HVAC Help!

I just noticed that my HVAC unit outside was running when it is turned off at the thermostat inside my house.

There’s no air coming out of the vents, but it does blow air through them if I engage the a/c. Why is the fan still running when its set to off?

So far I’ve tried:

Replacing the batteries in the thermostat with new ones.
Hitting the reset button on the unit outside.

I can turn it off at the breaker box in the basement, and I suppose I will have to do that to shut it off. Right now I have the a/c on and am hoping that once it reaches the temperature I’ve set it at that it will turn off, but I don’t think it will.

What is causing the fan outside to keep running when it shouldn’t be?

Do you mean that the compressor and condenser fan on the outdoor unit are running?

I’m assuming you have a split a/c with an indoor and outdoor unit. If so, it’s likely that the low voltage control signal that operates the units is powered from the indoor unit. Try turning off power to the indoor unit and leaving power on at the outdoor.

If the outdoor unit shuts off then you have some sort of short that’s making the control circuit for cooling. Kind of unlikely.

More likely is that the outdoor unit keeps running, probably because of a stuck contactor that powers the compressor and fan. If it’s not welded shut you may be able to unstick it by tapping on it or the like. ISOLATE POWER FIRST.

Reads your OP again.

If the compressor is running and the indoor fan isn’t you’re in a bad situation that may freeze the coil or damage the compressor, turn power off to the outdoor unit immediately.

If only the outdoor fan is running but not the compressor that’s trickier. Rarely a motor winding can fail and short to the case with enough winding remaining to turn the motor. The motor casing is now a current carrying conductor. Again, secure power.

I’ll keep an eye open if you can provide more details. Good luck.

eta spelling

I have the a/c on now and it works fine, so I assume that the compressor is working okay, because the air is cold. I don’t need my a/c on right now, but I’m trying to see if the thermostat will signal the unit to shut off once it reaches temperature.

I do have a split unit: outside is the fan/compressor portion, and inside is the blower and emergency heating coils.

The issue is that if I just turn the thermostat switch to “off”, the fan outside keeps going…but I don’t think the compressor is. In fact, when its doing that I can’t feel any air coming out of the vents at all. But I do feel the cold blast of air right now that the a/c is engaged. I’d bet the heat works as well.

I’m more worried about my electric bill than anything else. I have extended homeowners coverage, so if the unit dies I will get a new replacement for the $50 price of a service call. I can in fact shut it all off with the breaker, but the thermostat isn’t getting the outside fan to stop running.

There is most likely a relay that energizes the compressor fan, which is separate from the compressor relay. The fan relay is probably welded on. Unless you know electricity, you are not going to be able to fix this yourself.

Yeah, I kinda figured. If in fact something with the unit is shot, this is the time of the year for it. Its 73 degrees and is supposed to be nice for days on end here. If it doesn’t shut off when the a/c reaches temperature, then I will force off the power at the breaker and call my homeowner’s warranty company.

I’d like to save $50 if this is something I can resolve myself though.

…aaaaaand wouldn’t you know it, when the a/c reached its temperature the unit shut off like its supposed to. Still, something isn’t right. My unit and house are from 1996, so maybe something needs looked at. I have no idea if I can replicate these symptoms for the repairman, however.

Relays can go “intermittent.”
I had to replace a compressor relay on my unit for the opposite problem - it would fail to turn the compressor on every now and again.

Hopefully if I call in someone they can find the issue. I need to do it now before the sweltering Summer days hit though.

The next time your compressor fan is running on it’s own try setting your thermostat to FAN ON (as opposed to AUTO). If you get cold air coming up through the vents then we’ll know your compressor is running as well. If it’s just ‘room temperature’ air then most likely it’s just the fan running.

ETA, when you do that also make sure the HEAT/COOL setting is set to OFF.

If the compressor is running as well, I think we can narrow it down to a problem either with the T-Stat or the control board or relay in the furnace. If the compressor isn’t running it’s probably an issue within the compressor/condenser unit itself.

Looks at location
I know what you mean by “sweltering.”
Good luck.

I’m pretty certain that the compressor was not running until I actually engaged the a/c with the thermostat. What I noticed was that the unit’s fan outside was running and everything was set to “off”. I did not know that the outside fan could run like that with NO air blowing through the vents.

Once I turned the thermostat to “cool” and set the temperature below ambient the air started to blow through the vents like normal and it was cold, so I assume the compressor kicked on.

Actually, the outside fan didn’t turn off on its own. I looked at the thermostat, which I had set to 77 degrees, and it was reading the same temp so I just flicked the switch from “cool” to “off” and everything turned off like it was supposed to.

I don’t know what any of this means. I may have an intermittent short as beowulff mentions.

I’m not sure if you realize this or not, so I’ll mention it anyways. It’s the fan in your furnace that makes air come up through the vents. So even if the outside unit is spinning away, no air will come up through the vents until the furnace blower starts spinning.

So here’s the thing. If the compressor is running the (evap) coils in your furnace will be cold, if it’s only the fan and not the compressor running then the coils in the furnace won’t be cold. The easiest way to figure this out is to turn on your furnace fan and see if the air coming out of the vents is cold or not.
Once we know if the compressor is running or just the fan motor we can continue troubleshooting, but IMHO we can’t really move forward until we figure that out.

But since I activated the air conditioning at the thermostat and it worked, doesn’t that in fact verify that everything else is working properly? The air was forceful and cold.

The issue was that earlier in the day that the fan in the outside unit was running for no reason as everything inside (thermostat) was set to “off” because its nice enough out here to just have the windows open. I heard it running and was like “WTF?”.

You do need to call someone in on your problem. Banging on relays or fiddling with it may make the problem go away today. But it will probably return and according to Murphy it will be on the hottest day when all the tecks are up to their eye balls in work and they will get to you when they have the chance.

Well clearly something is wrong, I’m just trying to isolate it. If everything is working in proper order, the fan on your outside unit shouldn’t be running unless the fan on your inside unit is also running.
The way I see it (and raindog can confirm or correct me)
If the compressor is running:
1)There is a problem with the control board or a relay in the furnace and it’s turning the AC on.
2)There’s a problem with something in the condenser (outside) unit that it thinks something inside the house is calling for it.

If the compressor is not running
1)There’s a problem with something in the condenser unit that is causing the fan to turn on. Bad relay, short somewhere, don’t know.

The fact that when you turned the AC on at the TStat and then everything shut down properly, makes me think there’s a bad relay somewhere. Figuring out if the compressor was running also will help tell us which relay it was (the one in the furnace that says “Hey, I need some freon down here” or the one in the outside unit that says “Hey, the compressor is running, I need the fan running too”).

ETA, until you figure this out, you should probably turn the AC unit off at the breaker. Even if the compressor isn’t running and it’s just the fan there’s no point in wasting the electricity.

Thank you man. Seriously. So far the fan has not turned back on, but I am going to call in a service order tomorrow. No sense in waiting on this thing. I just found it highly odd that the unit outside was running, and I didn’t have anything turned on inside with the TStat. This is the time of year I’m supposed to user very little electricity with that thing!