Problems After AC Replacement
Long runtime on a new system is normal if it is a variable-speed or multi-stage unit designed to run longer at lower output for steadier temperatures and humidity control. It is a concern if the house never reaches setpoint, if runtime is continuous at full capacity, or if it is markedly longer than the old system's — which points toward undersizing, duct leakage, or a load the equipment was never sized for.
Modern equipment often runs at low capacity for extended periods to hold temperature and humidity steady, rather than cycling on and off like older single-stage units. Long runtime alone is not evidence of a problem with this type of system.
If sizing was based on the old unit's tonnage rather than a fresh load calculation, and the house's actual heat gain is higher, the new unit may run continuously without ever getting ahead of demand.
A unit sized correctly on paper can behave like an undersized one if a significant share of the air it produces is lost through leaky ducts before reaching the rooms, forcing longer runtime to achieve the same result.
Poor insulation, extensive west-facing glass, or air leakage from unconditioned spaces adds heat continuously, requiring more runtime to offset regardless of the equipment installed.
Aggressive setpoints, especially on hot afternoons, or humidity-control settings on equipment with dehumidification modes can extend runtime without indicating any fault.
Reduced airflow from a dirty filter, closed registers, or return restriction limits the amount of heat the system can remove per hour, extending the runtime needed to reach setpoint.
Stick to what you can see, feel and hear. Leave electrical panels, refrigerant, combustion appliances and equipment internals to a licensed professional.
Duct leakage effectively reduces the capacity of the equipment as experienced inside the house, without changing what the equipment produces at the plenum. A system rated correctly for the load can still run continuously if a meaningful share of its output never reaches the conditioned space.
This is particularly relevant with attic ductwork, where leaked air is not just lost but replaced by hot attic air being drawn in on the return side, adding to the load the equipment has to fight.
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:
In Southeast Louisiana's long, humid season, even correctly sized variable-speed equipment will show long daily runtimes as normal behaviour, since low-capacity, extended operation is how it manages both temperature and humidity through the summer.
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If extended runtime coincides with rooms that never quite feel right, a duct leakage test can determine whether lost air is contributing.
Continuous runtime without a satisfied thermostat means capacity or delivery is falling short of the heat load.
A gradual increase in runtime usually points to a gradual loss somewhere — capacity, delivery, or load — not a single sudden failure.
New equipment fixes what's inside the cabinet — it does not automatically fix distribution, sizing, or envelope issues elsewhere in the house.
Efficiency ratings describe the equipment, not what actually reaches the rooms — losses elsewhere in the system erase the expected savings.
Yes, leaking supply and return ducts can raise energy use — but the size of the effect depends on where the leaks are and how the ducts are routed.
New equipment doesn't change the ducts, the load, or the envelope — and any of those can leave a house hot.
Browse every symptom in the troubleshooting center.
Schedule a duct leakage evaluation and get measured numbers for your home — not a guess.
