AC Running but Not Cooling? A Colorado Troubleshooting Guide

Your air conditioner is humming, the fan is blowing, and the house keeps getting warmer. It's one of the most common summer calls an HVAC company gets in Denver, and the cause runs from a $10 filter to a failed compressor. This guide puts the possibilities in the order a technician would check them.

First, what does "not cooling" look like?

Be specific before you start, because each symptom points somewhere different:

• Air from the vents is warm or room temperature. The system is moving air but not removing heat. Common causes: the outdoor unit isn't running, refrigerant is low, or the condenser coil is blocked.
• Air is cool but weak. This is an airflow problem: a clogged filter, a frozen coil, closed or blocked registers, or a failing blower.
• The AC runs all afternoon and the house creeps up a few degrees on the hottest days. That may be normal at outdoor temperatures above design, or a sign the system is undersized or losing capacity.
• The outdoor unit hums or buzzes but the fan doesn't spin. This often points to a failed capacitor or motor. Turn the system off and call for service.

Step 1: The safe homeowner checks

Thermostat and power

Confirm the thermostat is set to Cool, the fan is on Auto (in "On," the blower runs even when the compressor is off, so you feel uncooled air), and the setpoint is several degrees below room temperature. Replace thermostat batteries if the display is dim.

Central AC normally has two breakers: one for the indoor furnace or air handler and one for the outdoor unit. A tripped outdoor breaker is a classic cause of "the fan blows but it isn't cold." Reset it once. If it trips again, leave it off and call. Repeated trips mean something is drawing too much current.

Also check the disconnect box mounted on the wall near the outdoor unit. Its pull-out block or switch sometimes gets bumped by landscapers or kids.

The filter

A dirty filter is the single most common cause of weak cooling and frozen coils. The Department of Energy recommends cleaning or replacing it every month or two during cooling season, and says swapping a clogged filter for a clean one can lower the AC's energy use by 5% to 15%. If yours is gray and you can't see light through it, change it before anything else. More on filters on our filter replacement page.

Registers, returns and the condensate drain

Make sure supply registers are open and return grilles aren't blocked by furniture or rugs.

Many systems have a float switch on the condensate drain or drain pan that shuts the AC off when water backs up. If the pan under the indoor coil is full of water, or the switch has tripped, the system may stop cooling while the fan keeps running.

Step 2: The outdoor unit and cottonwood season

The outdoor unit (the condenser) dumps the heat your AC pulls out of the house. To do that it pushes a large volume of air through a thin finned coil. Anything that blocks that coil raises refrigerant pressures, cuts capacity and makes the compressor work harder.

Along the Front Range, the classic blocker is cottonwood fluff. Cottonwoods line creeks, greenbelts and older neighborhoods across the metro, and in late spring and early summer their cotton-like seed fluff drifts into yards. The condenser fan sucks it onto the coil, where it mats into a felt-like layer that may not look like much but blocks a lot of airflow. Grass clippings, lint and dust do the same.

What you can do safely

1. Turn the power off at the thermostat and at the outdoor disconnect.
2. Clear leaves, fluff and debris from around the unit. DOE recommends keeping foliage trimmed back at least 2 feet from the condenser.
3. Rinse the coil with a garden hose at normal pressure. Never use a pressure washer, which flattens the soft aluminum fins.
4. Let it dry a few minutes, restore power, and give it about 15 minutes of run time before judging the result.

If the coil is heavily packed, has bent fins over large areas, or the matting is on the inside of the coil, it needs a professional coil cleaning.

Step 3: A frozen evaporator coil

It seems backward, but an AC that isn't cooling is often covered in ice. The indoor (evaporator) coil normally runs just above freezing. If too little warm air crosses it, or refrigerant pressure drops, the coil falls below 32°F and moisture from the air freezes on it. The ice blocks even more airflow, and the problem feeds itself.

Signs: weak airflow, frost on the larger insulated copper line at the outdoor unit, ice visible at the indoor coil, or water around the furnace as it melts.

What to do:

• Set the thermostat to Off for cooling and the fan to On. Moving room air across the coil speeds the thaw. Full thawing can take several hours.
• Replace the filter and open all registers while you wait.
• Once thawed, restart cooling. If it freezes again within a day, the cause is not just the filter. It's likely low refrigerant, a weak blower, a dirty indoor coil, or undersized return ducts. That's a service call.

Don't chip at ice or use a heat gun on the coil.

Step 4: Low refrigerant means a leak

An air conditioner is a sealed system. It doesn't burn or use up refrigerant, so if the charge is low, it leaked out somewhere: a corroded coil, a rubbed line set, a loose flare fitting, or a failed valve core. Low charge shows up as warm air, a frozen coil, hissing or bubbling sounds, and higher bills as the system runs longer.

DOE advises having a technician check that the charge is correct and test for leaks with a leak detector. Federal rules require refrigerant to be handled by EPA-certified technicians, so this is never a DIY job.

Why "just top it off" is a bad deal now

Adding refrigerant without finding the leak buys a few weeks or months. It has become more expensive, too. Under the federal AIM Act, production of HFC refrigerants such as R-410A has been capped at 60% of the historical baseline since 2024 and is scheduled to drop to 30% in 2029, according to a 2026 industry summary in Service Magazine. Since January 2025, new residential AC equipment ships with lower-impact refrigerants, mainly R-454B in ducted systems. R-410A remains legal for servicing existing equipment, but supply is tighter.

The right sequence is: find the leak, repair it, pull a vacuum, then recharge by weight. Our refrigerant leak repair page explains the process. On an old system with a leaking coil, replacement often makes more sense.

Step 5: Capacitors, contactors and other electrical parts

The capacitor gives the compressor and outdoor fan motor the jolt they need to start. In summer heat it's one of the most common parts to fail. A bulging or leaking top is a giveaway, but many fail without visible signs. Typical symptoms are a humming outdoor unit with a fan that doesn't turn, or a unit that clicks and shuts off.

Videos that show homeowners swapping capacitors leave out the risk. Capacitors store energy and can deliver a serious shock even after the power is disconnected, and the panel also holds the contactor and high-voltage wiring. Leave it to a technician with a meter and the right discharge tools. Repeated capacitor failures can point to a struggling motor. See our AC repair page for what a diagnostic visit covers.

Step 6: Altitude and undersizing

Some Colorado systems never cooled well, because they were sized with sea-level assumptions. At altitude the air is thinner, so each cubic foot carries less heat. An ACHR News design example shows that moving the same 36,000 Btu/h of sensible heat takes about 1,200 cfm at sea level but about 1,430 cfm at 5,000 feet. The same article uses a cooling altitude adjustment factor of about 0.92 at 5,000 feet for a dry coil. Equipment makers publish their own derates. Fujitsu's high-altitude bulletin for its Airstage heat pumps lists a capacity correction factor of 0.83 at 5,000 feet, for both heating and cooling.

In plain terms, a "3-ton" system in Highlands Ranch or Castle Rock may deliver noticeably less cooling than the same box in Phoenix. If ducts weren't sized for the extra airflow, it loses more. Our altitude guide for furnaces, ACs and heat pumps covers this in depth, and our Manual J sizing guide explains how a proper load calculation accounts for it.

Signs a system may be undersized or under-ducted for your home:

• It runs constantly on ordinary 85–90°F days, not only during heat waves.
• Upstairs bedrooms run 5°F or more warmer than the main floor, even with a clean filter.

Bigger is not automatically the fix. An oversized AC short-cycles. Duct fixes, attic sealing and shading west-facing glass sometimes do more than a larger unit.

Step 7: Heat waves, design temperature and realistic expectations

Residential AC is designed for a 1% cooling design temperature: a temperature exceeded only about 1% of summer hours. ENERGY STAR's design temperature guide, based on ASHRAE and ACCA data, lists about 91–92°F for Denver-area counties. When it's 100°F outside, a correctly sized system is beyond its design point. It will run continuously and the house may drift a few degrees above the setpoint by late afternoon. That's expected behavior, not a breakdown.

Denver is seeing more of those days. National Weather Service Boulder records show:

How to help your AC through a heat wave

• Set the thermostat a few degrees higher and leave it there instead of making big swings. A hot house takes hours to pull back down.
• Close blinds on south- and west-facing windows in the afternoon.
• Cool the house down overnight and in the morning when Colorado's dry air cools off quickly, then close up before the heat builds.

When to call a professional

Call if a breaker trips twice, the outdoor unit hums but the fan won't spin, ice returns after a thaw, you hear hissing, or you smell burning (shut it off at the breaker). A good diagnostic visit should include measured refrigerant pressures and temperatures, a capacitor and contactor check, airflow and static pressure readings, and a look at both coils. Ask for the readings, not just a verdict.

Repair or replace?

If your system is old, uses R-22, needs a compressor, or has a leaking evaporator coil, compare repair cost against replacement before you commit. Our AC replacement and compressor replacement pages explain the trade-offs. If you'll replace anyway, a cold-climate heat pump can cool in summer and handle much of your winter heating. Our heat pump vs. furnace guide and Denver HVAC cost guide help you compare. If another company has already told you the system is finished, we offer a free second opinion.

MrFixer Heating & Cooling is based in Castle Pines and serves the Denver metro. See our service areas or call (720) 466-7983.

Frequently asked questions

• Why is my AC running but the house won't cool down? The most common causes are a clogged filter, a dirty outdoor coil (often matted with cottonwood fluff on the Front Range), a frozen indoor coil, a tripped outdoor breaker, low refrigerant from a leak, or a failed capacitor. Check the thermostat, breakers, filter and condenser first, then call a technician if it still isn't cooling.
• Can I hose off my AC unit? Yes. Turn off the power at the thermostat and the outdoor disconnect, clear debris, and rinse the coil gently with a garden hose. Never use a pressure washer, which bends the fins. If the coil is heavily packed or the fins are badly bent, schedule a professional coil cleaning.
• How long does a frozen AC take to thaw? Usually several hours with cooling turned off and the fan set to On. Replace the filter while you wait. If the coil freezes again after you restart cooling, the likely causes are low refrigerant, a blower problem or restricted ducts, and it needs service.
• Is it normal for my AC to run all day in a heat wave? Yes, on days hotter than the design temperature, which is about 91–92°F for most Front Range counties. The house may drift a few degrees warm by late afternoon. If the AC can't hold temperature on ordinary 85–90°F days, something is wrong.
• Can I add refrigerant to my AC myself? No. Refrigerant must be handled by an EPA-certified technician. Low refrigerant also means there is a leak, which should be found and repaired before recharging, especially now that R-410A production is being phased down.
• Does altitude make my AC less powerful? Yes. Air at Denver-area elevations carries less heat per cubic foot, so systems need more airflow for the same cooling, and manufacturers publish capacity derates. Fujitsu, for example, lists a 0.83 correction factor at 5,000 feet. Proper sizing accounts for this.