Common HVAC/Central Air Problems
Am covering in my experience both from past home situations, and helping out my neighbors a tip on central air conditioners going.
Important Note: If you are not comfortable and want to err on the side of being safe, and/or not damaging your equipment, please do contact an HVAC company who has licensed professionals.
In my experience, the four top causes of a central air conditioning unit is no longer functioning at all or partially, especially over the summer when encountering extreme temperatures, will vary, but listed below is the order in which I check since candidly the easiest fix first and simplest to check are as follows:
1. Fuse tripped.
This can occur either due to fluctuation of power and/or the amount of power being drawn. Best part easy to diagnose as NO power runs to the unit, and easy fix is to find the fuse box, and the right switch to move to ON vs tripped state (which usually is middle vs right to OFF position). If the fuse was literally blown, after turning it to the ON position and checking the unit is indeed still not receiving power, you may need a fuse replacement. Never been the case for me, but figured add.
2. Auto shut off switch triggered by an Air Handler in the attic
The Air Handlers main function is to blow the cold air through the vents, and many have a drip pan that collects the condensation from the cooling effect and a connected drain pipe to the unit and the drip pan too. I have found that if you don't check and blow out the drain pipes from the drip pan, the drainage pipe can literally get clogged with dust/insulation/algae build up. When it is REALLY hot and REALLY humid outside, the amount of water generated by condensation effect is much higher than usual and can result in the drain pan literally filling up IF the drainage pipe is partially or fully clogged. The easiest fix for this is to use a shop vac that can blow out air, and simply attach the vacuum nozzle attachment that is closest in diameter to the drain pipe coming from the unit plus the secondary one from the drain pan. I have used duct tape to create an air tight seal in the past, and simply blow the lines out and validating outside (where the drain pipe usually runs out to) that debris is indeed cleared. If you really want to also ensure all debris is removed, a garden hose can be hooked up and secured to the drain pipe and flushed with water making sure you can control the flow closest to you, with a brass single nozzle being the easiest (plus it fits in the pipe too). I have only had to do this once for an older home that had not been used much and I found the air flow out from the drain pipe was restricted outside.
Note: The use of duct tape is super handy since when hooked up to the pipe and blowing air, allows you to go outside while the shop vac is running to feel the pressure coming out the other end and of course the blast of debris removed that varies. I have also used it for flushing with the hose/brass nozzle after insterting into the pipe to ensure no backsplash happens before rotating the nozzle to the on position to allow the flow of water through the hose.
3. Capacitor going or completely gone
I have dealt with this situation which usually happens after years of sustained usage. For example, on my prior 20 year old unit it had started to stop during extreme temperatures and like the above fuse situation, it would not turn on, although when tested I did see power being drawn by the unit. I used a voltmeter (set to the proper setting and most voltmeters have guides or just search online to learn which setting works best) to test the power of the line hooked into the capacitor and from the then the lines going from the capacitor to the contactor switch to validate if any major discrepancy.
Keep in mind to access the capacitor you do need to remove the protective panel, and are exposing live wires. Standard procedure for me is to literally pull the main power switch (usually attached to the house above the unit), put on thick rubber gloves and remove the panel and clean out the cob webs and other debris that can accumulate since power is shut off. To test the unit, I do then put back on the main power switch, ensure gloves are on and take my time being careful to not cross the leads on the voltmeter is key when testing each connection or worse touching the top of the leads on the capacitor with bare skin which will shock you. Switching a capacitor out is a simple operation and most important thing is to ensure you order the right one to the same specs of the old capacitor being put in and paying careful attention to the wiring before and after, as there are usually have three (single capacitor) or four leads (dual capacitor) and cables connected to the capacitor leads (common, hermatic, which connects to the compressor and fan).
Note: Capacitors do store residual power and can shock you if not careful as most units have 240 Volts of power. Like stressed above IF not comfortable and to err on the side of safety, call a licensed HVAC person first vs just jumping in.
4. Contactor switch going bad or shorted out
Just this month, I had this situation crop up at home where my central air conditioner unit (5 year old Bryant) outside the house was making a loud clicking noise. I was able to determine that the contactor switch (which engages the fan and condenser, when cool air is requested from inside the house) was actually erratically toggling between on and off hence no cooling inside. Interestingly when I pressed the switch on the actual contactor in, both the fan and condenser immediately engaged but once I removed pressure on the switch it started to act up again and the clicking was actually the switch receiving a current, but not continuous, so it simply dis-engaged and continued to repeat the pattern.
I shut off the power (master switch), put on my trusty Carhartt thick rubber gloves and carefully taking note and pictures of the wires, I removed the contactor switch. I then removed the cover on the actual contactor switch with my smallest flat head screwdriver based on the plastic notches seen on the sides. Sure enough, the contactor switch was bad due to the magnets that it uses to when engaged had worn and the sparking had burnt the switch out. I had to order a new switch and voila, problem solved.
5. Faulty Control Panel on the inside of the house
I have never experienced this issue, but know a neighbor whose house had encountered several lightning strikes which fried the control panel inside. We swapped his control panel with mine, and confirmed was the culprit. Not a common issue, but if you happen to have issues on one control panel, and have multiple identical ones in the house, swapping to test with one you know is working is the easiest fix. Also when identical, you do not have to re-wire, as many these days allow you to pop out manually if they have a battery compartment for when power goes out to keep all settings saved.
By checking the above, you usually can isolate most of the common problems unless something more serious like a bad electrical line or a serious issue on the actual AC unit and/or Air Handler itself (control panel on Unit burnt out, motor of fan burnt out, etc.). However, I did have one interesting case recently which is why I re-ordered what I would usually check to hopefully help you out.
6. Combination of problems above...
I did have a very new problem on the same unit that had the replaced Contactor switch as it was acting up as if the contactor switch had gone bad (#4 above). Interestingly after examining the switch and not seeing anything visibly wrong, I checked all the wiring and then decided to check the air handler in the attic too. Interestingly I heard a similar 'clicking' noise and upon closer examination saw the drip pan was slightly filled up (since we had a heat wave with 90 degrees of humidity, hence plenty of condensation), and the Air Handler Auto-Shut off switch (reminder...that is in place to prevent spill over of the drip pan associated to a clogged drain pipe per #2 above) was actually toggling between on and off making a clicking noise as opposed to forcing the unit into the off position. What this actually had resulted in was sending an on and off signal to the unit including the contactor switch based on the wiring done. Sure enough some insulation had fallen into the drip pan restricting just enough the drain pipe to slightly fill up the drip pan. Once I addressed the clog, and to be extra cautious blew out the drain pipe line, I now plan to order a new drain pan auto-shut off switch to solve this problem since it was faulty. Keep in mind if left unchecked, it would have damaged the contactor switch or worse the unit itself if it encountered this same problem and no one was home to address.
Hope these tips help, and remember if you are not comfortable and want to be safe, and/or not damaging your equipment since high voltage and shock or getting hurt is possible if not careful, recommend to contact an HVAC company who has licensed professionals. Feedback always welcome on anything I missed too!