Air Conditioner Troubleshooting – Common Hvac Problems

by Dave on January 21, 2009

Like many of the hvac training topics that are covered here in this blog, this one will be dedicated to central air conditioning. I will make more posts on hvac training that are referring to cars and window units but for this particular post I will go over the most basic and most common hvac problems that I encounter day in and day out as an hvac technician working on central air conditioning systems, so let me go over some basic air conditioner troubleshooting tips that may be of some use to you.

The most common complaint that I get from customers is that their a/c not cooling. Now this could be many different reasons for this, and this is the whole point of the post as I am going over the common problems that prevent the a/c from cooling, and most of them you can take care of yourself even if you have had no hvac training at all.

Dirty A/C Filter – First thing that I do when I get a air conditioner not cooling call is that I check the air filter, or furnace filter whatever you want to call it. I do this before anything. If you live in a humid spot that requires you to run your air conditioner all of the time then you need to keep up on your air filter, they should be changed once a month depending on what type of air filter it is that you are using. A dirty air filter will cause your central air conditioning system to not function as effective as it should, that is the basics of hvac training. Think about it the way that hvac works is that the warm air is removed from the room that is trying to be cooled and it is escorted outside. A dirty air filter will cause a restriction in air flow trying to get out of the house, which will make the cooling process a lot harder and a lot less efficient, so check your air filter monthly at least. There are many people out there with no hvac training that have no clue about this, and most of these people have lived in apartments all of their life and they do not know better, if you are one of these people that did not know do not feel bad it happens all of the time. That is what I am here for to give you the proper hvac training for the beginner, so with that being said let me go on to the next most common problem that I run into with central air conditioners not cooling.

A/C Coils Dirty – Now there are two sets of coils that are on a central air conditioning system, one set of the coils will be inside and the other is on the condenser itself which is located outside. So for the second hvac training tip is that you want to keep both sets of coils clean, for if they get dirty they will cause a restriction of air flow. For the inside air conditioning cools you want to make sure that you are always keeping your air filter clean, as explained in the above statement. If you do not have an air filter in at all that means that there is nothing to catch the dirt, dust, and all the other stuff that will clog up your inside air conditioning coils and prevent your system from cooling efficiently. Your inside air conditioning coils should be bright and shiny, free of debris, if they are not and they are dirty, there are many types of coil cleaners that are out there that will do a good job. I will cover coil cleaning in future posts as this is a whole other type of hvac maintenance and will require some basic hvac training before you attempt to do this. For the outside coils it is a little different, since there is no filter on the outside condenser. But the outside condenser is constantly blowing out air and there is no suction, and outside coils compared to the inside ones rarely get clogged up. Just make sure that wherever your condenser is located to make sure that it is not smothered by anything that would restrict the air being released, such as bushes and plants surrounding the condenser, leaves, shrubbery etc.

No Power – Another hvac training tip is if the air conditioning unit is not turning on at all, it is missing power somewhere. Most central air conditioners have two types of power that make the unit run, this is 240 volt and 24 volt. The 24 volt line tells the unit to kick on, while the 240 is the main power behind it all, without both working properly the central air conditioning unit will not work at all. The first thing that I do when I run into this problem is I check the breaker for the unit. If it is tripped then I simply reset the breaker by turning it off all the way and pushing it back to the on position. Then I will wait a couple of minutes to see if it trips again, if it does then there is a loose wire somewhere in the system, a bad breaker, or further electrical troubleshooting that I will get into in detail later. If the unit is running fine after the breaker is reset then the problem is usually fixed, unless it keeps tripping, which could be a amp overload and it might require a bigger breaker, you will need an amp meter though to determine this for sure though.

Another reason that there may be no power to the central air conditioning unit is that the float switch inside the air handler might be tripped. The float switch is located inside of the air handler in most cases and is usually positioned close to the coils, since there must be a pipe running from the drip pan. What happens is that over time your condensate line will get clogged up with sludge slime eventually if you do not do routine maintenance on your hvac condensate drain system, this will cause water to eventually build up, or condensate. When the drain line is clogged up with slime sludge the condensate will have no place to go and this will cause it to back up and flood your home, unless you have a float switch installed which you should, its regulation in most states. The water will push up on the float switch, thus cutting the power to the 24 volt brain of the hvac system, which will cause the a/c to lose all power. The way to cure this is to clean out the drain line, there are many ways to do this, but the best way that I know of is a wet dry vacuum.

These are just some of the most common problems that I run into when trouble shooting an hvac system here in the sunshine state Florida. There are many more possible reasons that the air conditioning system may not be working, like the unit could be low on Freon, electrical issue (could be so many), bad contactor, bad relay, bad blower fan, blown fuse, the list goes on and on and I will cover all of these I promise. But if you came here for some hvac training for the beginner, then you need to take all of this in slowly and learn the basic principles. I do not want to overwhelm you with too much information at once but if you need immediate assistance please just leave a comment and I will get to you as soon as possible, that is today’s lesson in hvac training I hope you all enjoyed it and learned something.

{ 25 comments… read them below or add one }

Jeff February 1, 2009 at 9:47 pm

Great article man definitely learned a couple of things from it :)

Ulysses Brownridge July 3, 2010 at 5:32 am

Their is a way to check the 24 volt and 240 volt components to confirm what is bad or broken or bad connection using the color code. Any information using your meter.

HVAC & R Fast Track Service Training January 17, 2011 at 2:57 am

Yes, Dave is right. Learning HVAC & R for the beginner can become overwhelming. I have been in the HVAC & R field since 1978 and teaching it in Parts houses as seminars, Union Halls and I also have taught 13 years in High School/College. It takes time for students to learn, although a modular approach with hands on Lab time is essiential. I have never seen any of my 2000 or more students say they learned it right and applied it without hands on training. I provide hands on training in Northern California in more than 20 different skill specific service technician diagnosis and troubleshooting problems. I will have a website soon and will be teaching the seminars starting in febuary 2011. Please feel free to email me if you would like more information on upcoming (John Ketelhut’s HVAC & R Fast Track Service Training) seminars/workshops.

Tammie Briggs May 1, 2011 at 3:14 pm

The problem that I have is that : I have central air condition and the outside motor is blowing but the inside air isn’t blowing. I need to know what could be the problem.

john May 19, 2011 at 5:26 pm

iam new in the hvac field and need some pointers about prices.ive been doing this line of work about a year now and need some pointers about how i should charge.

Tim June 9, 2011 at 12:33 pm

I would also be interested in knowing what to charge a customer.

joseph July 1, 2011 at 7:09 pm

good man that was great there ,but i have a window 1hpwr but will not start

Bill Brock August 1, 2011 at 3:33 pm

How do I fix tubing from freezing (stops the flow of air)?
Thanks, Bill

tony August 6, 2011 at 10:43 pm

hi i have a train unit and it only hums when turned on three days ago it was working fine i understand these can have many things it could be but i know power is getting to the unit outside i can here it hum i do have an understanding of electrical wiring i will check the drain hose and such that you mentioned in this blog thank you for your time Tony.

Javen Glazener August 15, 2011 at 12:06 am

I checked the breaker, and nothing. usually you can try to turn it on and off and it wouldnt click (like the unit itself) so i changed the thermostat. is there anything else i can try?

Julio August 16, 2011 at 4:31 am

New at this trade still taking the course I enjoy your page it answer a lot of questions thanx

Mark August 20, 2011 at 3:56 pm

The circuit breaker tip saved me a lot of time and effort.


Manuel Rivas August 20, 2011 at 5:53 pm

My central heating/cooling air conditioner condersor went out and
I am having a tough time located the condensor. The unit is 18 years old. Will it be cheaper to replace the part or purchase a new air conditioner?

Your help will be greatly appreciated

Marillyn August 27, 2011 at 12:50 am

My fan outside is not coming on but unit inside is blowing out warm air and the temp in my home is 91 what to do?

joe September 3, 2011 at 10:38 pm

when i turn the air on auto it shuts off but when i put on fan it stays running and cool air comes from the air vents . is it ok to let it run on the fan or should i shut it off.. also on the condencer out side in the back of it on the nut where the copper lines come in to the house when its on auto the start getting frost on them then the furnace shuts off only when i put on auto is it safe to keep it running on fan please let me know asap thank you joe bouchard

Marcel September 7, 2011 at 11:28 am

I have a Gibson central unit, I have just changed the condensor fan motor and for some reason I do not have the 24 volt control power.
I have checked incoming power at the contactor and have L1 to ground 123, L2 to ground 123, L1 to L2 244 vac. There is no 24 v at the t-stat terminals on the circuit board.

Dale Richter September 10, 2011 at 10:50 pm

Unit has ice buildup on line running into house and also on unit inside house

doug October 6, 2011 at 3:22 am

Thanks for info.I want to start my own bus have a elect degree outboard motor tech for 13 yrs car tech for all my life very good at all these thing but not big on opening my own business with the world like it is i have worked on three h/a out of two of three was very easy but the third one was the hardest one i had ever ran into i have a friend that been in the field for 30yrs plus and he help but still could not get it going he had to come over and still never ran into anything like this before we got it running but with out him not sure if i would of got it.He said i should get into my own business but scared how many people work on h/a that has 15/30yrs xp that still run into stuff that they never seen how do u cope with this in the field and what is ur resource? thanks your friend Doug

Christina October 6, 2011 at 12:03 pm

My A/C (inside) is dripping water outside of the pan and my filter is soaked. What is causing this? I have researched to no avail. :-(

mitch October 10, 2011 at 9:02 pm

I have central A.C. and the outside motor is blowing but the inside air isn’t blowing. Your help would be greatly appreciated. Thanks Mitch.

Dave October 19, 2011 at 11:17 pm

Marcel – check the transformer should have 24 coming out of it

Christina – either clogged drain, broken drain pan, undersized ducts, etc, could be alot of things best to call service tech.

Mitch – is the filter or evap coils clogged? bad blower motor or cap for motor? Could be several things…

Doug – I work for a big company and there are dozens of senior techs that know just about everything…so I always got someone to help, smaller companies usually just rig the equipment to work or rig it to work temporarily..residential is easy commercial and refrigeration gets tricky for me ….

robert November 23, 2011 at 10:38 am

my central heating is working but at the same time it is calling for central air. Do i neet a relay to shut off my central air when it is caalling for heat?

Jay March 28, 2012 at 5:18 pm

Hi Robert,

I have a problem with my residential AC system. In the air handler the fan is working but there is no coolant coming out of the system. I went to the condenser and checked the high voltage and the low voltage but the low 24 voltage is not working. But the fan on the condenser is not turning on unless I press the switch on the contactor. What can be the possible issues with the system?

weley April 16, 2012 at 3:44 am

I would like to learn more about rheem a/c air cool units , like what rules don’t apply to them

Sharon May 21, 2013 at 4:22 pm

I have a Heil central air unit and the fan stopped turning in the unit. I have tried all the suggestions on the internet for troubleshooting but nothing works. I cannot even find a replacement fan of the model number on the fan unit. EGADS!

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