Why Is My AC Running but Not Cooling?

It’s a sticky July evening, the AC is humming, the outdoor unit is running, and the house just won’t get below 80. That’s a frustrating spot, because everything sounds like it’s working. Usually one piece of the system isn’t doing its share. Here’s how to narrow it down and what to check before you call.

How your AC actually cools

A central air conditioner moves heat, it doesn’t make cold. Inside, warm house air blows across a cold evaporator coil, usually sitting on top of the furnace. Refrigerant picks up the heat and carries it to the outdoor condenser, where a fan blows it out. If airflow is blocked inside, the coil can’t pick up heat. If the outdoor unit can’t dump it, the whole loop stalls. Most not-cooling problems fall on one side or the other.

The common causes

Thermostat settings

Check it first. The system should be set to COOL, the setpoint should be several degrees below room temperature, and the fan should be on AUTO. If the fan is set to ON, the blower keeps running between cycles and can push warmer air around, which feels like the AC isn’t keeping up.

A dirty air filter

A clogged filter starves the indoor coil of airflow. Less air across the coil means less heat removed and, eventually, a coil that freezes. It’s the cheapest fix on this list and the one we see most.

A frozen evaporator coil

If you see ice on the copper line going into the house, frost on the indoor coil, or water around the furnace, the coil may have frozen. Common causes are low airflow (filter, closed vents, a weak blower) and low refrigerant. Turn the system to OFF and the fan to ON to thaw it. That can take a few hours. If it freezes again, there’s an underlying problem.

A dirty or blocked outdoor unit

The condenser needs room to breathe. Cottonwood fluff, grass clippings, leaves, and shrubs crowding the unit all cut the airflow across the coil. Keep about two feet clear around it. With the power off at the disconnect, you can gently hose the fins from the inside out if you’re comfortable doing it.

A failed capacitor or contactor

If the indoor blower runs but the outdoor unit is silent, or the outdoor unit hums but the fan doesn’t spin, a capacitor or contactor is a likely suspect. Capacitors wear out and often fail during heat waves. They’re a common, relatively inexpensive repair. They also store a charge even with the power off, so this one is for a tech.

Low refrigerant

An AC doesn’t use up refrigerant. If it’s low, there’s a leak. Signs include ice on the lines, hissing, and long run times with warm-ish air. Adding refrigerant without finding the leak is a short-term patch. We’ll tell you where the leak is, what fixing it costs, and whether it makes sense.

Undersized system or duct problems

If the house has never cooled well on the hottest days, or one floor is always hot, the issue may be the ductwork or system sizing. Upstairs rooms in the newer two-story homes around Dyer and St. John are a common example. That’s a different conversation from a sudden failure.

What you can safely check yourself

• Set the thermostat to COOL, fan to AUTO, and the setpoint 3 to 5 degrees below room temperature. Replace batteries if it uses them.
• Replace the filter if it’s gray or you can’t see light through it.
• Walk the house and open supply vents. Make sure return grilles aren’t blocked by furniture.
• Check the breakers for both the indoor unit and the outdoor unit, plus the disconnect box by the condenser.
• Look at the outdoor unit. Is the fan spinning? Is it packed with cottonwood or crowded by plants?
• Look for ice on the refrigerant lines or water around the furnace. If you see ice, turn cooling off and run the fan to thaw.
• Check the condensate drain and pump. A full pump or clogged line can trip a float switch and stop cooling.

A good sign things are working: air from a supply vent should feel noticeably cooler than the air going into the return, often around 15 to 20 degrees cooler once the system has been running a while.

When to call a pro

Call if the outdoor unit won’t start, the fan doesn’t spin, the lines ice up again after thawing, or the breaker trips more than once. Don’t keep resetting a breaker that trips. That’s the system telling you something electrical is wrong. Refrigerant work, capacitors, contactors, and motors all need a licensed tech with the right tools.

If you have a gas furnace sharing the space with the indoor coil and you ever smell gas, leave the house and call NIPSCO or 911 from outside.

A few ways to help it keep up

On the hottest days, close blinds on sunny windows, avoid running the oven or dryer in the afternoon, and don’t set the thermostat way down to cool faster. It doesn’t cool any faster, it just runs longer. ENERGY STAR and the Department of Energy both have good tips on AC maintenance and efficient settings. A yearly spring tune-up catches dirty coils and weak capacitors before a heat wave does.

If the AC is running but the house still feels clammy, the problem might be humidity rather than temperature. See why a house stays humid with the AC on.

How we handle it

We find what’s actually wrong and quote the repair first, with the price on paper before we start. Our techs aren’t on commission. Most not-cooling calls end with a capacitor, a contactor, a cleaned coil, or a leak repair, not a new system. Visit our AC repair page, or go straight to AC repair in Munster, Dyer, or Schererville. Every town we cover is on our service area page. Call (219) 555-0142 and a real person picks up.