It's 90 degrees in St. Louis, your AC has been running for hours, and your house is still 78. This is one of the most common calls we get in summer — and the good news is that most of the causes are diagnosable without a service call. Here's how to work through it.
First: Is the System Actually Running?
Before diagnosing anything, confirm that both parts of the system are running — not just one. Your central AC system has two main components: the indoor air handler (in a closet, basement, or attic) and the outdoor condenser unit.
Quick check before you do anything else
- Go outside and confirm the condenser unit is actually running (fan should be spinning, compressor should be humming)
- Check the indoor air handler — you should hear the blower running and feel airflow at the vents
- Look at the refrigerant lines coming off the indoor unit — if you see ice, stop cooling immediately and switch to fan-only mode
- Check your thermostat setting: confirm it's set to 'cool' and 'auto', not 'on'
If the outdoor unit isn't running at all, you may have a failed capacitor, a tripped breaker, or a refrigerant lockout — those are technician calls. If both units are running and you still aren't getting cooling, work through the causes below.
Most Common Causes — In Order of How Often We See Them
1. Dirty or clogged air filter
Easy fix — do it yourselfA clogged filter is the most common reason an AC runs constantly without cooling properly. When airflow is restricted, the system can't move enough air across the evaporator coil — so it runs and runs without ever getting your home cool. Check your filter first. If it's gray and packed with dust, replace it.
Replace the filter (1-inch filters should be changed every 30–60 days in summer; thicker media filters every 3–6 months). If this was the problem, your system should recover within an hour.
2. Refrigerant leak or low refrigerant
Requires a technicianRefrigerant is what actually moves heat out of your home. If your system is low on refrigerant — usually because of a leak — it loses the ability to absorb heat effectively. Signs: the AC runs continuously, your home never reaches the set temperature, and you may see ice on the refrigerant lines or the indoor coil.
Low refrigerant isn't a DIY fix. A technician has to find and repair the leak, then recharge the system. Simply adding refrigerant without fixing the leak is a band-aid that won't last.
3. Frozen evaporator coil
Stop the system — call for serviceIf the evaporator coil (inside the air handler) freezes over, airflow stops and your system blows warm air — or barely any air at all. Causes include restricted airflow (often from a dirty filter or blocked vents) or low refrigerant. You may see ice on the indoor unit or refrigerant lines.
Turn the system to fan-only mode and let it thaw for several hours before running cooling again. If it refreezes, you have an underlying problem that needs professional diagnosis.
4. Condenser unit problems (the outdoor unit)
Requires a technicianThe outdoor condenser unit releases heat from your home to the outside air. If the condenser coil is dirty, the fan motor has failed, or the unit is running in a confined space without proper airflow, it can't release heat effectively — and the whole system suffers. A dirty condenser can reduce efficiency by 20–30%.
You can rinse the outdoor coil with a garden hose (gently, from the inside out) to clear surface debris. Anything beyond that — fan motors, capacitors, refrigerant — needs a technician.
5. Thermostat issue
Check yourself firstSometimes the problem is simpler than you'd expect. A thermostat set to 'on' instead of 'auto' will run the fan constantly even when the system isn't actively cooling — blowing uncooled air. A failing thermostat can also misread indoor temperature, causing the system to cycle incorrectly.
Confirm the thermostat is set to 'cool' and 'auto' (not 'on'). Set it 5 degrees below the current room temperature and see if the compressor kicks on. If replacing the thermostat doesn't help, the problem is in the HVAC system itself.
6. Ductwork leaks
Requires inspectionIf your ducts have significant leaks — common in older St. Louis homes — cooled air escapes into unconditioned spaces like attics, crawl spaces, or wall cavities before it ever reaches your living areas. Your AC works hard, your energy bills are high, and your rooms never feel right. Some rooms may be noticeably warmer than others.
A duct inspection can identify the worst leaks. Sealing ducts is one of the highest-ROI improvements for older St. Louis homes — and often dramatically improves comfort without replacing any equipment.
A Note on St. Louis Heat
St. Louis summers are genuinely hard on HVAC systems. Humidity regularly runs above 70%, and when the heat index is 105°F, even a well-maintained system is working near its limits. If your system is undersized for your home, it will run continuously and still not keep up on the hottest days — that's a design problem, not a mechanical failure.
A rule of thumb: if your system keeps your home within 20°F of the outdoor temperature during peak heat, it's doing its job. If you can't maintain a comfortable indoor temperature even when it's only 85°F outside, something is wrong.
Signs you may be dealing with a sizing problem, not a mechanical one
- Your AC never shuts off, even on mild days (75–80°F outside)
- The system was installed without a Manual J load calculation
- Rooms far from the air handler are always warmer than the rest of the house
- The system was oversized to 'play it safe' — oversized systems short-cycle, can't dehumidify, and wear out faster
When to Call a Technician
Handle these yourself first: filter check, thermostat settings, clearing debris from the outdoor unit. Those take 10 minutes and cost nothing.
Call a technician if:
- The outdoor condenser unit isn't running or makes unusual sounds
- You see ice on the refrigerant lines or indoor coil (after thawing — if it refreezes, there's an underlying problem)
- The system trips your breaker repeatedly
- Airflow at the vents has dropped noticeably
- You've replaced the filter and confirmed everything is set correctly but still aren't getting cooling after a few hours
Most AC issues that require a technician can be diagnosed and fixed in a single visit. The most common repairs — capacitor replacement, refrigerant recharge (after fixing a leak), and contactor replacement — are generally in the $150–$600 range depending on the part and labor. We give you a straight diagnosis and a flat-rate quote before we do any work.
AC not keeping up in the St. Louis heat?
We diagnose and repair AC systems throughout Chesterfield, St. Louis County, St. Charles County, and Jefferson County. Our service call fee is $125 — applied toward any repair. No upselling, no pressure. Just a straight answer on what's wrong and what it costs to fix it.