Capacitor failure during the first heat advisory
The single most common no-cool call in this metro arrives during the first stretch of 95-degree days. The outdoor fan hums but does not spin, or the unit trips the breaker on start. That is usually a failed run capacitor, a $15 part that costs $180 to $350 installed.
Heat accelerates the failure, which is why every local shop is booked solid during the first heat advisory of the season and why the same repair is easy to schedule in May. If your capacitor was replaced more than five years ago and your unit is over ten, having it tested during a spring tune-up is a genuinely cheap insurance policy.
Frozen evaporator coils in humid weather
You go to the basement and find ice on the copper line or a puddle under the air handler. In St. Louis this usually traces back to restricted airflow: a filter that has not been changed since spring, closed supply registers in unused bedrooms, or a coil coated in dust from a basement remodel.
The second cause is low refrigerant from a leak. Either way, turn the system to fan-only for two to four hours to melt the ice before a technician arrives. Running the compressor against a frozen coil is how a $300 visit turns into a compressor conversation.
Condensate drains and the summer humidity load
A properly working system in a St. Louis July pulls several gallons of water out of the air per day. That water leaves through a condensate line, and in older homes it often terminates at a floor drain or a small pump. Algae growth clogs that line every summer, the safety float switch trips, and the system shuts off with no error a homeowner can see.
Pouring a cup of distilled vinegar into the cleanout every month or two during cooling season prevents the majority of these calls. If you have a condensate pump, listen for it after a long run cycle; a pump that never runs on a humid day is a pump to check.
Storm season, power blips, and control boards
Missouri sits in a corridor with frequent summer thunderstorms, and brief outages or brownouts are common. Repeated power interruptions are hard on control boards and on compressors that restart under load. If your system loses cooling right after a storm, check the breaker and the furnace door switch before calling.
A surge protector at the disconnect is inexpensive relative to a control board. Homeowners on well pumps or long rural service drops in outer St. Charles County see this more often than those in dense municipal grids.
Upstairs that never cools
This is the most common comfort complaint we hear in the metro, particularly in 1950s and 1960s two-story homes in Kirkwood, Affton, and Florissant where the original system was designed for a single-story load or where a second floor was finished later.
The fix is rarely a bigger condenser. Look first at return air capacity on the second floor, duct sizing and leakage in the attic, insulation depth, and whether a damper adjustment or a dedicated mini-split for the upper level is a better investment. A contractor who proposes a load calculation before proposing equipment is worth keeping.
Frequently asked questions
How often should I change my filter in summer?
During cooling season, check a one-inch filter monthly and expect to change it every one to two months. Four- and five-inch media filters typically last six to twelve months, but pets and construction dust shorten that.
Is it normal for my AC to run all day in July?
On days above about 95 degrees, a correctly sized system will run nearly continuously. That is by design and more efficient than short cycling. Concern is warranted only when the indoor temperature keeps climbing.
Why does my house feel clammy even though the thermostat is satisfied?
That usually means the system is oversized and cools the air quickly without running long enough to dehumidify. Variable-speed equipment or a dehumidifier addresses it; a larger unit makes it worse.
Sources and further reading
Price ranges in this guide come from quotes and invoices shared with STL Home Pros by St. Louis area homeowners and contractors. Technical and climate references are listed below.
About the author
Nick Cascani is a New York native who has called St. Louis home for the last few years. He believes getting professional home service should be simple and convenient, and he writes these guides to help local homeowners make confident decisions about their HVAC, plumbing, and foundation projects.
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