Restart protection in the thermostat or condenser.

It waits to turn on until the high side pressure has dropped down to help unload the compressor at start up.

Yeah, I know about the restart protection in the thermostat, as the original post indicated. But when this problem occurs (the thermostat kicking in only triggers the blower and not the compressor), the high side pressure has been down for quite some time. I can turn the temp setting on the thermostat high enough that the A\C system is on standby for hours, but when I subsequently drop the setting down, only the blower runs. It's only when I turn the thermostat from "cool" to "off" and wait 10 minutes that the system runs correctly when I set it to "cool" again. How would this indicate that the restart protection is at fault?

Could be as simple as a bad timer relay on the high side sensor.

Could be a bad controller board in part of the system.

Nick - I have the exact same problem. I am using a Honeywell CT3500 programmable thermostat that is about nine years old. This problem only occurs very infrequently and I believe that it is just that the compressor doesn't kick on when the thermostat says go ( I do not think that it is the compressor stopping on its own).
My concern is that if this happens while we are away for an extended period of time, could this do damage to the compressor if left in this state? (As you probably know when this occurs the blower just keeps going and never shuts off because the temperature is never lowered.)

I am going to replace my thermostat with a new Honeywell RTH2300B and see if this problem happens again. I will post my results, but it might take a long time till I know definitively if it was the thermostat. Please let me know if you find out anything new on your end. Thanks, Todd

Well the new thermostat did not solve the problem. I had my AC guy come out and he showed me that the contacts on the contactor were slightly burnt and that even though it was pulling down, it was not making good contact at times. We have had to replace the contactor a few times in the past. This time he installed a new type of computer controlled contactor that was designed to reduce arcing and thus should have a longer lifetime. This device is an Emerson-White Rodgers 49P11-843 SureSwitch. (Cost is around 50-60 dollars on Amazon). So far this has worked very well.

I have the same problem with my new carrier air unit in Palm Springs, my contractor informed me that it something to do with the energy saver device from Edison?. When power has been down I have to reboot my compressor...my thermo is Wyfi and this causes my thermo not to connect with Wyfi....I'm fed up with this unit, it's the Cadillac of units and out here in the desert I've hadn't nothing but problems...Help!,,,,

Just to close the loop on this because topic was bumped recently... My original problem continued to worsen, and manifested itself first in the compressor sometimes not turning back on at all, even with a lot of downtime, and then later not shutting off at all, even when the thermostat was off It turned out to be the contactor. The contactor is a mechanical device that switches on the 240 volt power to the compressor when the 24 volt line from the thermostat energizes a magnet, causing a metal connection to move and complete the circuit. There were a couple of issues with the contactor, causing various problems at different times. In any event, a contactor is a cheap part, and replacing it myself wasn't particularly challenging. I wrote an explanation of exactly what I did at http://ths.gardenweb.com/discussions/3234157/compressor-continues-to-run-even-though-thermostat-and-blower-are-off?n=10 and hopefully someone will find the information useful.

Cheers,

Nick in Palm Springs

I am having the same problem as everyone else. When mine does it the thermostat is sending the proper voltage to the board but the board isn't sending voltage to the contactor. So that being said something whether it be the high or low pressure switch is keeping the unit from turning on until i let it stay off 5 to 10 minutes. then it runs fine for a little bit then same thing again. some days it will do this one time other days it will do it all day long.

To Jimmy Stanley who said "the thermostat is sending the proper voltage to the board but the board isn't sending voltage to the contactor"... If there's no voltage going from the blower's control board to the condenser's contactor when the system is supposed to be on, then the problem is with your board or the wires from same to the condenser, and not anything inside the condenser itself. You've got to have that low 24 volt juice from the control board reach the contactor so that switch can flip and let the 220 volt juice reach the major components inside the condenser. Just as important, the low volt flow from the board to the contactor has to continue uninterrupted until the thermostat senses that your interior air temp has reached the desired value. I wouldn't worry about "the high or low pressure switch" or anything else until you are able to confirm that the 24 volts are getting to the contactor as they should.

Nick in Palm Springs

Thanks nick_shirley, It only does this when it's hot outside and not everyday. I changed the board a while back and that wasn't the problem unless two boards were both bad and doing the same thing which i doubt. The last time it acted up i checked and had the 24 volts coming from the thermostat to the board but no voltage from the board to the contactor which is what made me think a pressure switch may be locking the voltage out. As stated previously by others turn the unit off for 5 to 10 min and it starts back up and runs fine for a while or a cycle or two. It varies how long from day to day. May run a week and no problem or do it 5 times a day. The one thing that confuses me is when it acts up i turn the thermostat off and the air handler doesn't turn off immediately it may run on for a minute or two. Any and all help is appreciated it's about to drive me nuts.

"The last time it acted up i checked and had the 24 volts coming from the thermostat to the board but no voltage from the board to the contactor which is what made me think a pressure switch may be locking the voltage out"

I want to be clear about something... When you say "the board", do you mean the control board that sits inside your furnace\blower? If so, that's what I mean as well. That board does a lot of things (including the timed shut-off delay mentioned below) but as far as I know, all it does to turn the condenser unit on or off is sent 24 volts over two lines to the unit, specifically the contactor within the unit. There certainly may be more complicated control systems out there but that's the way traditional A\C systems were set up. In such systems, the control board has absolutely no idea of what any "pressure switch" inside the condenser unit is doing, no idea of whether you have no coolant in the lines, no idea of any sort of problem or situation that might exist within the condensing unit. When that control board wants to turn on the condenser unit, it simply sends 24 volts down those little wires. If you turn your thermostat down far enough to signal that you want the room cooler, the thermostat sends a signal to the control board in the furnace, which then turns the blower on by the coils, and then sends the 24 volts to the condensing unit. If in fact you have "no voltage from the board to the contactor" despite you adjusting the thermostat to start the cooling cycle, then your first concern would be to understand why the board or the wires from the board aren't working properly.

If, on the other hand, you mean some sort of control board within the condenser unit itself, one that acts as a second brain (third if you count the thermostat) in the system and takes into account information from within the condenser such as high and low pressure readings before sending the 24 volts along to the contactor, then sure, you might well have an issue with that particular board or the pressure sensors or perhaps a whole bunch of things. Don't know. What I do know is that if you are indeed talking about this sort of internal control within the condenser itself and when you think it should be triggering the contactor but you can't detect any juice between the internal board and the contactor, then your problem isn't the contactor. The only way you can determine whether a contactor is bad is to send 24 volts to it. If you do that but the 220 volt lines within the unit aren't energized, then it's the contactor and indeed, because contactor issues can be all about carbon buildup on the contacts, you might have the symptoms you describe. But if the contactor isn't getting the 24 volts at all, you need to look for another answer. I strongly suggest that you turn this over to a repair tech given that you're no longer talking about replacing a $12 contactor in 15 minutes.

Nick in Palm Springs

Hello. I have what looks to be a similar issue. in very hot days when the ac unit runs very often the unit will just not restart at some point when the thermostat would demand it. the only thing that makes it start again is to set the thermostat above the current temp so that the contactor is not engaged, set the ac unit circuit breaker to off and then back on. Wait a bit and engage the thermostat again.

So far I replaced the run capacitor but the issue is still happening. I’ve checked 240v are present when the capacitor is engaged. i Can clearly see the capacitor engaged when the thermostat demands it.

Whats weird is that I tried to reset the unit by setting the thermostat over current temp and pulling the fuse next to the ac unit, wait a bit, put it back in and engage again the thermostat but the unit did not start. the only way is to reset it at the electrical panel box by setting the circuit breaker to off and on.

my unit is 16 years old and sometimes I can hear at start a bit of a sound coming from either the fan or compressor motor. However the unit runs smooth after start and cools very well.

I have another attempt to install a hard start kit considering the age of the unit and the start noises. i ordered the kit and will be it in 2-3 days. But I am really running out of ideas here and so far all I am getting from HVAC people is that I need to change it. combined with a furnace change as well due to the age of the system I am not looking forward to go for that unless the ac unit is really close to being done. still wanna give it a few tries that don’t involve service calls of $150-250 to only get quotes for a new system.

could It be?

1. A faulty circuit breaker that somehow does not give the the right amount of voltage? I’ve did reading with a multimeter and getting around 105-115 for both wires going to the ac at the circuit breaker and same at the unit?

2. Faulty contactor? I can for sure see it engaging when the thermostat demands it. Voltage past the contractor matches what’s coming from the box.

3. New capacitor is bad? For a he vented that matches spec requires by compressor but it’s just a bit higher. Instead of 50MFD and 370V it’s 55MFD and 440V.

4. End of life for older compressor?

any thoughts?

I have the same issue. I have replaced the capacitor with a 45/5 mf. I assumed that the old one was correct size. I need to make sure that is correct because what if it isn't the correct size.
My unit is 3.5 ton carrier. Is there a way to know for sure what is the right size capacitor needed? I also replaced the contactor. The compressor still doesn't run randomly. I know when I point my thermometer at it, it is hot. I measures about 220 F. I don't know how to solve this issue because it is intermittent and when the techs are here, it doesn't manifest it self so they can't diagnose it.

In my case I found the issue. During hot days when the compressor would run more than usual something would happen and one of the circuit breakers of the AC would trip. There are 2 breakers due to 240v. Anyway my panel will get replaced for other Reasons as well and hoping that will fix the AC issue.

Monther Wahhab yes mine to was getting hot and shutting down during all this. Thanks to all that have replied to this as i myself had the same problem for over a year. mine kept getting worse i checked everything that i could and had most items eliminated as the problem. I Bit the bullet and called my tech. It took a little while but we finally got it figured out. My pressure was going way high and shutting the system down. The problem was the reversing valve. yep it was apparently sticking slightly open allowing the pressure to go high shutting system down on high pressure. Changed out the reversing and expansion valve and it's running like new. I hope this helps someone, cause the problem drove me crazy.

Do you have a heat pump. Where is that reversing and expansion valve located? Are they in the condenser outside or in the furnace in the attic?

Update, I brought a different HVAC technician. He found out that the system was overcharged causing the compressor to overheat and then it the circuitry inside of it doesn't allow it to work. He removed some freon and told me that should do it.

Good deal hope it's that simple for you as i stated mine was way more complex. Yes i have a carrier unit and the reversing valve is in the condenser outside. the expansion valve is located on the inside unit (furnace)

Yes i have a carrier unit and the reversing valve is in the condenser outside. the expansion valve is located on the inside unit (furnace)

Air Conditioner Short Cycling.

https://www.minisplitunit.org/

Short cycling means that your air conditioner turns off and on too often, too rapidly, or at irregular intervals. That frequent on-off switching will ultimately damage the compressor motor, fan motor, controls, or other components, leading to system failure and costly repairs.

These specifics will help you decide if your air conditioner is short cycling or operating normally. Air conditioner short cycling in minutes: If your air conditioner turns off and back on at intervals of less than five minutes it is short cycling and needs repair.

If your air conditioner turns on and off at intervals of less than ten minutes it is probably short cycling and also deserves a service call.

Air conditioner short cyclingin seconds: Watch out: A "short cycling" air conditioner or heat pump compressor/condenser unit that turns on and then back off after just a few seconds is in serious trouble.

You should turn off the system and call your HVAC service company for repair.

https://www.cooling-heating.net

Irregular on-off A/C cycling: A short cycling air conditioner may also be heard running at irregular intervals over the course of an hour or so even though the indoor thermostat settings have not changed.

The good news is that some of these AC short cycling causes are trivial to repair and thus not costly. Some, like a dirty air filter or dust-clogging a thermostat can be addressed by the homeowner.