# Please explain this thing with my central air conditioning
**URL:** https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097
**Category:** In My Humble Opinion
**Created:** [October 22, 2014, 2:17am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097 "2014-10-22T02:17:26Z")
**Posts on this page:** 14
**Page:** 1
### Author:  [@ThelmaLou](https://boards.straightdope.com/u/ThelmaLou)
#### Post date: [October 22, 2014, 2:17am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/1 "2014-10-22T02:17:26Z")
The central a/c & heat system is in two parts. One part is in the attic; that’s the part where the filter goes. My landlord changes it. I was a homeowner for 20 years, but now someone else does a lot of stuff I used to have to care about. Yay!
The other part is outside and sits on the ground. There’s a fan in it. I’m not sure in which part the cooling/heating actually takes place.
The thermostat control in the house consists of a little screen. It’s probably programmable, but my work schedule is so erratic that it’s just easier for me to manually adjust it when I need to. Mostly in the summer I leave it around 76 all the time.
I have the system set to “off,” and the fan set to “auto.” The fan only has three settings: on, auto, circ (which I’ve learned, means the fan comes on periodically to circulate the site when the heat and a/c are both off).
Here’s my question: With everything off the thing outside on the ground still runs rather a lot. Why? The thing in the attic, which I believe is the fan, does not run when the system is set to “off.”
What is the function of the unit outside on the ground and why does it run when the system is off? Is that normal, or a sign that something is wrong?
At my former house, the outside unit was not where I could really hear it, so for all I know, it ran a lot, too. I dunno. Yes, I’m going to ask my landlord about it, but I like to ask stuff here, too. Thanks.
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### Author:  [@friedo](https://boards.straightdope.com/u/friedo)
#### Post date: [October 22, 2014, 2:45am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/2 "2014-10-22T02:45:17Z")
The thing outside is the compressor. Its job is to compress the refrigerant gas in the system, which makes it hot. The hot refrigerant gas then radiates its heat to the outside air, which is why the compressor has a big fan in it. The high-pressure gas is then let through a pressure-reducing valve whereupon it goes into your inside unit. Now under low pressure, the gas is cold and absorbs heat from the inside unit, which blows your house’s air over the cold refrigerant. This cycle happens continuously when the unit is running.
I don’t know why the compressor would be running when the AC is turned off. It sounds like something may be malfunctioning.
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### Author:  [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)
#### Post date: [October 22, 2014, 3:00am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/3 "2014-10-22T03:00:21Z")
And if your compressor IS running without the fan going on, you’ll probably see ice forming on the copper tube that runs to the fan.
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### Author:  [@burpo\_the\_wonder\_mutt](https://boards.straightdope.com/u/burpo_the_wonder_mutt)
#### Post date: [October 22, 2014, 3:07am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/4 "2014-10-22T03:07:34Z")
When I had something similar, it was a relay sticking (malfunction); when it stuck for good, the compressor never shut off (unless I pulled the “block” out of the power box), and everything outside frosted over. IANAHVAC.
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### Author:  [@ThelmaLou](https://boards.straightdope.com/u/ThelmaLou)
#### Post date: [October 22, 2014, 3:26am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/5 "2014-10-22T03:26:06Z")
Thanks. I believe something is amiss.
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### Author:  [@ThelmaLou](https://boards.straightdope.com/u/ThelmaLou)
#### Post date: [October 22, 2014, 11:30pm UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/6 "2014-10-22T23:30:18Z")
My landlord just told me that this system is a heat pump, not a regular compressor. Does that change any of the answers above?
He said he would get someone to look at it.
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### Author:  [@friedo](https://boards.straightdope.com/u/friedo)
#### Post date: [October 23, 2014, 2:10am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/7 "2014-10-23T02:10:27Z")
It depends on what the heat pump is doing, then. It may be configured to make hot water, in which case it would run when it needs to (basically doing the exact opposite of the refrigeration cycle described above.)
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### Author:  [@ThelmaLou](https://boards.straightdope.com/u/ThelmaLou)
#### Post date: [October 23, 2014, 3:09am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/8 "2014-10-23T03:09:32Z")
The a/c guy is here now and he’s harvesting great chunks of ice from the fan in the attic. The compressor outside was completely frozen, too. My landlord walked over-- he lives in the next block. A good time is being had by all. Well, me anyway.
I love being a renter.
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### Author:  [@friedo](https://boards.straightdope.com/u/friedo)
#### Post date: [October 23, 2014, 1:34pm UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/9 "2014-10-23T13:34:09Z")
You might not love it as much when the electricity bill comes. 😃
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### Author:  [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)
#### Post date: [October 23, 2014, 3:41pm UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/10 "2014-10-23T15:41:38Z")
> [@ThelmaLou](#):
>
> I love being a renter.
Darn you tenants!
I borrowed two grand to replace my tenant’s A/C and furnace three years ago. Now I have to come up with two for my A/C.
🙂
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### Author:  [@Cubsfan](https://boards.straightdope.com/u/Cubsfan)
#### Post date: [October 23, 2014, 3:44pm UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/11 "2014-10-23T15:44:15Z")
Sometimes you just have to flagellate it.
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### Author:  [@Razncain](https://boards.straightdope.com/u/Razncain)
#### Post date: [October 24, 2014, 1:09am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/12 "2014-10-24T01:09:40Z")
There are timed delays on most units, but they should eventually both shut off within five minutes or so. Being low on Freon will definitely keep the unit running. If you can reach up and feel the air coming out of the vent, can you tell if it is blowing cold enough? To be sure, you can reach in put a thermometer in the vent and get a reading.
BTW, even though your compressor is outside, putting the evaporator/blower in the attic is the worst location to put it, you’ll continue to pay dearly in electric bills until it gets moved to the inside of the house in a closet or some other out of the way area.
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### Author:  [@ThelmaLou](https://boards.straightdope.com/u/ThelmaLou)
#### Post date: [October 24, 2014, 1:50am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/13 "2014-10-24T01:50:48Z")
> [@carnivorousplant](#):
>
> Darn you tenants!
> I borrowed two grand to replace my tenant’s A/C and furnace three years ago. Now I have to come up with two for my A/C.
> 🙂
I hear ya. I’m also the landlord of a rental property I own.
> [@Cubsfan](#):
>
> Sometimes you just have to flagellate it.
Maybe later…
> [@razncain](#):
>
> There are timed delays on most units, but they should eventually both shut off within five minutes or so. Being low on Freon will definitely keep the unit running. If you can reach up and feel the air coming out of the vent, can you tell if it is blowing cold enough? To be sure, you can reach in put a thermometer in the vent and get a reading.
>
> BTW, even though your compressor is outside, putting the evaporator/blower in the attic is the worst location to put it, you’ll continue to pay dearly in electric bills until it gets moved to the inside of the house in a closet or some other out of the way area.
Not. My. House. Anyway, I’m fine with the bills.
The guy replaced some electronic doo-dad, and now it’s working like a champ.
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### Author:  [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)
#### Post date: [October 24, 2014, 1:58am UTC](https://boards.straightdope.com/t/please-explain-this-thing-with-my-central-air-conditioning/702097/14 "2014-10-24T01:58:58Z")
> [@ThelmaLou](#):
>
> I hear ya. I’m also the landlord of a rental property I own.
I didn’t raise his rent, for he is the best tenant I’ve had. 🙂