Brand New AC and the House Still Does Not Cool Properly

New equipment is not a new system

A replacement swaps the box. It does not touch the ductwork, the insulation, the return sizing, or the load the house actually presents. If the old system struggled because of any of those, the new one will struggle too — just more efficiently.

That is the single most common reason a homeowner spends five figures and is disappointed.

The four realistic causes

• The ducts were the problem all along. Leaky, undersized, crushed or badly routed attic duct throws away capacity before it reaches a room. New equipment attached to the same losses gives you the same hot bedroom.
• Wrong size. Both directions hurt. Undersized never keeps up on the hottest afternoons. Oversized cools fast, shuts off, and leaves the house cold and clammy because it never runs long enough to dehumidify. Oversizing is far more common, because systems get specified by matching the old label rather than by load calculation.
• Commissioning was skipped. The system was not evacuated properly, or the charge was eyeballed instead of weighed and verified with superheat and subcooling, or airflow was never measured. A modern system is far more sensitive to charge accuracy than an old one.
• Something else in the house. Attic insulation, west-facing glass, an addition nobody accounted for.

The measurements that settle it

Ask the installing contractor for the commissioning numbers. A properly completed installation has them:

• Refrigerant charge as weighed in, with superheat and subcooling
• Static pressure across the air handler
• Temperature split between return and supply
• Airflow at each register, or at minimum total system airflow
• The load calculation the equipment size was based on

If none of that exists, that is itself the finding. A contractor who cannot produce the numbers for a system they installed weeks ago did not take them.

Humidity is its own test

Buy a hygrometer. If the house sits at setpoint but reads above 60% relative humidity, you very likely have an oversized system short cycling. Note how long a cooling cycle lasts: satisfying the thermostat in under ten minutes and shutting off, repeatedly, is the signature.

That is a sizing and staging problem, not something a service call fixes with a screwdriver.

What to do about it

Go back to the installing contractor first, in writing, with specifics: which rooms, what temperatures, what humidity, how long the cycles run. Most reputable companies will return and check their work, and a charge or airflow correction is a straightforward fix.

If they will not measure, or the answer is that this is normal, get an independent assessment. Ask specifically for a load calculation and a duct evaluation rather than another look at the equipment — that is where the answer usually is.

Why this bites harder here

South Florida gives a system nowhere to hide. In a milder climate an oversized, badly ducted install underperforms for a few weeks a year. Here it underperforms from May to October, and the humidity failure shows up as mould risk rather than just discomfort.