Why Is My Air Conditioner Not Cooling?

AC Not Cooling

There are 16 potential reasons for an air conditioner not cooling. They are covered here plus the repairs to get your AC pumping out the cold air again – or more accurately, removing heat from your home to lower the temperature of the air in it.

The repairs for an AC not cooling range from simple to moderate to “you’ll probably need an HVAC tech to repair this one.”

Reasons Your Air Conditioner Isn’t Cooling

Why is my AC not cooling? The simple answer is this: Because HVAC systems are intricate and have plenty of moving parts, and they require maintenance.

While these integrated systems provide a great way to keep your house at a comfortable temperature during all seasons of the year, it also means that it involves several components that are pretty complex. Because of this, things can and do go wrong. This article will look at the cooling part of the system, and in particular, what might be the reason why the air conditioner isn’t cooling properly.

Let’s start with things that are quite easy for the homeowner to check and do something about. Then we’ll move into areas that may require the assistance of a competent service technician.

Easy Solutions for Every Homeowner

These are obvious but sometimes overlooked causes of the AC not cooling adequately.

1. Windows are Open

Some homes have very tight envelopes – house wrap/vapor barriers like Tyvek, new windows, doors with weatherstripping all around, etc. It creates little opportunity to air exchange between the outside. When the AC is running in a “tight” house, the indoor air becomes polluted and stale. The Environmental Protection Agency views this as a serious matter, and so do many homeowners. And some of those homeowners keep windows open even when the AC is running in order to get “a little fresh air.”

Air naturally travels from warm to cool. If you are running the AC with a window or door open, you are literally sucking the hot outside air into the house. This, of course, is not only making it harder for the AC to cool your house, but it will also increase your energy bill. Keep all windows and doors closed while running the AC. If you’re eager for fresh air, talk to an HVAC pro about a whole house ventilator that will provide it more affordably and comfortably. You have other air purification options too, and they’re explored in the Pick HVAC article Do I Need a Whole House Air Purifier?

2. Turning On AC After the House is Too Warm

You may be trying to save on your energy bill, so you turned the AC off before you left for work on a very hot day. Now you’re home and it’s 90F inside. No problem, you turn on your AC. Three hours later, you can’t understand why the inside temperature is only down to 80F. The problem is that you let it get too warm while the AC was off.

During hot weather it is best to keep the AC on, even if you set the thermostat at a higher temperature while you are away. When you return you can lower the temperature to what is comfortable and the system will be able to reach it within an hour or two. Another option is to install a programmable or WiFi thermostat that will allow you to set the thermostat to a lower temperature prior to arriving home, so the house will be cool when you get there – and you won’t waste energy cooling an empty house “all day.”

3. Tripped Circuit Breaker

Most central AC systems have two circuit breakers in the main power panel along with all other circuits inside the house. The unit has similar disconnect plugs in a box mounted on the side of the house near the outdoor unit. Make sure they are in the “On” position.

One circuit is for the inside air handler, usually along with the furnace. The other is for the outside component, the condenser unit. These are usually labeled so that you can tell which ones you’re looking for. If either of these breakers is tripped, the first indication may be that the system is not blowing cold air. The inside fan may be circulating air through your home, but if the outside unit is not operating, the air will not be cold.

If a circuit breaker is tripped, it will usually be obvious because the position of the switch is different from others in the same row. It also has “on” and “off” stamped on the front, and it may have a small window with a red or orange marker in it. 

If one or both of these breakers are tripped, simply flip it to the “off” position, making sure to push it as far as it will go; this resets the breaker. Then flip it back to the “on” position. If a circuit breaker trips back off almost immediately after you reset it, or if it will not reset at all, there is a more serious problem. Call a competent service technician to assist you.

4. Dirty Air Filter

Dirty Air Filter

When a filter becomes dirty, it will prevent enough air from moving past the evaporator to work efficiently. The system may be running, but not cooling properly.

The air filter on a typical central air system will be located in the steel cabinet that houses the furnace. Filters are rectangular – about the size of a small window - and about 1 inch thick. There are many types of filters, with some having a fiberglass or some sort of “spun” material. Some of the more expensive types are made of a pleated paper-looking material that will filter out smaller particles found in the air. The downside of this is that these “premium” filters will also become clogged more quickly, and can severely limit the amount of air passing through the system. You can learn more about HVAC filters, types and costs in our Guide to the Best Air Conditioner and Furnace Filters.

The best remedy for a dirty filter is to simply change it with a new one. It’s always best to purchase them in packs of two or more so that you have a fresh one handy to replace a dirty one. Do not run your system with no filter in place. This will allow dirt to get into the inside parts of your system and cause even greater problems.

For best results, change the filter at least once every three months. Remember, this same filter is used to circulate the warm air during the cooling season, so it gets a lot of use.

5. Dirty or Blocked Return Air Grills

This issue is much like the dirty filter problem.

There are several ducts in your house through which air is drawn from the living spaces to the central air system to be cooled and then blown back to the rooms through different ducts. These ducts that bring the air from the rooms are called “return” ducts since they return warmer air to be cooled by the AC unit.

The grills on the front of the return ducts are often ignored for many years and may accumulate a lot of dirt, dust, pet hair and other things that will restrict the proper flow of air through the system. This will result in your AC not lowering temperature the way you want it to – at least in some rooms.

Sometimes children might think it fun to see a piece of paper held against the grill by the suction of the air flowing into these ducts. A box, a piece of furniture or other object might also be placed in such a way as to at least partially close off one or more of these grills which, by the way, might also be in the floor next to a wall instead of in the wall near the floor.

Obviously, the way to solve this problem is to keep all objects away from these grills and to clean them periodically. If you can, it’s also a good idea to remove the grill (usually just a couple of screws) and vacuum the first few inches of the duct behind the grill. This will prevent that dust and dirt from getting into the main filter.

Handy Homeowner Fixes for an AC Not Cooling

OK, the ways to fix these reasons is that the air conditioner isn’t cooling are slightly more difficult, but not beyond the scope of anyone with decent DIY skills. The first two, like a dirty filter, prevent proper airflow and the dispersal of heat that the refrigerant removes.

1. Not Enough Air Circulation Around Condenser Unit

The condenser unit is usually at the back or side of your house and can easily become overgrown with weeds, bushes, trees or clogged with blown leaves and grass clippings. Too much congestion in this area can result in your system not blowing cold air because it cannot operate as it was designed to do.

Trim back all bushes at least 2 feet from the sides of the cabinet. Make sure there is at least 6-8 feet of clearance above the unit as this is where a lot of hot air must be able to escape. Do not allow leaves, weeds or any other debris to collect around the unit and it will be able to work properly.

2. Dirty Condenser Coil

Dirty Condenser Coil

Inside the outdoor component of your AC system is a condenser coil that has fins like those on a car radiator or the back of a window AC. The gaps between fins are small, so can become clogged by dirt, dust, leaves, plant seeds and other similar outdoor debris. This, as well as a dirty indoor filter is one of the most common issues that will result in your AC running but not cooling as it should.

The condenser coil should be cleaned annually. Here’s how the handy homeowner can do it fairly easily.

First, put the AC setting on your thermostat to “off”. Next, open the circuit box which is near the outdoor unit and pull out the disconnect plug to remove all power from the unit. Open the unit by removing the screws that hold the fan grill to the cabinet. Lift the fan out, being careful to not damage the power cord by which it is connected. Set the fan off one side of the cabinet.

If the coils are very dirty and clogged, gently brush them with a soft brush or spray them with one of the best coil cleaning foam products available today.

Coil fins are very thin and fragile, so be careful not to mash or bend them. Then spray the entire coil unit from the inside out with a hose on a spray setting (not a power washer). If the coils are only lightly dirty, just a spritzing with the hose will do.

Replace the fan on top of the unit, replace the screws and reinsert the plug into the disconnect box. 

3. Improper Thermostat Programming

Programmable thermostats are nice. They can be set to adjust the room temperature several times during a 24-hour cycle, and even different temperatures on different days of the week. But if not set correctly, your AC unit may not be lowering the temperature when you think it should. It might have reset to a default setting after a power failure. If you set it manually but later find that the room temperature is different, it may have defaulted again at its next program cycle, and the temperature is set higher than you want it to be.

If your AC is not blowing cold air at the times you want, you will need to reset the program on your thermostat. Carefully follow the instructions in the manual to accomplish this. You did keep the manual, didn’t you? If not, google it, and the manual should be available online.

4. Bad Thermostat

Symptoms of a bad thermostat can be several, including not reading the temperature correctly, not telling the AC system to turn on, defaulting to factory settings and so forth. If your thermostat has batteries, try changing them, resetting it and see what happens. If it is programmable, don’t forget to check the program and reset it if necessary. If all else fails, replace it. Our Thermostat Buying Guide will help you review your options and make a purchasing decision.

5. Plugged Evaporator Coil

This may be difficult even for the handy homeowner because this part of the system is located inside the air handler, usually right above or below the furnace. Dirt on the evaporator fins will restrict air flow through the unit and can even cause ice buildup on the evaporator coils if it is plugged badly.

Access to the evaporator is by taking the sheet metal plenum apart and carefully cleaning the coils after allowing enough time for any ice to completely melt. As with the condenser coil, use care in working on this part to avoid damage to the delicate fins. See our Evaporator Coil Cleaning Guide first. It might help you decide to DIY or call a technician.

6. Ductwork Leaking or Disconnected Ductwork

Disconnected Ductwork

After your AC unit draws the air from your living space and cools it, it then pushes it back into the house through “ducts”. These may be under the floor in a basement or crawl space or above the ceiling in an attic. They may also be in the walls, especially in a multi-story house.  Air escaping in any of these ducts will result in bad efficiency of your system. In fact, the Department of Energy suggests that in many homes, up to 30% of “treated” air, meaning heated or air conditioned, leaks. That’s a lot of waste!

You may not be able to find all these ducts, but the handy homeowner may be able to check many of them. In crawl spaces and attics, look at the connecting joints, especially wherever the ductwork makes a 90 degree turn. If the joints at these locations were simply taped when installed, the tape may have come loose, causing a leak or even a complete disconnect. If the cool air is going into one of these areas, your AC will not be lowering the temperature where you want it to.

Air Conditioner Not Running - Professional Help May Be Needed

Some of these are doable for those with good skills. You might not anticipate a few of these, but a home and HVAC system work together and must be in sync for your home to be as cool and comfortable as you expect in summer.

1. Bad Capacitor in the Condenser Unit

This is a problem that will be in the outdoor unit. A capacitor is used to give the compressor a “kick start” each time it starts up. Some brands have a 2-part capacitor that also helps start the fan motor at the top of the condenser. It is possible that one part is bad and the other part good. If the part of it that controls the compressor is bad, the fan may run, but the compressor won’t – and vice versa.

While the homeowner who is familiar with electrical components may be able to troubleshoot and fix this problem, the services of a competent technician is usually the best way to go. Here’s our AC Capacitor Cost and Replacement Ultimate Guide for more information.

2. Low On Refrigerant

This should not normally happen. The AC unit is sealed and the refrigerant should stay in for the life of the unit. If low refrigerant is the reason your unit is running but not cooling, it means there is a leak somewhere in the system. This is a problem for a professional to fix since handling refrigerants requires a license.

3. Poorly Ventilated Attic

This may be the result of blocked soffit vents or not enough other vents to cool the attic in your house. On very hot days this equals a super-heated attic which then radiates heat down through the ceiling into your living space. If your upstairs seems uncommonly warm on hot, sunny days, a bad attic setup might absolutely be an issue.

The very handy homeowner may be able address this problem, or a professional should be consulted.

4. Poorly Insulated House

If a house doesn’t have enough insulation it will result in air leaks up and sideways through ceiling and walls. It can also have leaks around doors and windows, through the fireplace, and even electrical outlets. Remember, hot air wants to get into your cooler house. Conversely, your warm air wants out in the cooler months.

If you are not proficient at adding insulation to an existing home, contact at least two competent professional insulators for prices.

5. Undersized AC Unit For Your Home

Are you new to the house? Do you just get a new central AC? If you’ve been there for a few years or the AC is several years old and this is a “new” problem, this probably isn’t the issue.

This is a situation you don’t want to have because it means “somebody goofed” when the system was installed. AC units are rated by the “ton”, which is a measurement of how much heat the unit can remove from a home in 1 hour. For efficient cooling, a house should have an AC unit rated at 1 ton for every 600 sq. ft. of living space.

If the AC system installed in your house is quite a bit below that, the only fix is to either replace it with a larger unit, or have an additional unit – perhaps a mini-split system – installed in addition to your existing unit.

Conclusion

Modern air conditioning systems are usually very efficient with a minimum of maintenance. By simply replacing the air filter regularly and keeping air grills clean, most homeowners will experience many years of trouble-free cool air in their homes.

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