Air conditioner humming outside but I have shut off the breaker

You apparently did not shut off the right breaker. Remember that your furnace is not likely on the same breaker as the A/C unit. The A/C unit should have a 220V breaker and there should be a shutoff in the immediate area of the unit.

My furnace has a seperate breaker, and on the furnace is the 'controler'. Since I have a heatpump/AC unit, I would guess in your situation, the controler is activating solenoids and relays, and that's what is humming.

Get a professional, you might find someone to help real cheap by networking. Check with your church members, coworkers, etc.

Good luck,

tom @

Thanks for this. It is definitely separate from my furnace. Im sure I have the right breaker though as it was the 220 and it stops the unit from actually running. I even pulled the breaker right out after pulling the wires. Would there be a separate power source for the heater? Just wondering if that is what might be humming. The panel of the unit (outside) notes that I need to leave it on for 12 hours before running the air if its been shut off, so Im assuming it has some sort of heating source for the fluid.

Thanks again

Joseph Meehan wrote:

The 240 volts that power the condensing unit is not the power that energizes the contactor in the unit. The contactor, which is the thing that would be humming or buzzing, is 24 volt and powered from the air handler (blower)

As was mentioned already, it's the low-voltage contactor you're hearing. The fact that it's buzzing, along with the fact that you had to shut down the 220 breaker to get it to stop, means that the contactor is getting power and calling for the A/C to run. If it were my system I'd be wondering why this is the case -- I'd probably start with the thermostat.

The contactor is powered by the same low-voltage transformer that powers your air handler (usually the furnace). It isn't really hurting anything to be on-- it takes very little power.

-Tim

Why not put the AC and the furnace on the same breaker? Course my furnace was electric so the power supply was 220 at 100 Amp. I just extended the line to where I wanted the AC unit and put the box and AC breaker there. No reason to use separate lines since the AC and the furnace can't run at the same time.

Course if you have a gas furnace and a 120V line that's another story.

My only concern would be what size breaker was specified for the A/C. If it specified a 50 amp breaker you are not protecting it (any the wiring if it was sized to 50 amp) with a 100 amp breaker and that would not be good.

It has been my experience which does not include electric furnaces as I live in an area where gas is cheap, that furnaces are 120V devices.

Thanks Tim. I tried shutting off every single breaker (including the line where the thermostat is attached) and none of them stopped the humming. I pulled out each of the wires (one at a time) from the contactor and it still hummed. Perhaps I'll try pulling the cold switch wire on the thermostat and see if that will stop it. My only concern was really that this buzzing had just started and that if it was a short, there would be the potential for a fire??

Thanks again

Tim Fischer wrote:

If you've disconnected all wires to the contactor and it's still humming, I'd probably call a paranormal expert or a bee/wasp/hornet guy

-Tim

This is what Im getting at. Its really odd that if I have all the wires out, it still hums. So Im wondering whether there would be a separate power source for the fan on the unit that is giving power to the thing. In fact, I had bees living underneath the unit this summer and had them killed off. Last night when I went out to look at the contactor, there were small worms crawling on the points - still alive, so there wasnt much power going to them but I wonder if they have shorted something.

Im off to get my palm read and maybe that'll help.

Thanks

Mano Tim Fischer wrote:

The first concern is duly taken care of as the

Lots of electric furnaces in the Northwest but may not be in many other places, that is why I specified that I had an electric furnace. Electric furnaces are usually 240 V, otherwise you would need huge amp breakers.

In my house, the outside AC unit has a separate electrical connection to the meter, and does not depend on the main panel. This normally makes no difference since turning off the main panel would prevent the

Around here that's a code vio -- the fire department wants a single place to kill power in the entire building. Unless, of course, you have a shutoff out by the meter (we don't).

-Tim

I wasn't here when the house was built (about 1969). Maybe that electrician wanted to save some money (100A panel instead of 150A, and a little less wire). This does provide a disconnect next to the outside unit (the gas meter is there too). Maybe it would be better to have it routed through the main panel (although that would require a larger panel). I suppose it would be expensive (If I did have it done, it could be time to add AFCI breakers).

Inside, I have a 100A main breaker, 11 20A 120V breakers, and 3 30A double-pole breakers (stovetop, oven, dryer). All control things other than the A/C which has a 50A double-pole breaker outside (and another thing, the unit says not to use more than a 30A breaker).

Probably the contactor. When the furnace indoors is calling for cooling, it sends a 24 volt signal to the outdoor unit.

To silence this, change your thermostat from "cool" to "off".

It's best if you can read the whole thread before posting. We've already ruled this out...

-Tim

I’m sure this is 14 years late, but I’m certain for someone in a similar circumstance reading through this ANY helpful response should be welcomed. I too experienced a similar situation where the AC breaker was switched off and the disconnect pulled out yet a buzzing sound wad heard around the capacitor area. Turned out to be that the THERMOSTAT was still left on AUTO and was sending a signal to the switch next to the capacitor hence the buzzing!! Funny how you think there’s no chance for electricity to be at the unit when specific breakers are off and yet here you have a situation where it’s the opposite. Granted, it is low voltage in it being a 24v signal, but still when you’re dealing with capacitors that could potentially arc up and cause serious damage, the last thing you want is mistakes. So definitely be weary that electrical systems are indeed nothing to be taken lightly and if precautionary measures aren’t in your vocabulary please call a professional. In the end turning the thermostat off got rid of the buzzing and the faulty capacitor was removed after discharge to be replaced. While it was buzzing, since there was a signal at the switch, any attempts to remove cables from the capacitor (especially the yellow lines to/from the switch( will result in a potential arc! Even though 24v, last thing you want is more firepower for the capacitor so please make sure everything is off and silent!

And if it wasn't discharged? Worst case, it's just 24V.

While it was buzzing, since there was a signal at the switch, any attempts to remove cables from the capacitor (especially the yellow lines to/from the switch( will result in a potential arc! Even though 24v, last thing you want is more firepower for the capacitor so please make sure everything is off and silent!

I agree the hum was from the coil buzzing due to the signal from the thermostat. Yes it is only 24V, and an arc from that isn’t a big deal. The precaution is intended for the many that are already on the edge about dealing with the capacitor that is indeed right there in that vicinity; and no I did not design the system. An arc, no matter how small, occurring while dealing with a capacitor that is deemed “deadly” in many forums and comments sections is enough to strike fear. The whole point was to describe a safe situation for the diy’ers. Its weird to see a statement such as “So What” from a professional such as you.