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I'm not suggesting doing this or whatever, I just want to know the answer. Let's say it is 80 deg F outside and I put the AC on 50 deg F. Obviously this will cause the AC to run all day, would this running all day cause the motor to burn or would the AC be fine? Would the Compressor lines freeze? Again this is a general, model agnostic, question. You can assume it's a good functioning unit with no obvious problems.

  • The only likely problem is the evaporator coil inside your furnace/air handler could ice up. This sometimes happens when humidity is high and the unit runs continuously for many hours, as the evaporator coil has no opportunity to defrost. And it's most likely to happen during the first 6 hours or so of unit operation. For this reason it's best to slowly reduce the thermostat temp rather than dialing it down to 50 from the start. Give it a setting where it will turn off occasionally. – Hot Licks May 23 at 13:22
  • A properly operating system will not ice up, modern high efficiency units use thermal expansion valves this valve regulates the temperature at the evaporator preventing icing in most cases , units with an improper charge or running outside design criteria (improper operation / properly functions unit will not freeze) if we change to old school simple orifice when properly set up the compressor will cycle based on pressure and again it will not freeze. A system that runs for longer times may remove more moisture it just depends on the size of the evaporator and temp it runs at. – Ed Beal May 23 at 15:08
  • Assumed it's a good functioning unit; there's no question here other than if continually running motors and compressors is better than stop-start cycling. Which is usage dependent, and under the constraints of this question we have no second variable to test. "2 stage equipment is for comfort, not economy." – New Central Air Install: choosing single-stage vs dual-stage condensers. – Mazura May 23 at 19:01
  • Same with cutting your gas valve back which "will cost you more in electricity, as it'll run longer to come up to temp, but it will bring you a more even comfort level." – How to Lower the Ouptut of Oversized Furnace .... The word is comfort. If you're not comfortable, you might as well just open the windows. – Mazura May 23 at 19:01
  • It does beg the question of if running a unit for 20y straight is better than not running it at all. But IME, no unit lasts much more than 20y, so I'd rather be the guy who's comfortable for those 20y. - We'd have to get into things like dry-rot, [w/e the word is for, 'sitting too long'], proximity to the ocean, etc. – Mazura May 23 at 19:16
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With a properly functioning system no the motor will not burn up. And the coils and line set may show some frost but would not freeze up.

If you leave the system running and there is a drastic temp drop things might ice up a little but the throttle valve will start restricting the flow of refrigerant so the compressor may start cycling , systems usually have a minimum off time so the cycling will not over heat the motor. For systems with simple orifices the pressure switch turns off the compressor until time to start again. Most all residential compressors have internal thermal overloads that stop the motor from operating if they get two hot. This is yet another safety so no a properly operating hvac system will not burn up if left on all the time.

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Compressor motors are self-cooling and designed to work for extended periods. Obviously more runtime means more wear, but there's no point at which it'll typically overheat or suffer other damage.

Refrigerant lines shouldn't freeze. In theory enough heat is taken out in the exchanger that it never gets out of operating range. Refrigerant stays liquid far below where it should ever be in use--around -150°F.

What can happen, though, is ice buildup. If you actually cool the building to such low temps, relative humidity rises dramatically. This leads to condensation and ice. Eventually the heat exchanger becomes completely clogged and the system breaks down. Then it runs without actually doing much, just wasting energy. If the weather is warm outside you'd be hard-pressed to get that to happen, though.

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    The liquid state is based on the pressure and temp. Only liquid helium 2 stage systems get -150 that I know of . Most refrigerants are compressed to 150-400 psi and cooled to room temp on the condenser and are in a liquid state when cooled. Then the pressure is released in the evaporator creating the cold (over simplified terms for op). – Ed Beal May 22 at 19:49
  • The refrigerant lines definitely shouldn't freeze, but the evaporator coil (inside the building) could become covered in ice if the system is low on coolant. This can block the airflow and cause it to not remove any more heat or humidity from the building until it's shut off & defrosted. – Eric Simpson May 24 at 15:42
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It shouldn't. When our previous house was for sale an incompetent house inspector set the thermostat to about 55 F. We were not home and it ran about 36 hours. The outside temp was only in the low 70's so the thermostat had to be turned low to run . The house was cold when we got home but otherwise no problem. The incompetent part was about a water heater TP valve.

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This often occurs when an a/c system is undersized. It will run continuously and never cycle off because the system does not have the cooling capacity to reach a reasonable set point like 75 F. This does not cause any problems except I think it can cause excessively dry air inside.

We have a 3.5 ton unit in a 2000 sq ft poorly insulated house. On a very hot day with the set point at 75 F, the unit will come on at 11 am and not cycle off till 4 pm or later. The unit has been in service for 29 years (orig compressor and air handler motor).

EDIT

The explanation of excessive drying of indoor air with an undersized a/c is that when the evaporator is cycled on it still removes nearly all the moisture from the air passing over it. The claim is that an evaporator with more cooling capacity does not remove significantly more moisture than one with less cooling capacity while it is cycled on. And since a properly sized evaporator will be cycled off a lot of the time, then the relative humidity in the house will be higher with a properly sized evaporator. Some fancier units can control both temp and humidity. I think they do this by regulating the cooling capacity of the evaporator which affects the relative duty cycle.

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  • Isherwood says the opposite about humidity. You can't both be right. – Tim May 23 at 12:32
  • I could well have this wrong. "I'm not the man I was . . .", Julien Baptiste, – Jim Stewart May 23 at 12:54
  • I agree with Jim. An undersized unit will run continuously and not be able to drop the temperature below a certain point. However, most of the time the building will be leaky enough that the unit can't remove all the humidity either. – Eric Simpson May 24 at 15:38
  • An oversized unit will quickly cool the temperature and shut off (short cycle) but not remove enough humidity to feel comfortable. The right size will drop both temp and humidity, then shut off, and be able to maintain both by cycling periodically. – Eric Simpson May 24 at 16:03

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