Replacing Compressor On Ac Unit: A Step‑by‑Step Guide (Tools, Safety & Pro‑Level Checks)
- Adam Haas
- 5 days ago
- 10 min read
Before You Start: Safety, EPA 608, and When Not to DIY
Replacing a home AC compressor isn’t like swapping a thermostat. It involves electrical isolation, refrigerant recovery, brazing, deep vacuum, and charging to manufacturer specs. In the U.S., it’s illegal to vent refrigerant; you must recover it with proper equipment and, in many cases, hold EPA Section 608 certification to service systems containing regulated refrigerants. Even if you’re certified, check local codes and permits; some jurisdictions require a mechanical permit for compressor change‑outs and may have line‑set length and pad/elevation rules to protect against flooding or vibration.
My process starts with absolute safety: kill power at the disconnect, verify with a meter, and lock out the breaker so it can’t be turned back on while you’re inside the condenser. I also remove the fan shroud and control panel for clear access to wiring and tubing. As I like to say when I’m on site, remove the power from the condensing unit before touching anything else. From there, every step is about protecting the new compressor’s oil from contamination (moisture, acid) and ensuring the system is sealed and dry before the first ounce of refrigerant goes back in.
Be honest about your tooling and skill: if you don’t have a recovery machine and tank, nitrogen with a regulator that has a braze setting, a vacuum pump with a micron gauge, and a refrigerant scale, stop here and hire a pro. Also consider warranty: many OEMs require licensed installation for parts coverage, and some will deny claims if the change‑out wasn’t commissioned to spec (new filter‑drier, proof of deep vacuum, etc.). Finally, if you suspect compressor failure caused by contamination (burnout), you’ll need extra cleanup measures (e.g., suction‑line driers and oil acid remediation). In my case on a recent swap, I ran an acid test—“This test is instant, and it came back negative for acid.” That allowed me to proceed with a standard drier change and evacuation instead of a full burnout protocol.
Bottom line: If you lack the tools, certification, or confidence to recover and weigh refrigerant, braze with nitrogen, and verify a deep vacuum with a decay test, don’t DIY this specific repair. The rest of this guide assumes you can do those things safely and legally.
Is the Compressor Really Bad? Quick Diagnostics to Confirm
Before you condemn the compressor, confirm it’s truly failed. Common symptoms include repeated breaker trips, humming without starting, high amps, or pressures that never reach expected splits. Check the run capacitor and contactor first; they’re simpler and often the culprit. Look for open windings (ohm test across common/start/run), short to ground (megger or resistance to chassis), and locked rotor (LRA spike with no start). In systems with a TXV, don’t confuse a stuck valve or a restricted filter‑drier with a bad compressor; both can mimic low flow.
In my recent case, the unit was no longer pumping refrigerant. After verifying controls and capacitor, I performed that quick acid test (negative), then connected recovery gear to see what I was working with. The recovery told a story: “I only pulled out three pounds from this unit… with an additional 50 feet of line set.” That’s below what you’d expect for the factory charge plus a long line. Chronic undercharge is a compressor killer—poor cooling, oil return issues, and overheating shorten compressor life.
Other quick tells:
Suction pressure barely moves with the compressor running, or it rises/falls abnormally when you block airflow—often points to pumping issues.
Discharge line isn’t hot relative to suction on start‑up—again suggests poor compression.
High superheat + low subcooling at steady state can point to undercharge or leaks upstream from the compressor diagnosis.
Once you’ve ruled out the simple electrical parts and confirmed the compressor is the problem, proceed with a methodical plan that includes refrigeration best practices (nitrogen purge, drier replacement, deep vacuum, and measured charging). That’s what follows.
Tools, Parts, and Materials You’ll Need
You can’t do a proper ac compressor replacement without the right kit. Here’s what I lay out before I touch a torch:
Refrigeration & recovery
Recovery machine and DOT recovery cylinder—sized for your expected charge.
Refrigerant scale—for both recovery logging and precise charging by weight.
Virgin refrigerant matching the nameplate—no mixing.
Brazing & nitrogen
Oxy‑fuel or air‑acetylene torch, silver/phos‑copper rods suited for copper and service fittings.
Nitrogen cylinder with a regulator that has a “braze” setting—I’ll “purge a generous amount of nitrogen” to start, then back it down for the braze to prevent oxidation inside the tubing.
Soap solution for leak checks.
Vacuum & measurement
Quality vacuum pump (I use a Fieldpiece VP67) with vacuum‑rated 3/8" hoses.
Micron gauge—non‑negotiable.
Valve‑core removal tools (I use three Appion tools) so cores stay out during evacuation and one port is dedicated to the micron gauge. Full‑flow evacuation setup (cores out, big hoses) → why removing cores matters.
Electrical & components
New compressor (correct model/orientation), crankcase heater if used, new filter‑drier (liquid line), new Schrader cores, contactor (and capacitor where appropriate).
Wire labels, heat‑resistant zip ties, and terminal hardware.
Access & handling
Compressor tote/lift—a simple but game‑changing tool for safe removal/install.
PPE: safety glasses, gloves, hearing protection, sleeves for brazing.
Pro tip: I prep the enclosure first—remove the four screws holding the fan motor, slide the control panel out to label wiring, and cut old zip ties so the fan assembly can be lifted out cleanly. That gives full access for clean tubing cuts and compressor extraction.
How to Replace a Home AC Compressor (Central Split System)
Power Down, Open the Condenser, and Recover Refrigerant
De‑energize the condenser at the disconnect and breaker; verify with a meter.
Remove the fan shroud and control panel; photograph and label all wiring.
Connect your recovery machine to liquid and suction service ports. Pull to near‑vac on the low side and monitor scale to log total recovered.
If your recovered amount is well below nameplate plus line‑set adders, note it. In my case, only about three pounds came out on a system with ~50 feet of line—likely a cause of failure.
Cap lines and keep the work area clean.
Legal note: Do not vent refrigerant. If you’re not equipped for recovery or not certified where required, stop and hire a pro.
Remove the Old Compressor (Cut vs. Sweat), Crankcase Heater & Mounting
With refrigerant recovered, cut the copper stubs to free the old compressor.
I used to sweat them out; “I feel cutting is a cleaner option” that avoids overheating nearby components and keeps the joint area tidy.
Disconnect electrical leads, remove mounting bolts, and use a compressor tote to lift it out safely.
Transfer parts: slide the crankcase heater back over the new shell before piping, and place the compressor on the pad.
Hand‑tighten the mounting bolts to align the stubs before brazing—much easier than fighting alignment afterward.
Remove the compressor plugs, dry‑fit the copper so it’s snug and tight—you don’t want a joint shifting mid‑braze.
Nitrogen Purge, Brazing the Joints, and Replacing the Filter‑Drier
Remove valve cores from both liquid and suction ports to open flow.
Hook up nitrogen with a braze‑mode regulator. I’ll “purge a generous amount of nitrogen” first to clear the lines, then back it down to the braze setting (a slight whisper of flow) to keep oxygen out while brazing. Keep the liquid line open so nitrogen has an exit path. AHRI research on nitrogen purging while brazing → shows oxidation reduction with nitrogen flow.
Replace the liquid‑line filter‑drier. I cut the old drier out rather than sweating it, since heating can release trapped moisture back into the system. Sporlan guidance on replacing the liquid‑line filter‑drier → product bulletin with best practices.
Braze all joints smoothly with appropriate filler; avoid overheating valves and protect components with wet rags/heat blocks as needed.
Pressure Test to ~150 psi and Bubble‑Check for Micro‑Leaks
Introduce dry nitrogen to ~150 psi (or as appropriate for your system and ambient) and let it stabilize.
Soap every joint—look for slow bloomers and listen for faint hiss.
Hold 10–15 minutes; watch for pressure drop. If it holds steady, release the nitrogen safely (it’s inert), and move to evacuation.
Deep Vacuum & Decay Test: Hitting Target Microns the Right Way
For evacuation I use a Fieldpiece VP67 with two 3/8" vacuum‑rated hoses and three valve‑core removal tools so I can dedicate one port to the micron gauge. Keep cores out during evacuation—it materially speeds down to the low microns.
My sequence:
With core tools closed, start the pump with the gas ballast open to vent vapors.
Open all core tools and watch the micron gauge. “Once the micron gauge gets around 3,000 microns, I close the gas ballast.”
Pull to your target—many techs aim for ≤500–700 microns on the gauge connected directly to the system, not through hoses/manifolds.
Now do a decay test:
Valve off the core tools to isolate the system from the pump.
Watch the micron rise for 10 minutes. I use this rule of thumb I mentioned on site: “It should not go above 1,000 microns over the period of 10 minutes.”
A small rise (e.g., ~100 microns in ten minutes) indicates a dry, tight system. If it spikes, chase leaks, change the drier if contaminated, and pull again.
This step is where many DIY attempts fail. The decay test gives you the real dryness of the piping and compressor shell—don’t skip it.
Charging by Weight, Then Dialing In Subcooling (TXV) or Superheat (Piston)
With the system isolated and verified dry, break the vacuum with refrigerant. I purge any air from manifold hoses and use virgin refrigerant. My workflow:
Weigh in the initial amount—typically what you recovered as a starting point—then dial to spec. On TXV systems, charge by target subcooling; on fixed‑orifice systems, charge by target superheat.
Account for line‑set length beyond the nameplate training charge. Manufacturers publish add‑on tables (e.g., many 3/8" liquid lines require ~0.5–0.6 oz/ft, but confirm for your model). In my example job, I added about two pounds to reach final targets due to the ~50' line.
Stabilize. I let the system run 15 minutes before making big adjustments. As I noted in the field, “after 15 minutes of running, I had a very high superheat with a very low sub‑cooling”—a classic low charge indicator—so I added refrigerant slowly in small increments.
Watch pressures, line temps, SH/SC, and compressor amps. Keep ambient conditions in mind (sun, airflow, indoor load).
When you’re close, install brand‑new valve cores, remove temporary gauges, reassemble panels, and tidy wiring with new zip ties and labels.
Commissioning Checklist: Numbers to Record (SH, SC, Amps, Ambient)
Commissioning is your proof the job was done right and your baseline for future service. I log:
Outdoor ambient, return/supply temps, and indoor wet‑bulb if available.
Suction/Discharge pressures, suction line temp, liquid line temp.
Calculated Superheat (SH) and Subcooling (SC) against targets (TXV: SC; piston: SH).
Compressor amps vs nameplate RLA/LRA.
Voltage at the contactor under load.
Final charge weight and line‑set length.
What Good Looks Like: Example Final Readings
On my recent swap, I finished with “13° superheat, 11° subcool, 4.5 amps.” That’s comfortably within typical targets for a TXV system in moderate ambient, and the system cooled smoothly. Targets vary by brand, coil match, and weather, so always charge to manufacturer specs—not someone else’s numbers. ANSI/RESNET/ACCA 310—grading HVAC installation quality → modern standard for field verification.
Printable quick‑check (copy/paste):
Safety: power off/lockout verified ☐;
recovery complete ☐;
acid test (if needed) ☐
Mechanical: compressor mounted/bolts torqued ☐;
crankcase heater positioned ☐
Brazing: nitrogen flowing ☐;
new liquid‑line drier ☐;
joints cooled/inspected ☐
Leak test: nitrogen ~150 psi ☐;
bubbles checked ☐;
hold 10–15 min ☐
Evacuation: cores out ☐;
<700 microns ☐;
10‑min decay <1,000 microns ☐
Charging: hoses purged ☐;
break vacuum with refrigerant ☐;
weigh in ☐
Commissioning: SH ___°F;
SC ___°F;
Amps ___A;
Ambient ___°F;
Final weight ___ lb/oz
Common Mistakes to Avoid (and How to Fix Them Fast)
Skipping nitrogen flow while brazing → black flakes/scale inside lines. Fix: always use a low nitrogen sweep during braze.
Not replacing the filter‑drier → moisture and acids circulate into new oil. Fix: cut out the old drier and install a new one in the liquid line.
Leaving Schrader cores in during evacuation → slow, shallow vacuum. Fix: core tools plus a dedicated port for the micron gauge.
No decay test → you think it’s dry when it isn’t. Fix: isolate and verify <1,000 microns rise/10 min.
Charging blindly by pressure → poor performance and repeat failures. Fix: weigh in, then set SH/SC after a 15‑minute stabilization.
Ignoring line‑set length → undercharged systems run hot and die early. Fix: add per OEM tables for your liquid line size and length.
Temperature clamps in direct sun or on dirty copper → bad readings. Fix: clean/sand clamp spots and shade them (I always do this).
Not replacing the contactor/capacitor on a tired unit → early call‑back. Fix: replace worn electrics during the swap and record values.
Cost, Time, and Warranty: DIY vs Hiring a Pro
DIY can make sense only if you already own (or can borrow) recovery, nitrogen, vacuum, charging, and brazing gear—and you’re comfortable using it. Otherwise, tool costs alone can rival the labor line on a professional invoice. The compressor price depends on tonnage, brand, and refrigerant (R‑410A vs legacy blends), and warranties often require licensed installation and proof of proper commissioning (new drier, deep vacuum, charge to spec).
Timewise, I plan a half‑day to full day for a clean swap: recover, remove, braze with nitrogen, pressure‑test, evacuate to low microns with a decay pass, charge, and commission. Add time if you’re dealing with burnout cleanup, hard‑to‑access condensers, or long line‑sets. If you’re on the fence, get a quote and ask the contractor to list what’s included (drier, nitrogen brazing, evacuation specs, commissioning numbers). That checklist tells you whether they do it right—the same way I outline here.
FAQs: Compressor Replacement
Can I replace my home AC compressor myself?
Only if you can legally recover refrigerant, braze with nitrogen, evacuate to low microns, and charge by weight. Lacking any of those means you should hire a pro.
What tools are absolutely required?
Recovery machine/tank, nitrogen with regulator (braze setting), torch and rods, vacuum pump + micron gauge, refrigerant scale, core tools, new filter‑drier, and standard electrical tools (plus PPE).
What micron level is “good enough”?
Aim for ≤500–700 microns on the gauge attached directly to the system, then run a 10‑minute decay. I expect the rise to stay well under 1,000 microns.
How long should I let the system run before adjusting charge?
At least 15 minutes. I don’t add refrigerant until the system is stable. If I see high superheat and low subcooling, I add refrigerant slowly and recheck.
Do I always need to replace the drier?
Yes—any time the system is opened. I cut out the old drier to avoid releasing trapped moisture and install a new liquid‑line drier.
How much refrigerant should I add for a long line‑set?
Follow your manufacturer’s add‑on table for the liquid‑line size and length. Don’t guess. In my example with ~50', I ended up adding roughly two pounds to hit targets.
Conclusion
Compressor change‑outs live or die on discipline: nitrogen while brazing, new drier, deep vacuum with a decay pass, and charging to spec. In my field example, the winning steps were simple but strict—purge nitrogen generously, close the gas ballast around 3,000 microns, verify a tiny decay rise, then stabilize 15 minutes before dialing in charge. The payoff was clean numbers—13° SH, 11° SC, 4.5 A—and a system that runs the way it should.
If any step above feels like a stretch (or illegal without your local certification), bring in a licensed pro and use this guide as your quality checklist.
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