Air conditioner or heat pump won't start:
What to check if your air conditioner or heat pump just won't start at all. Here we give a diagnostic sequence of things to check if the A/C is just not working.
Checking these simple items, switches and controls, including some you may not know about (overflow pan switch for example) may get your system running without a costly service call.
This article forms part of our series on how to diagnose an air conditioner or heat pump that is not cooling: this article explains how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperature, or an air conditioner that won't start.
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See MOTOR OVERLOAD RESET SWITCH just to be sure that the air handler blower fan motor has not shut off on thermal overload.
And if your blower fan is driven by a fan belt and an electric motor, of course check to see that the drive belt is in place and un-damaged. If the blower fan belt is broken the electric motor will run just fine (you may be able to hear it) but the blower fan assembly itself won't be turning.
Details are
at BLOWER FAN OPERATION & TESTING
If the indoor air handler blower runs but the outdoor compressor/condenser unit never starts then the problem is more likely there. If the indoor air handler runs but the outdoor compressor/condenser does not,
see COMPRESSOR / CONDENSER DIAGNOSTICS
1. Check that electricity is on for the equipment, everywhere. Check all switches and controls, including service switches, including outside by the compressor, inside at the air handler, and fuses or circuit breakers in the electrical panel.
See KEY HVAC SWITCHES service switches, circuit breakers, fuses, power
2. Check all of the safety and interlock switches: For example, condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner.
There are several switches and controls, both manual and automatic that can leave an air conditioner or heat pump turned "off" such as a blower compartment door interlock safety switch
Electrical problems: air conditioning system won't start: it may sound silly, but is the air conditioner turned on?
Has the cooling thermostat been set to "cool" and the temperature set below ambient room temperature?
Do both the outdoor compressor/condenser unit and the indoor blower fan/evaporator coil unit have electrical power?
Has the air conditioning electrical wiring been physically damaged or cut? Photo courtesy of Tim Hemm.
Are the power switches on at these units, are the fuses good, are the circuit breakers in the "on" position, and is the thermostat set correctly?
Air conditioner compressor problems, including compressor noises, hard starting, and burned-out compressors, are explained in detail beginning
at COMPRESSOR & CONDENSING COIL and including topics such as
If an electric motor hums but won't start or won't keep running the problem could be a bad start/run capacitor.
Of course a frozen bearing or burned-up compressor or motor or even low voltage can also mean a motor hums but won't start - we give links to those diagnostics in step 2 above.
See CAPACITORS for HARD STARTING MOTORS
Condensate pan switch lockout: an attic or other air conditioner air handler condensate drip tray or drip pan located under an air handler is installed to catch air handler condensate leaks if the normal condensate drain system fails.
Some condensate pans have their own separate overflow drain (a proper installation) or share their drain with the normal condensate drain (an improper installation).
But on some air conditioning air handlers the installer may provide a condensate overflow pan switch rather than a separate pan drain line.
In this installation the switch is designed to turn off the air conditioning system if it finds condensate water in the overflow pan. The idea is to shut down the air conditioner before there is a more costly leak into the building insulation or ceiling.
Condensate leaks into the condensate pan can shut down the air conditioner.
In tracking down an air conditioner condensate leak, I found that the the condensate pump drain line, a small-diameter PVC pipe, was clogged with water mold.
That was why my air conditioner wasn't kicking on. It would of ended up costing at least a hundred dollars just for a service tech to fix something as simple as that. I had to take the pump apart and clean it. I took off the PVC drain line coming out of inside air conditioner and blow and clean it all out too. - Jacob Behrends, FL
(July 24, 2014) Randy said:
The central a/c in my Florida home (in late July) would not turn on and the temp in the house was well above the preset temp of 78. Fearing a complete failure (the system is 17 years old) I googled my problem and eventually found this very clear and concise Inspectapedia site.
The past few days I could hear water dripping in the condensate drip pan and new something was up as this never happened before. I climbed my ladder at 11:30p and sure enough the condensate float switch was parallel to the tray meaning it tripped and turned the a/c system off. Grabbed my shopvac and within minutes the immediate problem was solved - Now to clear the drain...Excellent DIY website!
So if your air conditioning system seems to be normal in all other respects but it simply won't turn on, check for a flooded condensate pan or a defective condensate switch.
See CONDENSATE PAN OVERFLOW SWITCH in our description of HVAC switches & controls
See CONDENSATE DRIP TRAY DEFECTS
Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
On 2021-08-28 by inspectapedia.com.moderator (mod)
@Fred deel,
As you replaced the motor and capacitor, I'd look next for a bad fan bearing that's binding and loading the motor, maybe worse when it's run for a while and is heated up.
On 2021-08-28 1 by Fred deel
Ac fan wouldn’t start could manually press contractors an it takes off a few hrs later can motor slows up an hums replaced motor an capacitor took right off whe it shuts down it won’t start back unless pressing contractors then it tAkes off but eventually motor comes to a crawl could it be fan relay or control board it’s a trane xl 1500 xi
On 2021-08-26 by inspectapedia.com.moderator (mod)
@Giamatteo,
You're correct, it looks as if I haven't correctly understood your comments and question.
I apologize.
Let me take another look at think about it a bit.
Keep in mind that I have only a microscopic bit of information compared to the people who've been to your site. I can't be smarter than they are.
...
Would you say AC and furnace shutting off I'm a little confused.
Are you saying that in heating mode and then cooling mode your system shuts down and stays down for a long.?
And are you saying and heating mode that happens even when the outdoor compressor condenser unit isn't running?
In other words are you running on some other heat sources such as gas or oil?
So you can see that from afar, without a bit more detail we're not sure just what equipment we're discussing.
If I were to GUESS that by "furnace" you're referring to the indoor air handler that blows both cool air in cooling mode and warm air in heating mode,
AND
if I were to GUESS that you're asking about a heat pump system that provides both heating and cooling,
THEN
I would ask if your investigators included, in troubleshooting, a shut down caused by high head pressure at the compressor that in turn can be caused when a refrigerant metering device like a capillary tube or a TEV (Thermostatic Expansion Valve) freezes up or clogs with debris.
I really want to be helpful but beyond this we know so little about your installation that I worry about wasting your time sending you off in wrong direction.
On 2021-08-26 by Giamatteo
inspectapedia.com.moderato not sure what made you assume I haven't contacted someone or read the post prior to commenting. 2 different well known companies have been out. Even the owner of 1.. numerous times now. I also reviewed the page before I wrote the comment. I didn't see anything pertaining to the A/C and furnace both having the same issue in shutting off randomly and not starting for 12 or more hours. To me that wouldn't be "short" cycling. No errors on the circuit boards eirher. Thanks anyhow.
On 2021-08-23 by inspectapedia.com.moderator (mod)
@Giamatteo,
I agree that what you describe is really frustrating and annoying. But I've very little to go-on here.
Our most complete list of things to check for the air conditioner not starting problem that you described are given on the page above, better than I could type it a new here.
Please take a look, if you haven't already done so. It sounds as if you need an experienced Heating and AC refrigeration service technician on site.
Have you tried a conversation with the local A/C service company service-manager to ask for a senior technician.
On 2021-08-23 by Giamatteo
Hi! I hope someone can help. For almost 2 years the unit in my rental shuts down after running and will not start back up for hours. They keep sending someone out and it fixes nothing. They've tried everything and can't figure it out. I'm getting sick of it. It happens during heating and cooling. Please help. ♥️ Thank you.
On 2020-09-24 by (mod)
Let's start by checking the diagnostic steps given on the page above, Dorie.
On 2020-09-24 by Dorie
My heat and central air suddenly stopped today. My thermostat clicks but nothing happens. Hmm.
Any ideas?
On 2020-09-09 by (mod)
Still this sounds like a control board problem to me, but I'd start by reviewing the thermostat wiring;
it's also diagnostic to note whether this is a new problem or is one that's been present since installation
On 2020-09-09 by PETER
If I switched to the Heat mode on thermostat control, the fan blower to blow air inside the house works fine on Auto/on selections. When I switched to A/C mode, the fan blower NOT working on Auto and On selections, but the compressor Fan and everything seems working fine outside the house, The furnace control board status lights "RED" (Normal).
Please help !
On 2020-08-18 by (mod) - Sometimes my AC condenser unit will not turn on.
Todd:
Have you done a detailed diagnostic of just where power is present or not?
At the control switch, at the board, at relays, etc.?
What about a slow-draining condensate pan and a condensate pan switch that shuts off the unit?
And incidentally, you can have a failed start capacitor that isn't bulged. It's pretty easy and inexpensive to try swapping in a new one.
On 2020-08-18 by Todd B.
Sometimes my AC will not turn on. I have tried a number of things to troubleshoot this problem. To be more specific my thermostat(s) will call for cooling, the (attic located) furnace blower will start but the condenser unit fan will not be running and the system will just circulate warm (ambient temperature) air. The issue can be temporally resolved by removing power to the (attic) furnace and then connecting it again.
Unplugging the heater and plugging it back in. I typically accomplish this by flipping the furnace breaker in my electrical panel, though I have gone into the attic and simply unplugged the furnace from the electrical outlet just as a test. After power cycling the furnace, the system will cool normally. That fix typically lasts about 24 hours before I have to power cycle the furnace again.
A couple years ago we had a problem with the system which turned out to be the Furnace Control Board. The technician that I called to come out fixed the board by reflowing the solder at one point on the board. A problem he had seen before. This fix worked. Thinking that this board was the source of my problems
, I replaced the board with a new board from ICM Controls. This, unfortunately did not fix the problem.
After installing the new board everything was working normally, the test sequence worked as indicated, heating and cooling checked out when I set my stats for heat and then cool. After a period of time though, the same issue with the AC condenser fan not kicking on returned.
I inspected the electronics for the condensing unit yesterday and didn't see anything that stood out as a problem.
The capacitor is not bulging and everything appears to be connected properly. I even disconnected all the leads, applied dielectric grease to them and reconnected them. Still last night the problem returned. I tripped the breaker for the furnace and cooling resumed again.
I also have a zone controller in my attic. Two thermostats connect to the zone controller, zone controller connects to the furnace control board and drives two dampers in the ductwork to direct cooling to upstairs, downstairs or both. I see no signs of any issue with the zone controller.
I checked the LEDs on the zone controller while the AC was not working properly and found no fault or error indications. The zone controller is powered from a transformer inside the furnace so when power cycling the furnace, the zone controller is power cycled as well. I have not yet tried bypassing the zone controller.
...
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