Why Is My AC Not Cooling?

Talk to Shemtov Systems

An AC not cooling in a Maryland summer almost always comes down to one of five things: a blocked filter or coil choking airflow, a low refrigerant charge from a leak, a failed capacitor stopping the compressor or fan, a frozen evaporator coil, or a thermostat calling for the wrong thing. Two of those you can check yourself in ten minutes. Shemtov Systems repairs air conditioning across Silver Spring and Montgomery County, MD. Call 240-355-3333.

Key takeaways

  • If the outdoor unit is running but the air from the vents is not cold, start with the filter and the outdoor coil.
  • Ice on the refrigerant line means shut the cooling off and run the fan; running it frozen is how compressors die.
  • An AC that needs refrigerant has a leak. Refrigerant is not consumed, so topping it up without finding the leak just delays the same failure.
  • A humming outdoor unit with a still fan is usually a failed capacitor, which is a cheap part.
  • In a Maryland heat wave an undersized or dirty system can run continuously and still lose ground; that is a capacity problem, not a fault.

Is the air warm, or just not cold enough?

These are different faults. Air that is barely cooler than room temperature usually means the compressor is not running or the charge is badly low. Air that is clearly cold at the vent but never gets the house to setpoint usually means airflow, duct loss or capacity: the system is cooling, just not enough of it is reaching the rooms. Put a thermometer in the supply air and another in the return. A healthy split between them is roughly 16 to 22 degrees Fahrenheit. Below that points at charge or compressor; well above it points at airflow.

Start with the filter

This is the single most common cause and it costs nothing to check. A loaded filter starves the evaporator coil of air, the coil gets too cold, and moisture on it turns to ice. Pleated media loads through its depth, so resistance climbs long before the filter looks dirty. If yours is more than three months old, change it before anything else, then give the system an hour.

While you are there, note the size and write the date on the frame. Our guide to how often to service an HVAC system covers sensible intervals.

Check the outdoor unit and the coil

The outdoor condenser rejects the heat your house is losing. If it is packed with cottonwood, grass clippings or leaf litter, it cannot, and the system loses capacity in exactly the weather you need it. Switch the system off at the thermostat, look at the fins, and clear anything within about two feet of the cabinet.

Then look at whether the fan is spinning. A unit that hums but whose fan sits still is the classic failed-capacitor symptom. That is an inexpensive part, but it is inside a cabinet with capacitors that hold a charge even with the power off, so it is a call rather than a DIY job.

Is there ice on the refrigerant line?

If the larger insulated copper line into the house is frosted, the coil has frozen. Switch cooling off at the thermostat and set the fan to ON so the ice melts, which takes a few hours. Do not keep running it. A frozen coil sends liquid refrigerant back to the compressor, and compressors are not built to compress liquid; that is how a repairable fault becomes a replacement.

Once it has thawed, the cause is one of: a restricted filter, closed or blocked registers, a weak blower, or a low charge. The first two you can rule out yourself.

What if it needs refrigerant?

Then it has a leak. Refrigerant is not a consumable like fuel or oil; a sealed system that is low has lost it somewhere. Adding more without finding the leak buys a season at most and vents refrigerant to the atmosphere in the meantime. The right sequence is leak search, repair, then recharge to the nameplate weight.

This also matters for older equipment. Systems built before 2025 use R-410A, which is being phased down, so the calculation on repairing a badly leaking older system has changed. Our guide to HVAC replacement cost in Maryland covers where that line usually falls.

Could it be the thermostat?

Sometimes, and it is worth thirty seconds. Confirm it is set to COOL and not to FAN ON, which blows room-temperature air continuously and is often mistaken for a broken AC. Confirm the setpoint is actually below room temperature. If the screen is blank, change the batteries. If the thermostat was recently replaced, a miswired common wire or a heat-pump setting on a conventional system will produce exactly this complaint. See thermostat installation and service.

Why does this happen more in Montgomery County summers?

Because an air conditioner here is doing two jobs. Maryland summers are humid, and a large share of the system's work is pulling moisture out of the air rather than dropping its temperature. A system that is oversized, low on charge or short on airflow reaches the thermostat setpoint before it has run long enough to dehumidify, so the house reads the right number and still feels clammy. That is a comfort complaint that no amount of turning the dial down will fix; see what size air conditioner a home actually needs.

When should you stop and call?

Call once you have changed the filter, cleared the outdoor unit and confirmed the thermostat, and it is still not cooling. Call sooner if you see ice, smell burning, hear grinding or repeated clicking, or if a breaker keeps tripping. A breaker that trips more than once is a fault, not a nuisance, and resetting it repeatedly is a fire risk.

Shemtov Systems has repaired air conditioning across Montgomery County since 1981, and advertises a 24-hour answering service. See air conditioning services or call 240-355-3333.

Frequently asked questions

My AC runs constantly but the house never gets cool. What does that mean?

On the hottest afternoons a correctly sized system running continuously is normal, because it is sized for a design temperature rather than the worst day of the year. If it runs constantly on a mild day, suspect airflow, a low charge, or duct losses through an unconditioned attic. Measure the temperature split between supply and return air: roughly 16 to 22 degrees Fahrenheit is healthy.

Should I pour water on the outdoor unit to help it?

Rinsing the coil fins gently with a hose, with the power off, is legitimate maintenance and can help a dirty coil. Pouring water over the cabinet while it runs is not, and pressure washing bends the fins and makes the problem permanent.

Can a dirty filter really stop an AC from cooling?

Yes, and it is the most common cause we find. Restricted airflow drops the coil below freezing, ice forms, and once the coil is iced the system moves almost no air at all. The symptoms look dramatic for a cause that costs a few dollars to fix.

How long should I wait after resetting or thawing before deciding it is broken?

Give it a full hour after a filter change and several hours after thawing a frozen coil, with the fan running. If it is still not cooling after that, the remaining causes all need gauges or meters to diagnose.

Is it worth repairing an older air conditioner that is not cooling?

It depends on the fault and the age. A capacitor or a contactor on a ten-year-old system is worth fixing. A compressor or a significant refrigerant leak on a system past fifteen years usually is not, particularly with the R-410A phase-down changing the maths on older equipment.

Talk to Shemtov Systems

If your air conditioning is running but not cooling, call Shemtov Systems at 240-355-3333. Family-owned in Silver Spring since 1981, serving Montgomery County and the Washington DC metro, with a 24-hour answering service.

Call 240-355-3333

Shemtov Systems has served Montgomery County and the Washington DC metro since 1981. Call 240-355-3333 to talk to a technician.