What happens if an AC gets overworked?

Suppose that an AC unit is put in an environment where it will always be hot (i.e., in the middle of July and someone left the house door wide open for the entire 3-month summer.) What effect will that likely have on it? (say, the central HVAC unit in the walls)
Or, conversely, if someone leaves the front door wide open throughout all of winter and the HVAC has to heat 24/7?

Your electric bill will be immense.
A/C’s are designed to run with 100% duty cycle. In the summer out here, ours will run for hours in the middle of the day.

A good piece of equipment will work 24-7

Most consumer equipment is designed for a specific duty cycle (hours/per day) and environment. Parts may simply fail by wearing out. Parts may overheat, because they were designed for specific cooling environment. A motor that overheats may lose all its lubricant. Worst case, the insulation on the wires catches fire. If it’s a heating unit, the enclosure may reach the temperature where it starts to burn surrounding material.

Depends exactly what is happening, what the unit is. For example, a heater unit sitting in a cold environment should be just fine - it just won’t do its job since the room to be heated loses heat faster that it can replace it. There is a risk that the wiring will catch fire - not sure what electrical design standards are for that stuff. A fan motor may overheat or bearings wear out. An AC unit in a hot environment - well, the outdoor temperature is what they were designed to run in. Whether they can run continuously or whether the heat-shedding fins on the condenser were designed to run continuous - depends on design.

So short answer - it depends.

Right, hours no doubt, but can they really handle running 2000 or 3000 hours in a row nonstop?

3000 hours is about 8 hours a day for one year. That’s 2-3 years worth of usage for our heatpump, and it typically runs for 3 years between failures. And, the last failure was the contactor, which would last many times longer if the unit wasn’t cycled.

Depending on the specific type of system, using an air conditioner at a temperature higher than the air temperature outside can actually destroy the system, if the temperatures outside are low enough (I don’t remember what the specifics are on my system are but do know its in the 40s-50s approximately). I learned this the hard way by blasting my AC at night during the fall a few years ago. Since i didnt know any better, I had no idea what i had been doing until my AC stops working and i ultimately need to spend mucho dollars to replace it.

A/C running for days/years nonstop w/ compressor engaged, in a proper environment that should get the longest number of operational hours out of it. It is the ideal case for longevity (in terms of total hours running, not total life in years)

You can cause problems if the outside coils are not allowed to cool, like placing the A/C in a fireplace (I wasn’t that person). It eventually tripped the breaker. A/C properly installed in the window ran great without any signs of damage.

Do you mean running it in a situation where the indoor temperature is lower than the desired target temperature?

This. Running hours is one measure of life, and start/stop cycles is another. There are five main failure modes I can think of for a conventional air conditioner:

  1. evaporator fan seizes up
  2. evaporator coil develops cracks, loses refrigerant
  3. condenser fan seizes up
  4. condenser coil develops cracks, loses refrigerant
  5. compressor fails (seizure, loss of compression)

(this ignores all the niggling little things that can happen, like loose wires or factory defects)

#1, 3, and 5 pretty much are dependent on total operating hours. #2 and #5 are far more dependent on the # of start/stop cycles; every time you start/stop the unit, it has to develop the thermal gradient across the evaporator/condenser, and this causes thermal stresses. Those stresses are no big deal if applied once, but repeatedly applying/relieving them subjects a part to fatigue failure; tiny cracks will develop over time, and eventually the refrigerant inside will start to leak out. So if you start your AC once and leave it running, fatigue failure is no longer an issue: you’re left with failure modes #1, 3, and 5, which are really only dependent on operating hours.

I once read about a similar issue with gas turbine engines. When used for ground-based power generation, you can run them for a very long time between servicing, since they tend to run at approximately constant power output for days or weeks on end. When used for pushing airplanes through the air, gas turbine engines need servicing far more frequently because they are shut down and started every few hours, and operated at a wide range of power settings, which puts the parts through far more thermal cycles per hour of operation than ground-based gas-turbine engines.

In my experience the most fail-prone part is the capacitor, which would get zero workout in your scenario. I think you would get (many) more hours of operation out of an AC running constantly than out of a unit in normal (start-stop-start) operation. That is operation hours, not calendar days.

Interesting. I guess this is like appliances like the fridge and freezer where they’re better always on than sometimes on/off. Guess fluorescent lights are the same way, like the ones on 24/7 in Walmart?

One thing worth mentioning is most AC units are either on or off. If you come home and it’s 80 degrees and you set the the thermostat way down to 60 degrees, it will not cool any faster than it would if you set it at 70. It will just run longer to reach 60 degrees.

That is per design. Earlier capacitors used PCB based oil which gave them great life at the cost of the environment. Nowadays a lot of substitute vegetable oils such as castor oil, etc. are used which are great for the environment but the life of the capacitor is significantly reduced.

One potential issue is that a relatively non-sophisticated A/C has the potential for the evaporator coil to freeze over. I’ve seen this both on window units and whole-house central units. Once that happens, I don’t know if it can un-freeze itself without being shut down. That has the potential to damage the compressor because of liquid slugging or perhaps oil separation or who knows what else.

All those things. Also consider that small frozen droplets could choke the expansion valve (i’ve Seen that happen)

I have to disagree with you here. I spent many years at the General Electric factory that produced capacitors. There was no detriment to capacitor performance or life when we switched from Pyranol to the non-PCB based oil, Dielektrol. The thing that made Pyranol such a good dielectric was that it, unlike the replacement oil, was not flammable.

(Too bad that more calendar days is the actual goal…)

If there is an improper charge or air flow.

Any electrical motor will have a service life, and the main factors are number of starts, stops, and run time.
If the evaporator coil is continusally exposed to a high temp and does not bring the temp of the coil with in range for a long period of time the motor may over heat and this will effect the running life. On any sealed unit the motor is designed to be cooled by the refrigerant.

If the outside coils are too hot, the refrigerant will not condense as expected. What effect will this have on an AC unit?

If the inside coils are too cold, the refrigerant will not evaporate as expected. Does this ever happen in real life?