AC Installation Troubleshooting: Common Problems and How to Fix Them

AC Installation Troubleshooting: Common Problems and How to Fix Them

Here’s something that surprises most Los Angeles homeowners: the brand-new AC system that won’t cool on day three usually isn’t broken. In our experience across thousands of installations, roughly eight out of ten post-installation complaints trace back to a refrigerant charge error, a miscalibrated thermostat, or an airflow setting that takes ten minutes to correct. The unit itself is fine. What you’re seeing is an installation detail that got missed during the final walkthrough, not a warranty claim. This guide walks you through the five AC Installation Warning Signs Every Homeowner Should Know that show up most often after a new AC goes in, what each one actually means, and how to tell whether you’re dealing with a contractor fix or a manufacturer defect.

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Quick Answer

Most AC installation problems in the first 30 days are not equipment failures. Weak airflow, warm air from vents, water leaks, loud startup noises, and unresponsive thermostats typically stem from refrigerant charge errors, duct sealing oversights, or calibration misses that your installing contractor should correct at no charge. Before calling for a warranty claim, verify the technician completed a full AC Repair Maintenance Checklist for Los Angeles Homeowners and ask for a copy of the refrigerant charge verification.

Table of Contents

• The Five Most Common Post-Installation Symptoms
• Refrigerant Undercharge: The #1 Performance Complaint
• Warranty Defect vs. Installation Error: Who Pays for What
• The Commissioning Checklist Every Homeowner Should See
• Thermostat and Ductwork Issues That Mimic Equipment Failure
• What the Kingswell Guarantee Covers After Installation
• Common Mistakes to Avoid
• When to Call a Professional
• Frequently Asked Questions
• The Bottom Line

The Five Most Common Post-Installation Symptoms

After a new AC installation in Los Angeles, these five complaints show up more than all others combined. Each points to a specific category of problem, and knowing which is which saves you from an unnecessary service call fee.

1. Weak airflow from vents. If the air coming out feels faint compared to your old system, the culprit is rarely the blower motor itself. More often, the ductwork wasn’t properly sealed at the new connection points, or the static pressure (the resistance the air meets as it travels through your ducts) wasn’t measured and balanced during startup. In older Los Angeles neighborhoods like Highland Park or Eagle Rock, where ductwork may be original to a 1950s home, mismatched airflow is especially common. The new unit pushes more air than the old ducts were designed to carry, and without proper balancing, that pressure escapes through gaps or gets bottlenecked at sharp turns.

2. Warm air blowing when cooling is called for. This is the symptom that sends homeowners into a panic, but it’s frequently a simple reversal. The reversing valve on a heat pump system, or the low-voltage wiring between thermostat and condenser, may have been misconfigured. We’ve also seen this happen when a dual-fuel system defaults to furnace mode because the outdoor temperature lockout wasn’t set correctly for Los Angeles’s mild shoulder seasons.

3. Water leaking from the indoor air handler. New installations should not leak. When they do, the cause is almost always a condensate drain line that wasn’t pitched correctly, a missing P-trap, or a float switch that wasn’t tested. In multi-story Los Angeles homes, where the air handler sits in a hot attic, an uninsulated drain line can also sweat profusely and pool water where it shouldn’t.

4. Loud noise at startup or shutdown. A brief rattle or bang when the system cycles on or off usually indicates loose hardware on the condenser pad, an unsupported refrigerant line vibrating against framing, or a compressor that wasn’t properly isolated on its mounts. These are installation details, not equipment defects.

5. Thermostat won’t respond or shows error codes. Modern communicating thermostats need to be paired correctly with the control board inside the condenser. If that handshake didn’t happen, or if the C-wire (the common wire that provides continuous power to the thermostat) wasn’t properly connected, you’ll get erratic behavior or a blank screen.

Refrigerant Undercharge: The #1 Performance Complaint

If your new AC runs constantly but never quite reaches the set temperature, refrigerant charge is the first thing to check. This is the single most common installation error we encounter, and it’s also the easiest for a homeowner to verify if they know what to ask for.

Every AC system ships from the factory with a partial refrigerant charge, enough for a specific line set length, usually fifteen to twenty-five feet. If your Los Angeles home requires a longer run between indoor and outdoor units, common in hillside properties or homes with rooftop package units, the technician must add precisely measured refrigerant to match. Too little, and the evaporator coil (the indoor coil that absorbs heat from your air) won’t get cold enough. Too much, and liquid refrigerant can slug back to the compressor and damage it.

A proper charge verification involves three measurements, and you should ask to see the recorded numbers:

1. Superheat. This measures how much the refrigerant gas has heated above its boiling point at the evaporator outlet. It tells the technician whether the right amount of refrigerant is reaching the indoor coil.
2. Subcooling. This measures how much the refrigerant has cooled below its condensation point at the condenser outlet. It confirms the outdoor unit has enough refrigerant to fill the condenser coils properly.
3. Pressure-temperature correlation. The technician compares actual pressures against manufacturer specs for the outdoor temperature at the time of testing. In Los Angeles, where afternoon temperatures can swing twenty degrees from morning startup, this must be done during stable conditions.

These three numbers should be recorded on your installation paperwork. If they aren’t, the charge was likely set by guesswork. A reputable contractor will show you the readings without hesitation. At Coldharbour Heating & Air, we leave a copy of the charge verification with every homeowner before we pack up our tools, as part of The Kingswell Way.

The cost to correct a refrigerant undercharge on a new installation should be zero. This is installation workmanship, not a warranty repair. If you’re being quoted a refrigerant recharge fee within the first year, ask specifically whether the original charge was verified in writing.

Warranty Defect vs. Installation Error: Who Pays for What

This distinction matters because it determines who returns your call and who pays for the fix. Homeowners who don’t know the difference often end up paying out of pocket for problems that are their contractor’s responsibility.

Manufacturer warranty defects cover the equipment itself: a failed compressor, a defective control board, a leaking evaporator coil due to a factory flaw. These are rare in the first 30 days but not impossible. The manufacturer supplies the replacement part, and the contractor typically handles the labor under their own installation warranty terms.

Installation errors cover everything the contractor did or didn’t do: improper refrigerant charge, unsealed duct connections, incorrect wiring, missing condensate safety switches, failure to level the condenser pad, or skipping the commissioning process entirely. These are the contractor’s financial responsibility, period.

Here’s where Los Angeles homeowners get tripped up. Some contractors blur this line by telling you to “call the manufacturer” for any problem in the first year. The manufacturer will ask for your installation paperwork, and if the commissioning checklist wasn’t completed, they may deny the claim and refer you back to the installer. Meanwhile, the installer has already been paid and may be slow to return.

Protect yourself by getting three documents at installation completion:

• The manufacturer’s equipment warranty registration, with serial numbers recorded
• The contractor’s written workmanship warranty, with duration and coverage spelled out
• The commissioning checklist, signed by the technician who performed the work

At Coldharbour Heating & Air, every AC Installation in Los Angeles comes with our 365-Day Done Right Promise. If anything we installed doesn’t perform as it should within that full year, we return and correct it at no charge. That’s written on paper before we leave, not offered as a verbal afterthought.

The Commissioning Checklist Every Homeowner Should See

Commissioning is the quality control process that separates a professional installation from a swap-out where the old unit was removed and the new one was dropped in its place. In Los Angeles, where Title 24 energy codes require specific efficiency standards and verification, commissioning isn’t optional, it’s legally required for permit compliance. Yet many homeowners have never heard the word.

A complete commissioning checklist for a split-system AC installation includes these verified items:

1. Refrigerant charge verified by superheat and subcooling - not estimated, not topped off by sight glass, measured and recorded
2. Airflow measured at each supply register - with a balancing damper adjustment if rooms vary by more than 10%
3. Static pressure tested across the evaporator coil and filter - high static pressure strains the blower motor and reduces efficiency
4. Thermostat programmed and tested through all modes - cooling, heating if applicable, fan-only, and emergency heat
5. Condensate drain flow-tested and float switch verified - water poured through the pan should exit freely and trigger the safety switch
6. Electrical connections torqued to manufacturer spec - loose connections cause heat buildup and premature failure
7. Condenser pad leveled and vibration isolation checked - a rocking condenser wears refrigerant lines and makes noise
8. System cycled through three complete on/off sequences - intermittent problems often show only after multiple cycles

In Los Angeles’s coastal zones, like Venice or San Pedro, salt air corrosion makes proper electrical torquing especially important. In the San Fernando Valley’s hotter inland climate, airflow verification matters more because systems run longer and harder. A commissioning checklist that doesn’t account for your specific microclimate wasn’t done thoroughly.

Ask to see this checklist before you sign the final walkthrough. A technician who completed it properly will be glad to review it with you. One who didn’t may deflect or claim it’s “internal documentation.” It isn’t. It’s your proof that the job was done to standard.

Thermostat and Ductwork Issues That Mimic Equipment Failure

Not every post-installation problem lives in the outdoor condenser or indoor air handler. Two of the most common culprits, thermostats and ductwork, sit between the equipment and your living space, and their symptoms look identical to serious mechanical trouble.

Thermostat mismatches. If your new system is a two-stage or variable-speed model but your thermostat only supports single-stage operation, you’ll get short-cycling, temperature swings, and higher energy bills. The equipment is capable of precision; the control signal is blunt. This is a specification error at the time of sale, not an equipment failure. Similarly, if you upgraded to a communicating system like certain Lennox or Rheem models, the thermostat must be from the same brand family to exchange data properly. A generic smart thermostat won’t suffice.

In Los Angeles’s older housing stock, we frequently find that the existing wiring between thermostat and equipment lacks the C-wire needed for modern thermostats. Contractors who don’t test for this may install a battery-powered thermostat as a workaround, which loses programming during power outages and can behave erratically as batteries weaken.

Ductwork leakage and sizing. A new AC rated for three tons of cooling capacity needs roughly 1,200 cubic feet per minute of airflow to operate efficiently. If your ducts were sized for a two-ton system, or if 20% of your conditioned air is leaking into the attic through disconnected joints, the new unit can’t deliver its rated performance. The equipment isn’t underperforming; the delivery system is.

Los Angeles has some of the oldest residential ductwork in the country. Homes built before 1970 often have galvanized steel ducts that have separated at seams, or early flexible duct that has collapsed internally. A proper installation includes a duct pressure test, and the results should be shared with you. If your contractor never mentioned duct condition, that conversation should happen before you blame the new equipment.

What the Kingswell Guarantee Covers After Installation

The Kingswell network, which Coldharbour Heating & Air joined in 2003, was built on a simple observation: most HVAC companies ask homeowners to trust verbal promises, then disappear when problems arise. The Kingswell Guarantee puts four promises in writing on every job, and the fifth, our namesake value, is the reason we exist at all.

A cold harbour was a roadside shelter where travelers waited out bad weather. Comfort in any season, without worry about who would stand behind the work. That’s the job.

Here’s what those written promises mean specifically for your post-installation period:

Say the price before the work starts. On installation day, this means the quoted price is the price. If the crew discovers that your electrical disconnect needs replacement or your pad is rotted, that additional work is quoted separately and approved by you before proceeding. Nothing changes at the door.

Show up in the window given. Installation crews arrive when promised. If weather or traffic delays occur, you get a call with a revised time, not a no-show.

Leave the place cleaner than found. This includes hauling away the old equipment, vacuuming work areas, and wiping down any surface we touched. In Los Angeles’s tight lot lines, where equipment must sometimes come through a side gate or down a narrow walkway, this matters more than in sprawling suburban markets.

Put the guarantee in writing. Every repair and installation gets a written document before we leave. For installations, this includes the 365-Day Done Right Promise: if anything we installed doesn’t perform correctly within a full year, we return and correct it at no additional charge. Not a prorated credit. Not a referral to someone else. Us, back at your door, fixing what we missed.

Come back if it fails. This is the promise that separates contractors from sales operations. We’ve returned at no charge for a loose wire that vibrated free, a drain line that settled after the first month, and a thermostat that needed a firmware update. These aren’t failures of character. They’re realities of mechanical work, and we own them.

If your installing contractor won’t put similar terms in writing, that absence tells you something. AC installation in Los Angeles is competitive, and the companies that survive long-term are the ones that treat the post-installation period as part of the job, not the end of it.

Common Mistakes to Avoid

• Paying a diagnostic fee for a new-installation problem. Any issue within the first 30 days that traces to installation workmanship should be corrected at no charge. If you’re being asked to pay for a “service call” to diagnose a problem with work you just paid for, push back.
• Accepting “that’s normal” without specifics. A new AC should cool your home to the set temperature, cycle on and off in reasonable intervals, and operate at a conversational noise level. Vague reassurances without measured data are not answers.
• Skipping the permit and inspection. In Los Angeles, most AC replacements require a permit. The permit inspection is your independent verification that the work meets code. Contractors who discourage permitting may be cutting corners you can’t see.
• Installing a mismatched indoor and outdoor unit. Even if both are new, they must be certified as a matched pair by the Air-Conditioning, Heating, and Refrigeration Institute (AHRI). Mismatched equipment loses efficiency and may void the manufacturer’s warranty.
• Neglecting attic insulation and sealing. A high-efficiency AC in an unsealed Los Angeles attic is like a thermos in a bonfire. The equipment is capable, but the building envelope wastes the improvement.
• Choosing the lowest bid without verifying what’s included. A proper installation includes permits, commissioning, and disposal of old equipment. Bids that omit these line items often add them later or skip them entirely.

When to Call a Professional

Call your installing contractor immediately if you notice refrigerant leaks (ice on the refrigerant lines, hissing sounds, or oily residue at connections), electrical burning smells, or complete system failure within the first 30 days. These are not wait-and-see situations. For refrigerant issues specifically, federal EPA regulations require certified technicians to handle the material, and DIY attempts can void your warranty and release harmful chemicals.

If your original contractor is unresponsive or disputes that the problem is their responsibility, you still have options. See our more guides & resources for additional help. A second-opinion service call from a qualified technician can document the installation error in writing, which strengthens your position significantly. Coldharbour Heating & Air offers free estimates in Los Angeles for homeowners who need an independent assessment of a recent installation. Call (817) 402-2972 and we’ll review what was done, document what we find, and give you a written report you can use with your original contractor or as a basis for corrective work.

Frequently Asked Questions

The Bottom Line

A new AC that underperforms is frustrating, but it’s rarely a reason to panic about equipment quality. The five symptoms we’ve covered, weak airflow, warm air, water leaks, noise, and thermostat issues, most often trace to installation details that a qualified technician can correct quickly and at no charge. Your job as a homeowner is to know which problems belong to the contractor, which require manufacturer involvement, and what documentation you should have in hand before anyone leaves your property. Get the commissioning checklist. Get the charge verification. Get the written guarantee. In a market like Los Angeles, where summer heat arrives reliably and good contractors stay busy, the ones who put these in writing without being asked are the ones worth calling back.

Written by Victor Mancini, Owner at Coldharbour Heating & Air, serving Los Angeles since 2003.