Why does my AC drop into fan mode when temp has not been reached yet?

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Fretman

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I started noticing this about a year ago. There are times my AC, part of my HVAC unit, would drop into fan mode and stop cooling before the set temp has been reached. When my AC is on, I can hear an extra noise, something like a buzzing almost. That's when the cooling is happening. But at times, this would stop and the unit would go into fan mode and my house doesn't cool down at that point. I would have to turn the unit off and let it sit for a few hours. Then when I turn it on again, the AC will kick in again, and I won't encounter this issue again for a couple of days until it happens again. I do notice that this happens more often when it's extremely hot outside. Can anyone shed some light or observations on this? Thanks.
 

Sparky617

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The coil might be freezing, and then going into a defrost mode.
Good point. My coil was freezing up last week. Had my HVAC tech come by and I was 2.5lbs low on refrigerant. The units are 21 years old, I'm planning on replacing them later this year after the cooling season. My HVAC guy has been fantastic at keeping these things running much longer than my neighbors have with their units. Most have replaced theirs, some long ago. The cost of keeping my units running has been much lower than replacing them years before necessary. Not including routine maintenance I've probably put $1500 into the two units over the past 15 years. Refrigerant, a blower motor, a capacitor or two, a set of relay contacts. Neighbors that have used ARS to maintain their units are getting new ones after 8-10 years, so some are getting ready for their third units. ARS is really good at selling new units. Find a good independent HVAC tech to keep your units in good working order.
 

Jeff Handy

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Biggest problem with old units is that the older refrigerant is obsolete, hard to get, and the government is requiring a huge price increase on it every year, to encourage people to switch to new eco-friendly type of refrigerant.
One that causes less damage to the ozone layer above us.
 

Sparky617

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Biggest problem with old units is that the older refrigerant is obsolete, hard to get, and the government is requiring a huge price increase on it every year, to encourage people to switch to new eco-friendly type of refrigerant.
One that causes less damage to the ozone layer above us.
Yeah, the last visit was $359, the refrigerant is getting really expensive. As I recall the ozone layer has largely repaired itself over the past 20 years. I have a classmate from HS that owns his own HVAC company in MI and said my guy was very reasonable in his pricing. I'm just glad my units held on through my kids college years. Without the college expenses I'm in a much better position to replace 2 units and to add a new one for my soon to be finished basement.
 

Fretman

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@Jeff Handy , I think you are spot on. I now remember the same thing happening about 20 years ago when I had a window AC. The same thing happened. The AC would kick off but the fan would keep going. Then I looked at the coils and there was frost all over them. So now with my HVAC unit, if this is the cause, is it because I'm low on refrigerant, or is it just too hot outside, or is the unit getting too old. It's about 13 years old now. This sounds like something that I won't be able to tackle myself.
 

Jeff Handy

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I don’t know, I have just seen it happen to my old a/c and to others I know.
I just powered it off for an hour or so, that always worked for me.

Sometimes it happens if you set the temp too low.
Or if it runs for a very long time.

Pros on here can probably advise.
 

Fretman

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Yes, when this happens, I usually turn off the unit for 2 hours. After that, it may run for another few weeks without the issue happening again. Seems to happen when it's really hot outside...towards the late afternoon timeframe.
 

WyrTwister

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A/C and refrigeration work on temperatures and pressures .

First of all , check and replace as necessary , your filters . Is the indoor fan blowing well ?

Low on refrigerant is a common cause of the indoor coil freezing up . Not difficult to diagnose , once you get the gauges / hoses hooked up .
 

Fretman

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@WyrTwister , thanks. I just changed the filter recently and there's no blockage there. The indoor fan blows nicely. Low refrigerant is a good bet. The HVAC unit is about 13 years old so not sure if refrigerant needs replacing after all this time. From what I heard, refrigerant should never need to be topped up if everything is running properly.
 

Sparky617

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@WyrTwister , thanks. I just changed the filter recently and there's no blockage there. The indoor fan blows nicely. Low refrigerant is a good bet. The HVAC unit is about 13 years old so not sure if refrigerant needs replacing after all this time. From what I heard, refrigerant should never need to be topped up if everything is running properly.
As the unit ages leaks become inevitable. You don't replace the refrigerant you add to it. Eventually, the unit will need to be replaced. A slow leak a pound or two over the course of a year is manageable. At 13 years you can probably expect to replace this unit in the next 4-5 years, maybe sooner. I would not sink more than a few hundred a year into the unit. If the heat exchanger, coil, or compressor goes out I wouldn't replace the part, I'd replace the whole system. My units are 21 years old, I've kept them going longer than my neighbors, but I expect to replace them this fall or next spring. Both needed a couple hundred dollars in repairs this year and the visible rust is beginning to be worrisome.
 

WyrTwister

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Many / most A/C / Refrigeration systems leak . From a tiny amount that is not noticeable for years , to a major leak that necessitates topping off one or more times per year / season . A matter of degree .

The refrigerant does not " wear out " , it just , usually , leaks out .

Some leaks are next to impossible to locate , and then repair . Some are fairly easy . This often determines how a leak is addressed .

If the leak can be repaired / stopped , I would do so and have the refrigerant topped off . No point in replacing equipment that can be practically maintained and is still performing OK . Often , people replace equipment to acquire more energy efficent equipment . Some times this pays off , sometimes , not .

Best of luck to you , :)
God bless
Wyr
 

pjones

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There is a possibility that the compressor is over amping and turning itself off because the internal overload is protecting it from failure. Usually if the indoor coil freezes up on cooling mode and that causes the system to shut down then the outside condenser fan motor will also turn off. Because it is still running tells me that the compressor contractor is still pulled in but the compressor isn’t running, so off on internal overload. If the contractor overcharged the system or didn’t remove non condensibles from the system (if there are any that entered through the leak) then that could cause the compressor to run at a higher amperage and cause issues on hot days when it needs to work hard.

With a heat pump sometimes the outdoor fan will turn off while the compressor runs if in heat mode, but there is no time that the fan should run by itself unless they got creative with the wiring at some point.
 

Fretman

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@pjones , thanks for the input. The last few days, even though hot, I've been turning on the AC later during the day...mostly when the sun starts going down. I have not encountered the issue while doing this. But I will look into the overload comments you made.
 

Jeff Handy

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This might not be the ultimate fix for your problem, but you could try switching the fan setting to constant ON.
At least during very hot weather.

That might keep the coil from freezing, if a frozen coil is the reason it is shutting off.

Constant fan also equalizes the temp better all over the house.
And at night, it pulls cooler air from a basement to the upstairs, even when the compressor is off.
 

Fretman

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@Jeff Handy , interesting suggestion. I will try that the next time it gets really hot. But I believe the fan turns on each time the AC comes on?
 

Jeff Handy

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The constant fan would help unfreeze the coil, if it was approaching frozen, by warming it up with room temperature air, between times that the thermostat called for cooling and the compressor came on.
 

pjones

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For clarification, is it the indoor fan that is staying on or the outdoor fan that is staying on when the compressor turns off?
 
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