An air conditioner that runs constantly but does not cool the house enough usually has a restricted airflow problem (dirty filter, dirty coils, closed vents), leaking ducts, low refrigerant from a leak, or is simply undersized for the heat load. A quick way to tell: measure the temperature drop between the air going in and out of the system. A healthy system typically cools the air by roughly 15 to 20°F.
Measure the temperature drop
- Run the AC for 15 minutes
Set the thermostat well below room temperature and wait.
- Measure return and supply air
Use a cooking or digital thermometer at the return grille and at a supply vent close to the unit.
- Compare
A drop of 15 to 20°F is typical. A much smaller drop points to the unit itself (coil, refrigerant, compressor); a normal drop with a warm house points to the ducts or the house.
Airflow first: filter, vents and returns
A clogged filter is the most common cause. Replace it, check that supply vents are open and not blocked by furniture or rugs, and that return grilles are clear. Our AC troubleshooting guide explains why airflow explains so many faults. If frost shows up on the indoor unit, see air conditioner freezing up.
Dirty coils
Dirt on the indoor evaporator coil or the outdoor condenser coil reduces heat transfer. Rinse the outdoor coil gently with a garden hose with the power off, and clear leaves and grass clippings from around the unit, leaving about two feet of clearance. The indoor coil needs a technician to clean.
The outdoor unit
Check that the fan is spinning and the unit is not making unusual noises. If the outside unit is silent, see AC blowing warm air, outside unit not running; for noises, AC loud noise.
Leaking or uninsulated ducts
Ducts in an attic or crawlspace can lose a large share of the cooled air through leaks and poor insulation. Rooms far from the unit staying warm, or dusty vents, are typical signs. Duct sealing and insulation work is a job for a contractor.
Low refrigerant
Air conditioners do not use up refrigerant. If the charge is low, there is a leak, and topping it off without repairing the leak just delays the problem. Ice on the refrigerant lines and a hissing sound are common signs. Refrigerant work requires a licensed technician; it is never a DIY job.
The house itself
Sun-facing windows, poor attic insulation and gaps around doors all raise the load. Close blinds in the afternoon, and check the thermostat is not near a heat source or in direct sun. An older system, or one sized for a smaller house, may be at its limit; see repair or replace your AC.
When the weather is simply too hot
On very hot days, an AC may not be able to cool the house far below the outdoor temperature. If it holds a steady 15 to 20°F below the outdoors, it is probably working correctly. Using fans, closing blinds and avoiding heat-producing appliances such as the oven during the hottest hours helps. Preparing the system before summer prevents many problems; see how to prepare your AC for summer.
When to call an HVAC technician
If the filter and coils are clean, airflow is good and the temperature drop is small, call a technician to check the refrigerant charge and the compressor. Also call if you see ice on the lines, hear hissing, or the unit keeps tripping the breaker. Our not blowing cold air guide covers the related no-cooling fault.
