It's a hot and sticky 90+ degree day with the midday sun blistering through your windows and your AC unit is blowing nothing but warm air. You've tinkered around with the thermostat and adjusted the temperature beyond subzero levels, but to no avail, your AC is not cooling.
If the temperature controller (thermostat/remote) is correctly set with a status symbol indicating that the indoor fan is running and a cooling call command is present, you can ignore the basics like controller batteries and circuit-breakers.
Typically, if everything seems normal yet your AC is still not blowing cold air, the outdoor unit will be where you need to start your fault-finding.
Nine times out of ten, the outdoor unit will be in what's called "lockout" mode, which is a safety mechanism that prevents any potential major damage to any mechanical components.
Every AC unit utilizes a series of safety switches and overloads that are designed to protect both the compressor and fan motor(s) from unnecessary wear.
That said, how does all that technical mumbo jumbo help you now in a time of need and sweltering heat when your AC not blowing cold air? Well, let's breakdown the basic troubleshooting steps you should take before calling in the professionals, as some of the time, this could very likely be a quick 15-minute fix.
Why Is My Air Conditioner Running But not Cooling the House?
Just because the indoor fan is running, it doesn't mean your AC system is operating correctly.
From my previous 20+ years of experience in the HVAC industry, I've gained an immense amount of HVAC related knowledge. But the one thing I've learned over my latter years is that the internet is your friend when it comes to fault-finding the better-known air conditioner brand names.
The vast majority of AC units used for residential purposes utilize specific fault codes to indicate the error that has occurred with the system. Different manufactures use highly beneficial fault codes to assist with not only diagnosing the fault but, by also providing a solution to help rectify the existing fault.
To determine what fault code is present with your AC unit, simply check the temperature controller display screen. If a fault is registered, you should see a fault code, like what is illustrated below.
If you locate a fault code on the temperature controller display panel, Google your AC manufacture with the search term "fault codes" and your AC unit model number. This may seem like a pain in your moneymaker, but it will eliminate any redundant troubleshooting and sped up the entire process.
So if you've done all that and you're unfortunately none the wiser, you can start by conducting the following fault-finding methods.
What do I do if My Air Conditioner Isn't Blowing Cold Air?
What we're about to cover are some of the exact techniques that a qualified HVAC technician would perform to successfully resolve any would-be faults on a site visit.
Trust me, I've done this before.
Thermostat Incorrectly Set
One very common fault that will cause the compressor to trip on internal overload (overheating) is due to adjusting the set point on the thermostat abnormally low.
On a ridiculously hot day, you want your air conditioner to produce super cold air on demand. Although, if you adjust the set point on the thermostat below 68 degrees on a 90+ degree day, you'll put excessive strain on the compressor causing it to essentially overheat.
That's why for comfort cooling in a residential application, you should never set your thermostat below 72.5 degrees when experiencing extreme ambient temperature conditions.
If you do find that the thermostat has been mistakenly adjusted too low and your AC system is not supplying cool air, you can try power cycling your AC system to bring it back to life.
It's best to turn the power supply off at either the circuit breaker or power outlet on the outdoor unit to reset any safety trip switches like a compressor overload.
Note, I'd let the system cool down for an hour or so before turning it back on if this is the case. This is because some compressor overload switches close circuit when the compressor body cools down.
Although, if your system is of an older generation, you'll have to manually reset the compressor overload switch which will be located on the contactor, damn near identical to what is pictured below.
Blocked Evaporator Coil or Dirty Return Air Filters
If you've never had you'd your AC system serviced or have not conducted a DIY service yourself, you'll most certainly find that the return air filter for your air conditioner will be darn right filthy.
Dirty filters will greatly restrict the amount of airflow that can pass over the evaporator coil. This will either result in the system tripping the low-pressure (LP) switch and or cause the evaporator coil to freeze up if the system doesn't have an LP switch.
This is a simple fix as you can merely remove the filters from the return air grill, wash them with your garden hose, place them back in position, and reset the system by turning it off at the electrical isolator (power outlet) for 10 seconds and back on.
A good 95% of residential AC units are equipped with an auto-reset LP switch, so by turning the power off/on for a short period of time, you'll be able to reset the fault.
Keep in mind, that some AC units are fitted with disposable air filter(s), and will need to be replaced, not washed if they are dirty.
A dirty air filter is not only going to create problematic issues regarding the functionality of your air conditioner, but your air conditioners also going to be producing dirty air in terms of air quality that you and your family are breathing.
So for a routine maintenance task that only has to be done every 6-12 months and will take you less than an hour to complete, it is highly beneficial.
Restricted Airflow Over Condenser Coil
Condenser coils tend to collect a considerable amount of dust and dirt as airflow is drawn over the coil from its ambient environment.
A condenser coil merely acts as a temperature medium. It causes the refrigerant to change state from a high-pressure vapor to a liquid before entering the evaporator coil via a refrigerant control device, like a TX valve.
So, if you don't ensure that the condenser coil remains clean enough to complete this essential process, the discharge pressure from the compressor will continue to increase before it trips via a high-pressure (HP) switch.
If you do find that the condenser coil is dirty and the outdoor system isn't running at all, this could very much be the cause of the fault. To resolve this issue, simply turn off the power at either the circuit breaker or power outlet next to the outdoor unit and wash the coil with a regular garden hose.
There is no need to use a pressure washer as too much pressure will damage the aluminum fins of the coil.
I generally use something like Nu-Brite Coil Cleaner which will last you literally years as only a small amount is necessary for each coil clean. A Condenser coil clean is really an annual task (at best) so you'll have plenty of coil cleaner leftover for future spring cleaning.
That said, here's the exact step-by-spet process to correctly clean your dirty condenser coil:
- First, grab yourself a pair of household rubber gloves and put on some sunglasses to protect your eyes.
- Using your garden hose, set the nozzle to decent power spray function, and rinse the coil thoroughly.
- Next, mix the Nu-Brite Coil Cleaner solution with a 1-5 part ratio of water into a regular spray bottle.
- Spray the mixture onto the condenser coil and coil allow it to works its magic for around 15mins or so.
- Lastly, generously rinse the condenser coil with water again to remove the coil cleaner residue.
Once the condenser coil is clean and dry, you can restore power to your AC system. As previously mentioned, mostly all residential AC units use HP & LP switches with an automatic reset function, so once you've power cycle the system, the fault will reset.
If that's not the case and you're one of the unlucky few without an auto-reset HP switch, you'll have to dive a little deeper.
To do this, with the power off, you'll have to remove the main cover on the condensing until and hunt around for a switch identical to what is displayed below.
Once you've located the manual HP switch, depress the switch until you hear a "click", and you're good to go. Just remember to put the protective cover back on the AC unit before you restore power.
Why is AC not Getting Cool After Resetting it?
If you've attempted some of the suggested basic repair works but either had temporary success or none at all, your AC system may have a major malfunction.
Here are some of the more technical and labor-intensive faults that could be holding your AC back from delivering that much need cold air to your hot and stuffy house.
Burnt Out or Seized Condenser Fan Motor
Condenser fan motors are generally pretty robust. Although, after a few years of relatively serious runtime, motor bearings can start to wear which will eventually result in a seized fan motor.
I this occurs, the easiest way to diagnose a seized condenser fan motor is to isolate the power supply and spin the fan motor blade free-hand. If you hear a scraping sound while the blade is reluctantly spinning or the blade doesn't spin at all, the motor will need to be replaced.
A seized fan motor or even a fan motor will worn bearings will cause either a fuse, circuit breaker, or overload to trip. This safety mechanism prevents the motor from operating as it's amperage will be well over the maximum set range.
Electrical faults with condenser fan motors are quite common and whether it's a dead-short, damaged winding, or the motor is simply down-to-earth, such electrical faults will be impossible to verify without at least a multimeter or megohmmeter.
Regardless of the actual fault, if you don't have access to either of these electrical testing tools, you can still determine whether or not the motor is cooked by conducting the following test:
- Isolate the power supply to the outdoor unit and remove the main panel containing all the electrical gear.
- Locate the condenser fan motor (CFM) or relay and identify the "line voltage" leg out of the contactor or relay.
- Remove the line voltage wire(s) from the contactor or relay and temporarily restore power to the system.
- If the contractor pulls-in or the relay makes without blowing a fuse or tripping a circuit breaker, the motor is the problem.
I would really only attempt this fault-finding exercise if you have some limited experience working on electronic circuitry and have a basic understanding of electrical switching gear. Such a test will merely allow you to determine if the fault is either the relay/contactor or the motor its self.
Here's where things get tricky. Correctly evaluating an air conditioner compressor to ascertain whether or not it is functioning properly or even at all, will require a set of HVAC manifold gauges and a multimeter.
You will need to be certain of the suspected fault before you condemn a compressor due to the substantial cost attached to them. Not only that, depending on the age of your AC system, replacing the compressor may not be the most cost-effective option.
This is due to the considerable expense of the compressor and the labor necessary to replace it. The scope of works to replace a faulty compressor will look something like this:
- Reclaim all of the existing refrigerant in the system with a recovery machine.
- Unweld the compressor's discharge & suction ports using an oxy-acetylene torch.
- Remove the compressor from the condensing unit and install the compressor.
- Weld in the new compressor and install a new replace liquid line dryer.
- Fully leak test the system with nitrogen and a trace charge using a leak detector.
- Vacuum the system for a 24 hour period to remove any moisture or contaminants.
- Charge the system with refrigerant using refrigerant scales to the correct required amount.
- Install the existing pump heater or crankcase heater and test if it functioning correctly.
- Replace either the contactor or relay for the compressor and fully commission the system.
As you can see, there is quite a bit involved with replacing a faulty compressor. While you could conduct these works yourself to save some dollars, you'll want to be absolutely sure that the compressor is the issue before you go reclaiming refrigerant or firing up the oxy-acetylene torch.
Plus, what I covered above doesn't even discuss what is required if the compressor is burnt out. That's where things get really messy and even more expensive as the labor hours will increase and you'll also have to install a burn-out dryer for a limited amount of time before later removing it.
If you do suspect the compressor to be faulty, I recommend calling in the professionals to fully assess the problem and ask for a quote to both repair and or replace the system entirely.
AC not Blowing Cold Air and Leaking Water
While this specific issue could be related to either a blocked condensate drain pipe and or a faulty condensate pump, generally speaking, a lack of refrigerant will be the likely culprit.
If your AC system is not blowing cold air, yet your AC is leaking water that seems to be trickling from the indoor unit, you'll most certainly find the indoor evaporator coil (within the indoor unit) to be iced up.
This will happen if there is a refrigerant leak in the AC system, as there will be a reduction in pressure within the refrigeration system, as pressure and temperature are directly related.
Low-Pressure Fault - Refrigerant Leak
Alright, if you do encounter this specific fault, you're honestly not going to be able to do a whole lot without a particular set of professional-grade HVAC tools and a decent degree of refrigeration knowledge.
There is a massive misconception when it comes to refrigerant leaks in AC systems, and that is the wayward thinking that AC systems need to be "topped up" with refrigerant.
That's not the case, and you should never have to simply add more refrigerant to your system if there is a reduction in either head or backpressure.
Your AC unit is a sealed system that effectively pumps refrigerant around a pipeline through a series of components via a compressor.
Throughout this process, the refrigerant changes state leaving the compressor as a high-pressure vapor, to a liquid form within the condenser, to then a low-pressure vapor within the evaporator, to where it re-enters the compressor as a vapor and is compressed again repeating the same process.
Even though I left this last, this is one of the more common faults you encounter when your AC not cooling. While it is a task you could possibly DIY, you will need to get hold of a few tools before you can effectively diagnose and repair a refrigerant leak.
Here's a complete list of every piece of equipment and tool you'll need to leak test and repair any AC unit:
- Pump Down cylinder to recover the existing refrigerant in the system.
- Refrigerant weight scales to measure the amount removed from the system.
- Manifold gauges to test, reclaim, and charge the system with refrigerant.
- Nitrogen regulator with a small d-sized nitrogen cylinder to charge system.
- An electronic leak detector to pinpoint the exact location of the leak.
- Oxy-acetylene torch kit used to weld the copper pipework or components.
- Some brazing flux paste, abrasive emery tape, and silver solder 15% brown tip.
- A robust HVAC vacuum pump to remove any moisture and or contaminants.
- Lastly, a new cylinder of refrigerant to charge the system with once the leak is repaired.
Now that you know what tools and materials are mandatory to mend a leak on an air conditioning unit, here are the exact steps you should take to repair a refrigerant leak on your air conditioner.
- Isolate the power supply to your air conditioner at the circuit breaker or remove the fuses from the fuse base cartridge, and turn off the power outlet switch.
- Remove the main protective panel/cover to access the Schrader valves so you can connect the manifold gauges to the high and low-side ports.
- Remove the Schrader valve caps and connect the blue hose to the low-side (suction) and red hose to the high-side (discharge) port.
- Next, connect the yellow hose to the inlet of the recovery unit and using another hose connect that to the Pump Down cylinder.
- Now open the manifold gauge ports and begin reclaiming the system's refrigerant unit both gauges read around 10 PSI or 68 KPA (standing pressure).
- Once the standing pressure equalizes, charge the system with nitrogen until your gauges read roughly 200 PSI or 1350 KPA.
- Now it's time to break out the leak detector and rigorously go over every part of the system until you locate the leak (focus on the locations exhibiting oil residue).
- After locating the leak, remove the yellow hose from the nitrogen regulator and release whatever pressure is in the system into the atmosphere.
- Now prep the area where the leak is by cleaning up the pipework with abrasive emery tape and then apply some brazing flux paste.
- Using your oxy-acetylene torch, weld the area applying adequate heat, and by dabbing silver solder to the surrounding area seal the leak.
- Now you'll need to recharge the system with nitrogen back up to 200 PSI (1350 KPA) and check that your weld will hold under pressure.
- After that, you can blow off the nitrogen in the system and connect your vacuum pump by fitting the yellow hose to the inlet of the vacuum pump port.
- Vacuum the system for a full 24 hours or overnight, and after that period of time, make sure that the system holds negative pressure under a vacuum.
- Next, close the manifold ports and break the vacuum by connecting the yellow hose to your refrigerant cylinder and open the low-side port until the gauge reads zero.
- You can now charge the system using a set of refrigerant scales adding the necessary amount of refrigerant you'll need which will be located on the outdoor unit nameplate.
- Lastly, you can now remove the manifold gauge ports, fit the Schrader valve caps back in position, and return the system to normal operating by restoring power.
Obviously, this is a lengthy and labor-intensive process that is beyond a complete beginner's grasp. This is the kind of job that could literally make or break your air conditioning unit. So, before you attempt to master such an intricate repair job like a regular HVAC pro, do as much research as humanly possible.
There are a bunch of problematic faults that could have sidelined your air conditioner. Some of these issues can be remedied quickly, while others will require the services of a qualified HVAC technician.
By now you've likely come to the conclusion that air conditioners are very complicated pieces of equipment that utilize complex mechanical and electrical components with precise control technology.
These elaborate AC systems of today allow us all to enjoy an immense level of comfort as they effectively cool your home or even heat it up producing warm air in the colder months.
Hopefully, this in-depth DIY HVAC repair guide has helped you sort out your AC issues or at least given you a beneficial understanding of how AC systems operate and what is required to maintain one long term.
If you do set aside some time to conduct routine maintenance throughout the year on your AC system, you'll assuredly avoid a number of common AC faults. Honestly, you won't have to devote a whole lot of time and you don't have to rebuild the wheel either.
Just run through the basics like we covered here and you'll greatly prolong the lifespan of your AC unit. Plus, you'll also save a few dollars in the process by not having to pay unnecessary service fees. Best of luck.