Air Conditioning Diagnostics, Repairs and Maintenance

There are a number of different kinds of problems that can occur with most central air conditioning systems from time to time.

These are discussed including some of the most common problems and how they can be remedied by the homeowner with the right hands-on capabilities and some tools that are commonly found in most toolboxes.

Here are some of those problems that users commonly report:

AC not working at all
AC runs but not cooling well
Room temperature too cold or too hot
Water leaks from AC
AC makes strange noises
AC won't turn off

While a qualified air conditioning repair engineer should be called to handle certain kinds of central air conditioning repairs, there are a variety of methods you can try for yourself for taking care of simple repairs and maintenance in this article.

One of the most important regular maintenance jobs you should carry out at least twice each year is to replace the filters, shortly before the heating and cooling seasons are upon you.

Now let's expand on the problems listed above and explain some simple solution measures you can attempt yourself.

Central AC is Not Working At All

First of all, make sure the thermostat is set in the "Cool" position if the AC won't start working. If you have done that and the central AC still won't go on automatically despite the thermostat signalling the need for reducing the temperature:

Check in the main AC electrical panel as well as any secondary circuit panels to see if there is a tripped breaker or a blown fuse. Reset the tripped breaker (turn it off andthen on) or replace the fuse. A central AC will typically connect to a dedicated 240-volt circuit.

The breaker may continue to trip if there is a problem such as a short in the system. This could be in the compressor, the capacitor or the fan motor. You will need to hire an electrician to diagnose and solve this kind of problem.
Check the thermostat setting is "COOL" and the temperature setting is 3 degrees or more lower than the ambient room temperature.
Check the power is on (there is power to the furnace or air handler, as well as the outdoor condenser). Ensure the compressor's 240-volt disconnect has not been shut off. This can be typically located in a metal box that's mounted near to the compressor.
After turning off the power to the AC, remove the thermostat's cover. Pull out the thermostat's body from its base. Check if it has batteries and replace them. Ensure all wires are securely attached to their terminals. Be careful that the cover doesn't pinch the wires as you replace it. Wait 3 or 4 minutes then turn on the system again.
If the problem persists, turn off the power, open the thermostat again. Unscrew the wire from the "Y" terminal. Now turn the power back on. Hold the wire ONLY by its insulation (CARE), touch the bare end of the wire to the "R" terminal and hold it in position for about 2 minutes. If in that time the compressor kicks on, that indicates the thermostat is faulty. You'll need to replace it. If the compressor does not kick on when doing the above, turn the power back off. You can check the capacitor or call an AC technician.
Check capacitor and wires attached to the compressor. This capacitor starts the condenser and the fan. The AC unit will not run if the capacitor has failed. It is easy to test if it works or not and is cheap and easy to repair if it doesn't. See below:

Test the Capacitor in the AC Compressor

For your safety, ensure all power to the unit is shut off before opening the AC unit's electrical cover. Remove the cover.

Set a digital multi meter to "Capacitance," then touch one lead to the "Common" terminal. Touch the other lead to one of the other 2 terminals. The meter readout should display a number. If it displays "OL" that indicates there's a short.

Note: AC capacitors can be configured as two capacitors in one. Both share the common leg.

That concludes our look at user diagnosing and solving the problem of the air conditioning not working at all. Now let's look at solving some of the problems associated with the AC working but not properly.

AC Not Cooling Properly

If you can hear the central AC running but it is not cooling properly, the first thing you should check is that there is nothing blocking or limiting the air flow anywhere in the system. This includes checking the air filters, the registers and the compressor.

You should also check for a kinked refrigerant line that could be restricting the flow of refrigerant. Check that there is not a blower fan that's not running properly as this can cause the system to fail to cool properly.

The next thing to check is the air handler.

How to Check the Indoor Air Handler

Another cause of indoor temperature problems is a faulty air handler. This can be checked as follows:

Turn off the electric power to the AC air handler and/or furnace. If the system's air handler is a gas furnace, be sure to shut off the gas at the gas valve serving the furnace.
There is an access door at the front of the air-handler's cabinet to allow access to the filter. Remove the door and pull out the filter. Clean or replace the filter as necessary and refit it.
Check for ice. If there is ice accumulated in the area around the coils, replace the door, turn on the power and turn on the fan. That should melt the ice within an hour or so.

What causes ice to form inside the air handler?

Ice in the Air Handler

There are 2 common causes of an AC air handler freezing up:

Reduced air flow cause by poorly working fans, dirty filters and/or coils
Low refrigerant level. This needs to be checked by a professional and, if necessary, recharged.

Mostly, this particular problem is caused by reduced air flow.

Clearing the Condensate Drain

All air conditioners, window, portable, mini split and full systems are capable of creating a lot of excess water because they are continually removing moisture from the air. To expel excess water, they have a plastic drain pipe coming out of the air handler on one side.

Over time, algae and other contaminates can block this drain pipe. When that happens, the AC stops working. Some condensate drains are fitted with a float switch that prevents the AC from running if water backs up.

Excess water can also form puddles around the unit and in extreme circumstances, flood the area. To deal with condensate problems, please see the chapter on "Water Leaking from AC" below.

How to Check the AC Outdoor Compressor

Sometimes the outdoor part of the system containing the compressor can cause problems. Here is how to check it for problems and solve them.

Before doing any work on the compressor, be sure to shut off the power to the unit and for safety, verify that it is off.

Clean the outdoor compressor. This involves making sure the external unit is free of fallen leaves and other debris that could be blocking the flow of air. A simple brush or broom is a handy tool to accomplish this job.
Ensure the compressor is working. When the thermostat is set to a temperature that is 3 degrees or more below the ambient room temperature, it should be possible to hear the outdoor compressor running and to see the fan revolving inside the top of the unit. If the fan is not turning, check for an overload switch (or button) to reset. Note: not all types of compressor have this. If the fan is not turning, attempt to manually spin the fan blades clockwise by poking a screwdriver or stick through the top grille. The fan should spin freely. If manually spinning the fan is enough to get it going, that indicates the unit has a faulty capacitor. This must be replaced (see Step 6 of the chapter on "Central AC Not Working At All" above).
If none of the above works, it may be that the coolant needs recharging. As this is not a job you can do yourself, you'll need to call a qualified local AC professional to check the coolant and if necessary, recharge it.

If the Air Handler Does Not Blow Air Properly

If the air handler is not blowing air properly, it could be that the indoor evaporator coil has become dirty, clogged or frozen up. Reduced air flow is commonly caused by dirty or blocked filters, a dirty coil, dirty or clogged squirrel cage blower or a malfunctioning fan motor. One or more of these symptoms can cause freezing up of the coil, resulting in reduced or blocked air flow.

If the air handler motor runs but the blower does not cause any air movement, there is a probability that the belt that connects the two has broken. Replacing the belt is fairly easy if you have some tools and handy-man skills.

Replacing an AC Blower Belt

If you discover that the AC blower belt is broken, here is how to replace it with a new one:

Turn off the electricity supply to the unit. In the case of the air handler being a gas furnace, shut off the gas supply at the gas valve.
Locate the air-handler cabinet and remove the door on the front to provide access to the blower (some units may have a slide-out drawer.) Inspect the old belt and write down the number stamped on it. Get an exact replacement from a hardware store or heating supply outlet.
To fit the new belt, it is usually possible to slip the belt on the smaller pulley of the motor first and then start it on the blower pulley. Do this by rotating the blower pulley by hand, being sure to hold the belt in place (take care to keep your fingers from getting caught up between the pulley and the belt). The belt should easily slip into place. It may happen that the fit seems too tight or the belt is difficult to slip in place. If that happens, the motor mount may be adjusted to provide more slack. Once the belt is fitted, re-tighten the tension by readjusting the motor mount. Observe the manufacturer's specifications for the correct tension. As a rule of thumb, the belt should move about an inch when pressing down on it.
Some blower motors need lubricating, although those with sealed bearings do not. Check the manufacturer maintenance manual to see if the bearings need oiling and carry this out by following the manufacturer's directions.

AC Blower Hums But Does Not Run

If the air handler blower motor makes a humming noise but doesn't run, it will probably need to be replaced.

AC Air Handler Does Not Run

If the air handler fails to run at all, there may not be any power getting to it. Check its circuit breaker along with any control switch at the air handler/furnace cabinet.

Check the door switch. This is a small switch located inside the cabinet door that is fitted to prevent the appliance running when the door is open. You should also check the thermostat is working correctly (see chapter on "Central AC is Not Working At All" above).

Water Leaking from AC

All central AC as well as mini-split, window,portable air conditioners vent free or vented and high-efficiency combustion furnaces will create a significant amount of condensation, especially in humid climates.

While this is handled effectively by free-standing auto-evaporating air conditioning models that expel most or all of the moisture, with most regular system this can amount to several gallons per day.

As a consequence, this excess water must have a means of exiting the air handler. This is typically achieved with a plastic pipe acting as a drain tube. That tube needs to go directly outside to allow the excess water to drain away.

The tube often terminates near the compressor or a floor drain or in some systems to a small electric "condensate pump" that is usually located adjacent to the air handler. In the case of a condensate pump being used, it is connected to a 1/2-inch rubber or vinyl tube that exits to the outdoors or to a drain.

Water Collecting around the Base of the Air Handler

When water is discovered dripping or pooling around the base of the air handler, it is a sure indication that:

There may be a leak from one of the pipes or tubes that carries it
There may be something blocking the water's flow
The condensate pump (if fitted) may not be working and is overflowing

On some air handlers, there is a small safety float switch connected to the condensation drain pipe. In case the drain pipe gets backed up with excess water, the switch shuts off the AC.

When this is discovered, it is a sure sign the time has come to clear the condensation drain pipe.

Clearing the Condensation Drain Pipe

If your HVAC system has an electric condensate pump, check to be sure it is connected to an electrical outlet that works. Look at the drain tubing to see if it come loose or is disconnected. If it has, tighten it up or reconnect it.

The most likely possibility is the tube or the pump has become clogged with algae. If this has happened, a wet-dry vacuum can be used to suck the water and whatever is clogging it from out of the tube. If the pipe is severely clogged, it might be easier to just replace it with new length of 1/2-inch tubing. This is something that can be purchased at an HVAC supply shop, a well-stocked hardware store or online.
Test the pump is working by pouring some water into its collector. The pump, which has an inner ball float that will rise with the water level and turn it on, should then start. If the ball float is stuck or broken, it won't rise. If it has become stuck, you should disconnect the pump from the electricity supply, then disassemble the top from the inbound tubes and clean it out to remove any blockage thoroughly. If the float is old or broken, it's probably better to replace the condensate pump. That will avoid needing to do it again in the near future.

Side note: To clear the algae blocking up things, dilute 1 part of bleach to 16 parts of water and pour it into the pump trough. Then pump it through the tube. Here's a tip: Avoid doing this on the lawn as it will kill off the grass.
If the pump runs but it does not empty the trough, the check valve located just before the discharge tube could be stuck. To clear it, first unscrew the valve and manually loosen the little ball inside. Check for any obstructions or algae build-up. If a condensation drain tube has become clogged with algae, clear it if possible by blowing it out or pushing a wire through it. Alternatively, simply replace it with new piece of tubing.
If ice is blocking the tube, the AC filters may need to be cleaned or changed. If the filters are clean, it could be the AC refrigerant supply is low. Call an AC technician to recharge the refrigerant.

Air Conditioner Makes a Noise

Sometime strange noises may occur within the system. They can come from the air handler, the compressor or from inside the ductwork.

Air Handler Noise

Problem noises in air handlers include a squealing sound, a grinding sound and a buzzing sound.

Squealing sounds: Most modern air handlers have direct-drive motors, but some older units are belt-driven and the noise can come from an old, worn, slipping belt-drive. In many cases, the belt has become misaligned or badly worn and needs to be replaced. Follow the instructions in the chapter above on "Replacing an AC Blower Belt" while referring to your owner's manual.

If there is a squealing or grinding noise coming from a direct-drive blower, shut off the unit. Call an HVAC professional as the motor's bearings may be shot. If you are experienced enough to handle this kind of work, order and replace the motor yourself.

Buzzing sounds: If you hear a buzzing sound when turning on the thermostat it will probably be coming from the fan relay or the air handler or furnace fan as these are the only parts that come on indoors when the thermostat is turned on.

Switch the thermostat's HEAT/COOL switch to the OFF position. Now switch the fan switch from the AUTO to ON position. This will make only the fan come on. If there is a buzz from the air handler, it likely has a bad fan relay or the blower fan itself is worn or damaged.

Outdoor AC Compressor Makes a Noise

Outdoor compressors sometimes make buzzing, humming, or grinding noises.

AC compressor buzzing: If the outdoor AC compressor unit makes a buzzing sound, sounding like it's trying to turn on but it can't, the run capacitor could be defective. If that capacitor fails, the motor overheats when trying to turn on but can't and a thermal overload switch shuts it off.

When you hear it buzz, poke a long screwdriver or stick through the grille and try to manually spin the fan clockwise (be careful). If it spins on its own for one cycle but stop working again, the indication is the capacitor has failed and must be replaced. See the chapter above on "Central AC is Not Working At All" and point 6.

AC compressor hums: If there is a humming sound from the outdoor compressor when the thermostat is switched to the "COOL" position, the low voltage transformer on that unit could be faulty. It will need to be checked by a professional AC technician.

AC compressor grinding noise: The bearings in the compressor motor can wear out over time. This results in a grinding sound. Often, the motor can overheat and shut off, signalling that the motor needs to be replaced.

Noisy Ductwork

Many HVAC heating/cooling ducts are metallic, so they can readily conduct noise from the air-handling unit to your rooms. The conduction of sound can be dampened and even stopped by inserting a flexible insulated ductwork between the metal ducting and the heating/cooling system. This work will need to be carried out by a professional heating contractor.

Ductwork pinging or popping: Thermal expansion or air blowing past a loose flap of metal can create a pinging or popping sound to emanate from metal ductwork. Diagnose by tracking along the duct runs and listening out for the sound. If you locate it, making a small dent in the sheet metal will create a more rigid surface that is less likely to move or make noises as it heats up or cools down.

Furnace or air handler ductwork rattling sounds: If rattling noises come from the furnace when it runs, check that cover panels are all screwed on tightly. If not, tighten them.

Air Conditioner Won't Turn Off

If the AC fails to shut off when the ambient room temperature reaches the thermostat setting, the problem may be with the thermostat or it could be a fault in the electrical system that runs the condensing unit outdoors. A quick, one-off solution would be to shut off the unit with the AC's circuit breaker.

Note: However, please be aware that circuit breakers are not light switches and should not be used as such. Continual use in this way will weaken them until they falsely trip or fail altogether.

If the thermostat readout is blank, or if the system shuts off completely when you switch the thermostat to the "HEAT" setting, the thermostat is most likely faulty and should be replaced. However, if the thermostat is working with the furnace, it may not be the issue unless one or more wires is disconnected or has arced.

You can check this by removing the low-voltage yellow wire from the thermostat when the AC is running.

If the unit shuts off, the thermostat is either broken or is wired incorrectly. Check for correct wiring and there are no cross-connected wires. If the wiring looks good, buy a new thermostat and follow the instructions to install it.
If the unit does not shut off, it is likely the contacts on the outdoor run relay have welded together. That can happen over time due to frequent electrical arcing occurring at the relay. Turn off the power to the furnace and condensing unit before checking out the run relay. The relay can be disassembled and the contacts pried apart.

This should work as a temporary fix until you can buy a replacement.

Get the Work Done

Keeping your AC equipment is good working order as this will save energy and you money. If a central air conditioning system is in a bad state of repair, there is a good chance it will fail when you need it most.

You can keep your system running well by following the several maintenance and repair tips above if you have some handyman experience, or get someone who has to do it for you. A well maintained, serviced and running system will reward you will longer service and fewer headaches over time.