Should a new AC fix a hot house by itself?

Only if the equipment itself was the limiting factor. If the old unit was undersized, failing, or losing capacity, a properly sized replacement can help. If the limiting factor was duct leakage, poor insulation, or restricted airflow, those remain after replacement.

Could my new AC be undersized?

It is possible if the installer sized it to match the old unit's tonnage rather than performing a fresh load calculation. Renovations, added insulation, or new windows since the original installation can change the actual load significantly.

How long should it take to notice if the new AC isn't fixing the problem?

Give the system a few full cooling cycles across a warm afternoon. A persistent gap between setpoint and actual temperature, or continuous runtime without recovery, is a clear signal something beyond the equipment is limiting performance.

Is it normal for a new AC to run more than the old one at first?

Some adjustment period is normal, especially with variable-speed equipment that runs longer at lower capacity by design. Continuous full-capacity runtime with no temperature recovery is not normal and warrants investigation.

Problems After AC Replacement

I Replaced My AC — Why Is My House Still Hot?

A new air conditioner replaces the equipment, not the duct system that delivers its air, the load the house places on it, or the envelope that gains heat. If the house was hot before because of leaky or undersized ducts, a restricted return, poor insulation, or solar gain, a new unit inherits every one of those problems and can still fail to keep the house cool.

Key takeaways

Replacing the condenser and air handler does not touch the ductwork, insulation, or windows.
A correctly sized, correctly installed unit can still be starved of airflow by the existing duct system.
If the old system struggled for a reason unrelated to its age, that reason is still present.
A duct leakage test tells you whether the distribution system is part of the problem.

Most likely causes

Duct leakage was never addressed

If ducts run through a hot attic and leak conditioned air before it reaches the rooms, a new compressor produces cold air that never arrives. Replacing equipment does nothing to a leaky duct run.

Undersized or poorly designed ductwork

A new unit sized for the house's load still needs enough duct capacity to move that air. Ducts designed for a smaller or different system can restrict airflow regardless of how good the new equipment is.

Load calculation issue at installation

If the replacement was sized off the old unit's tonnage rather than a fresh load calculation, and the house's insulation, windows, or additions changed the actual load, the new unit may be undersized for what the house now needs.

Building envelope and solar gain

Poor attic insulation, air leakage into the living space, or heavy west-facing glass add heat load no equipment change addresses. A house that gains heat faster than it can be removed stays warm regardless of the AC's age.

Incomplete commissioning

Refrigerant charge, airflow across the coil, and blower settings all need to be verified after installation. A unit that is undercharged or set to the wrong blower speed will underperform even though it is brand new.

Return air restriction

A return grille or filter too small for the new equipment's airflow needs can starve the system, reducing capacity regardless of how the outdoor unit is rated.

What you can safely check yourself

Check that all supply and return registers are open and unobstructed by furniture or rugs.
Replace the air filter if it has not been changed since installation — a clogged filter restricts airflow on any system.
Feel airflow at several registers to see whether the whole house is weak or only certain rooms.
Note whether the house cools reasonably at night but loses ground in the afternoon, which points toward heat gain rather than equipment capacity.
Check the thermostat's actual runtime — near-continuous running with little temperature drop suggests the system is undersized for the load or not delivering the air it produces.

Stick to what you can see, feel and hear. Leave electrical panels, refrigerant, combustion appliances and equipment internals to a licensed professional.

What a professional should measure

A duct leakage test to quantify how much conditioned air is escaping before it reaches the registers.
Airflow measurement (CFM) at the equipment and at individual registers, compared against the manufacturer's rated airflow.
A Manual J load calculation to confirm the new unit was sized for the house as it currently exists.
Static pressure measurement across the air handler to identify duct or filter restriction.
Refrigerant charge and coil temperature verification by the installing contractor.

When duct leakage could be involved

Ducts running through an unconditioned attic are exposed to extreme temperatures for most of the cooling season. Every leak in that duct run loses air that has already been cooled, at the point it is hottest outside the equipment's rated performance.

A new, more efficient unit does not seal existing leaks or resize undersized trunks and branches. If the duct system was undersized or leaky relative to the old equipment, it is often equally undersized or leaky relative to the new equipment, and sometimes more so if the new unit moves a different volume of air.

When this probably isn't a duct-sealing problem

The house cools evenly and simply takes longer than expected, pointing toward capacity or load rather than distribution.
Airflow feels strong and even at every register, but the space temperature still climbs during peak afternoon heat.
The installer has confirmed refrigerant charge and airflow are within specification and the issue persists.
The house is noticeably hot only in rooms with heavy west or south-facing glass, suggesting solar gain rather than air delivery.

When to call an HVAC contractor

Duct Daddy specialises in duct leakage testing and duct sealing. We do not perform mechanical HVAC repair, refrigerant work or equipment replacement. Contact a qualified HVAC contractor if any of the following apply:

If refrigerant charge, coil airflow, or blower settings have not been verified since installation.
If the unit runs continuously without reaching the setpoint, which may indicate undersizing or a charge issue.
If unusual noises, ice on the coil, or short-cycling appear.
If the load calculation used for sizing is unavailable and the house has been renovated or added onto since the old system was installed.

Why this matters in Southeast Louisiana

In Southeast Louisiana's long, humid cooling season, any duct leakage into a hot attic compounds daily, so a house that struggled before replacement often struggles the same way afterward unless the duct system is addressed separately.

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Frequently asked questions

Should a new AC fix a hot house by itself?: Only if the equipment itself was the limiting factor. If the old unit was undersized, failing, or losing capacity, a properly sized replacement can help. If the limiting factor was duct leakage, poor insulation, or restricted airflow, those remain after replacement.
Could my new AC be undersized?: It is possible if the installer sized it to match the old unit's tonnage rather than performing a fresh load calculation. Renovations, added insulation, or new windows since the original installation can change the actual load significantly.
How long should it take to notice if the new AC isn't fixing the problem?: Give the system a few full cooling cycles across a warm afternoon. A persistent gap between setpoint and actual temperature, or continuous runtime without recovery, is a clear signal something beyond the equipment is limiting performance.
Is it normal for a new AC to run more than the old one at first?: Some adjustment period is normal, especially with variable-speed equipment that runs longer at lower capacity by design. Continuous full-capacity runtime with no temperature recovery is not normal and warrants investigation.

Find out whether the ducts are part of the problem

A duct leakage test measures exactly how much conditioned air your new system is losing before it ever reaches a room, separating an equipment question from a distribution question.

Schedule a Duct Leakage Test

Find out what your duct system is actually doing.

Schedule a duct leakage evaluation and get measured numbers for your home — not a guess.

Schedule a Duct Leakage Evaluation