAE flare block and a listed flaring tool. Slide the flare nut on the tube before flaring (every tech has forgotten this at least once). Proper flare is a mirror-finish even taper with no cracks.
4. Torque to manufacturer spec with a calibrated torque wrench. The old "hand tight plus three-quarter turn" rule cracks fittings. Typical torques:

Tube Size	Typical Torque (in-lb)
1/4 inch	60-80
3/8 inch	90-110
1/2 inch	95-125
5/8 inch	110-130
3/4 inch	135-155

Confirm torque against the specific unit's installation manual - some brands call different numbers.

Line Set Prep - Brazing with Nitrogen Purge

For systems where you braze rather than flare (most central split installs, some mini-splits at the outdoor unit):

1. Use 15 percent silver alloy filler rod (Harris Safety-Silv 15 or equivalent) for copper-to-copper. 45 percent silver (Stay-Silv 45) for copper-to-brass.
2. Purge with nitrogen at 2-5 SCFH through the line while brazing. Set a nitrogen regulator on low flow, introduce at one end of the line, relieve at the open end. Without purge, copper oxide scale forms inside the line from the heat - black, flakey, and eventually plugs the TXV or chokes the filter-drier.
3. Clean the copper with emery cloth or a fitting brush. Flux is not used on pure copper-to-copper with silver braze above 5 percent silver.
4. Heat the joint evenly with the torch tip sweeping back and forth. Bring the copper to a dull cherry red. Feed the rod into the joint, not onto the flame - capillary action draws the solder into the gap.
5. Once the joint is flooded, pull the torch away and let it cool. Do not quench with water - it cracks the braze.
6. Leave the nitrogen flowing until the copper has cooled below red.

Every braze without nitrogen purge is a future compressor call.

Thermostat Wiring

Standard 18/8 thermostat cable (18 AWG, 8 conductors) carries the 24VAC control signals between thermostat, air handler, and outdoor unit. Conductor color convention for a standard split heat pump:

Wire	Color	Function
R / Rh / Rc	Red	24V transformer hot (Rh heat, Rc cool, R common on single transformer)
W / W1	White	Heat call (furnace) or aux heat call (heat pump)
W2	Brown/pink	Second-stage auxiliary heat
Y / Y1	Yellow	Cool call / compressor first stage
Y2	Blue/pink	Compressor second stage
G	Green	Indoor blower call
C	Blue/black	Common (24V return) - required for smart thermostats
O / B	Orange / Dark blue	Reversing valve - O energizes in cooling (Carrier/Trane), B energizes in heating (Rheem/Ruud)

Key rules:

• Every modern thermostat (Nest, Ecobee, Honeywell T-series, Lennox iComfort) needs a C wire. Do not rely on "add-a-wire" kits on a heat pump install.
• On a heat pump, run O/B to the correct terminal for the brand convention. Getting it backward reverses heating and cooling calls.
• Label every wire at both ends. A labeled panel saves you 40 minutes on the first service call three years from now.

Pressure Test with Nitrogen

Before evacuation, verify the new refrigeration circuit is leak-free by pressurizing with dry nitrogen. Oxygen or air is never used - compressed air carries moisture and oxygen, and can ignite refrigerant oils under compression.

1. Close both service valves on the outdoor unit (to isolate the factory charge from your test).
2. Connect a nitrogen regulator to the manifold yellow hose.
3. Slowly pressurize the line set and indoor coil (outdoor condenser is isolated) to 150 psig. Hold 15 minutes. Watch for pressure drop.
4. If no drop, pressurize to 300 psig and hold another 15-60 minutes. 300 psig finds small leaks that 150 will not.
5. Soap-bubble every connection you made. Any bubble is a leak. Repair and re-test.
6. Release nitrogen slowly through the recovery port. Do not dump - you will cool the line set and potentially frost the Schrader valves.

Evacuation to Below 500 Microns

A proper vacuum removes air, moisture, and non-condensables so the refrigerant circuit is clean at start-up. Target is below 500 microns (measured with a micron gauge, NOT a manifold gauge - manifold gauges cannot read deep vacuum).

Two methods:

• Single evacuation - pump continuously until micron gauge reads below 500 microns, isolate the pump, watch decay. If it holds below 1000 microns for 10 minutes, you are done. A fast rise to atmospheric = leak. A slow rise to 2000 and stopping = moisture.
• Triple evacuation - pump to 1500 microns, break vacuum with dry nitrogen to 5 psig, pump down again to 500 microns, break again with nitrogen, pump down one final time below 500 microns. Used when moisture contamination is suspected (compressor burn-out, open system).

Use a two-stage vacuum pump sized at 5 CFM for residential; 8 CFM for larger commercial. Change the pump oil every few jobs - dirty oil will not pull deep vacuum.

Connect the micron gauge to the system side of an isolation valve (a core-removal tool isolating the Schraders, or a dedicated isolation tee), not on the pump side. Reading pump-side vacuum tells you only that the pump works, not that the system is evacuated.

Release the Factory Charge

Once the system holds below 500 microns, isolate the pump, close the valves on the manifold, remove the pump. Open the outdoor unit service valves with an allen or valve wrench, counter-clockwise until fully open (back-seated). The factory charge floods in.

Replace the service caps with new o-rings if supplied. Tighten the caps - they are the final seal, not just dust covers.

Start-up Checklist

With power at the disconnect, thermostat calling cool:

1. Verify voltage L1-L2-GND at the contactor: 208-230V nominal, within 10 percent.
2. Contactor pulls in on call from thermostat.
3. Outdoor fan starts.
4. Compressor starts within 2 seconds of fan.
5. Measure compressor amps with clamp meter. Compare to RLA on nameplate.
6. Measure outdoor fan amps. Compare to FLA on nameplate.
7. Run 15 minutes, then read:
   • Suction pressure and suction line temp (superheat if piston, reference only if TXV).
   • Liquid pressure and liquid line temp (subcool, target 8-12 deg F per manufacturer chart).
   • Indoor delta-T across the coil (target 17-22 deg F).
   • Static pressure across the air handler (target 0.5 inch wc total).
8. Record all readings on the install sticker inside the air handler door.

Punch List and Inspection

Before you pack out:

• Condensate drain is sloped 1/8 inch per foot minimum, with a proper P-trap on the air handler side.
• Condensate safety switch (float switch or sensor) tested - lift the float, the system should shut off.
• Attic installs: secondary drain pan under the air handler with its own dedicated drain line, OR a float switch in the secondary pan that kills the unit on overflow (per IRC M1411.3 / IMC 307.2.3). Missing secondary protection is the number-one attic-install red-tag and a common water-damage callback.
• Line set is insulated continuously from coil to condenser, with no gaps or cracks.
• Line set wall penetration is sealed (putty, caulk, foam).
• Disconnect box is labeled and lockable.
• Thermostat programmed for the customer's schedule.
• Filter is new, correct size and MERV rating.
• Customer walk-through: show the filter location, the breaker, the disconnect, the thermostat, the condensate drain cleanout.
• Permit and inspection closed with AHJ (authority having jurisdiction). Final sign-off before the customer pays the balance.

Day 1 Checklist

• [ ] No install without a Manual J. "It was a 3-ton so we put in a 3-ton" is how you oversize by 30 percent.
• [ ] Match indoor coil to outdoor condenser per AHRI. Mismatched coils void warranty.
• [ ] Braze with nitrogen purge at 2-5 SCFH flowing, every time. No exceptions.
• [ ] Flare connections get a calibrated torque wrench, not a guess.
• [ ] Pressure test with nitrogen to 300 psig before evacuating. Never air, never oxygen.
• [ ] Evacuate below 500 microns and verify with a micron gauge (not manifold).
• [ ] Confirm O/B thermostat terminal matches the heat pump brand convention.
• [ ] Take all readings at 15 minutes of runtime and write them on the install sticker.
• [ ] Close the permit before you leave.

Expert Tips

• "Size by Manual J, not by what was there." The old system was likely wrong too.
• "Nitrogen purge during brazing is non-negotiable." Scale is invisible until it kills the TXV.
• "Torque wrenches do not lie. Technicians do." Flares fail from over- and under-torque both.
• "Micron gauge or you do not know." Manifold gauges cannot read real vacuum.
• "Write the commissioning numbers on the sticker." The next tech needs to know where you left it.
• "Close the permit, collect the check." An unclosed permit is a lien waiting to happen.