Thermostat settings or programming
A schedule, a hold, a fan set to ON rather than AUTO, or an incorrect mode can all produce air movement without cooling. Worth eliminating first.
HVAC performance
AC Running But House Not Cooling? Here's What to Check
When a system runs and the house does not reach setpoint, one of three things is happening: the equipment is not producing the cooling it should, the cooling it produces is not reaching the rooms, or heat is entering the house faster than the system can remove it.
Those three possibilities call for three different specialists, which is why guessing is expensive. The checklist below is ordered to eliminate the cheap and simple causes first, then to point you at the right kind of diagnosis.
Nothing here asks you to open equipment, handle refrigerant or work on wiring. If the checks below do not explain the problem, that itself is useful information.
Reviewed by Duct Daddy
Written and reviewed by the Duct Daddy team, a duct leakage testing and duct sealing specialist serving Southeast Louisiana. This article is general guidance, not a diagnosis of your system.
Why a running system may not cool the house
Filter restriction
A heavily loaded filter cuts airflow across the coil, which cuts capacity and can lead to coil icing — which cuts capacity further.
Closed or blocked supply registers
Closing registers in unused rooms does not redirect capacity the way people expect. It raises system pressure and can reduce overall performance.
Weak or uneven airflow between rooms
If some registers deliver well and others barely deliver, the distribution system is the more likely suspect than the equipment.
Equipment or refrigerant problems
Low charge, a failing compressor, a dirty condenser coil or a blower problem all reduce delivered cooling. These require a licensed HVAC service technician — Duct Daddy does not perform mechanical HVAC repair.
Insulation and building-envelope problems
Missing attic insulation, large unshaded glazing, and air leakage at the attic plane add load continuously. The system can be healthy and still lose.
Conditioned air lost through the duct system
Supply leakage delivers cooling into attics, chases and wall cavities. Return leakage pulls hot attic air into the system. Both make a functioning system behave like an undersized one.
A homeowner-safe checklist
- Check the thermostat mode, setpoint and schedule, and set the fan to AUTO.
- Check and replace the air filter if it is dirty.
- Confirm supply registers throughout the house are open and unobstructed.
- Walk the house and note obvious airflow differences between rooms.
- Check that the outdoor unit is running, clear of debris, and not iced over.
- Consider whether the problem is constant or only during the hottest hours.
- Consider whether anything changed recently — a renovation, an attic project, new equipment.
If you see ice on refrigerant lines or the indoor coil, turn the system to fan-only and call a licensed HVAC service technician. Do not attempt refrigerant work.
When to call a licensed HVAC service technician
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:
- The outdoor unit is not running, or runs and shuts off quickly.
- There is ice anywhere on the system.
- Air from the registers is not noticeably cooler than room air.
- The problem began abruptly rather than getting worse over seasons.
How duct leakage is separated from the rest
The distinguishing feature of duct leakage is that it can be quantified. Most of the other causes on this page are assessed by inspection and judgement; duct leakage is assessed with a pressure test that returns a number.
Duct leakage is one of the few items on this list that can be measured rather than estimated. A duct leakage test pressurises the duct system and quantifies how much air is escaping, so the duct system either is or is not part of your problem — and you have a number either way.
That number is useful even when the answer is "your ducts are fine." It removes an expensive unknown from the conversation with your HVAC contractor.
More detail on the process: how air duct sealing works and how leakage is measured and verified.
Measure your duct leakage
We test the duct system, tell you what we find, and only propose sealing where sealing is the right answer.
Related guides
Weak airflow at the vents
Barely any air at a register — and how to tell restriction, leakage and physical damage apart.
AC runs all day
Long runtimes are not automatically a fault — here is how to tell the difference between working hard and losing air.
One room stays hot
A single bedroom or upstairs room that never keeps up with the rest of the house — and what actually causes it.
Browse every guide in the comfort guide.
Find out what your duct system is actually doing.
Schedule a duct leakage evaluation and get measured numbers for your home — not a guess.