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Why a New AC Underperforms in Las Vegas
Last updated: October 25, 2026 · Reviewed by Colton Air Conditioning, NV license 0092120
Short answer: when a new AC doesn't cool well, the cause is usually the installation, not a defective unit. The usual suspects are equipment sized without a load calculation, low airflow through old or leaky ducts, an incorrect refrigerant charge, or a thermostat that wasn't set up for the new equipment. A NIST study found that duct leakage, refrigerant undercharge, low indoor airflow and oversizing have the most potential to cut performance (NIST).
First, rule out the normal stuff
- Recovery time. If the house heated up during the changeout, the walls, furniture and attic hold that heat. Give it a full day of normal operation before judging the system.
- Mode and schedule. Check that the thermostat is in Cool mode and that a new schedule or eco mode isn't overriding you.
- Filter. Some new systems use a different filter size or location. Use the one the installer specified.
- Returns and registers. Keep them open and clear of furniture.
The five installation causes, and how to tell
| Symptom | Likely cause |
|---|---|
| Runs short cycles, house feels clammy | Oversized for the house. It shuts off before it can remove much moisture. ENERGY STAR warns that oversized equipment cycles too often (ENERGY STAR). |
| Runs nonstop, rooms near the unit are fine but far rooms are hot | Old ducts that are too small or leaky for the new system's airflow. See ductwork problems. |
| Weak air at every register | Blower or airflow not set for the ducts, or a restrictive filter |
| Ice on the line or the indoor coil | Charge or airflow problem. Turn cooling off and call. See frozen coil. |
| Cooled fine at first, then got worse | Possible leak at a new brazed joint, or a drain or float switch tripping |
Why Las Vegas exposes a bad install fast
A small installation fault might go unnoticed in a mild climate. In the valley, long summer runs at high outdoor temperatures leave no slack. Attic ducts sit in some of the hottest air in the house, so leaks and thin insulation show up as warm supply air. Many 1990s and 2000s homes kept their original duct systems when the equipment was replaced. See HVAC in 2000s homes.
What to ask the installer for
Ask for the Manual J load calculation, the AHRI matched-system certificate, and the startup readings (refrigerant charge and airflow). The EPA's bid checklist suggests asking for these documents up front (EPA). Any warranty work should go through the original installer first. See what a professional install includes.
When a second look helps
If the installer can't show the measurements, an independent test of static pressure, temperature drop and charge will tell you whether the problem is the equipment, the charge or the ducts. See HVAC second opinion. Call (702) 738-6976 to schedule.
FAQ
Is it normal for a new AC to take a while to cool the house? On the first day, yes, after a long shutdown. After a full day of normal running, it should hold the house as well as it did before or better. Can a new AC be the wrong size? Yes. If the old tonnage was copied without a load calculation, the new unit can be too big or too small for the house. Will a bigger unit fix a hot back bedroom? Usually not. That is generally a duct or return-air problem. Colton Air Conditioning is a licensed Las Vegas residential HVAC contractor, NV license 0092120.