Home a/c systems – Why is it more efficient for it to run continuously a couple of hours rather than cycling on/off so often?

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For the sake of argument and discussion, let’s use a perfectly matched, balanced, and calibrated R-410a home a/c system as an example.

Why do folks say it’s more efficient for said home a/c system to run continuously for 2 hours and then off for 5 minutes instead of cycling on/off much more often to achieve the same interior cooling setting on the thermostat? (I understand how home a/c system work, but I’d like to know why some say longer run times = greater efficiency.)

I’ve heard this claim time and again, but I’d like to know the science behind it. I live in Texas where a/c units run for hours during the day from June through September.

In: Engineering

16 Answers

Anonymous 0 Comments

If you look on the data plate of your outside unit (the condensor) you’ll see a rating for LRA and FLA. Locked Rotor Amps (LRA) is the current draw that would occur if the rotor shaft were instantaneously held stationary within a running motor (a stuck compressor). For a couple seconds, this amperage is present as the compressor motor overcomes the static pressure in the system. LRA is typically five to seven times the motor full load amps (FLA) which is the nominal amperage when the unit is up and running at proper pressures. So if your unit has a 5 amp FLA, it will have as much as a 35 LRA. Although that LRA state is very brief, it’s sucking power like crazy for those first couple of seconds. Multiply that by 5 times an hour and you have a hot, tired, inefficient system.

Anonymous 0 Comments

Short cycles don’t have time to remove much humidity. The long cycle could remove a lot more water and allow you to raise the temp a couple of degrees at the same comfort level.

Anonymous 0 Comments

It takes more energy to get things moving, then it does to keep things moving.

There is also an energy cost to getting the refrigerant to ideal temperature, So if it cools down or heats up that process must be done again

Anonymous 0 Comments

Part of it is size, a more powerful unit that could cool faster would cost more money to build. Also, a larger unit would have larger coils and more refrigerant that’d have to cool before starting to cool the house. The first little bit after your AC starts, it’s just cooling the AC system down enough to really start working. Then you have to run the fan after the AC cuts off to extract all the cold from the already chilled AC system. Another part of it is humidity control. Longer cycle times tend to remove more humidity, making it feel cooler.

Anonymous 0 Comments

Thanks all. Very well explained.

Anonymous 0 Comments

This is why modern units with variable speed compressors are good. Instead of having one massive startup spike (that’s also why window and portable units will sometimes cause lights to dim when they kick on), it can run the compressor at a large range of speeds. So instead of turning on full tilt to cool and then shut completely off, it can run let’s say 25-50% constantly to maintain temp, and then dip to 10% power when there is less demand for cool. By having a variable speed compressor you are averaging out the power draw, running in a more efficient manner as far as cooling and dehumidifying, and you’re putting less stress on the electrical systems both in your unit and the grid.

Anonymous 0 Comments

It’s related to why 3 is usually the first speed on a fan.

It takes a lot of energy to get something like that spinning and running from a full stop, but less to just keep it going. but that initial startup takes a *lot* of power.

On top of this, full blasting your AC as hard as it can essentially go will overall be more efficient by making the house go *below* whatever setting you have set, giving you a bit of time to actually get above your set temperature and kicking back on much later. It’s much easier on the motor and compressor to just turn on once for an hour, get the house to 72 (if it’s set to like 75) then go off for a while, rather than trying to continuously turn off and on to keep it at a steady 75 exactly.

Anonymous 0 Comments

Ever notice the lights dim, then return to normal when you turn on a large appliance?

That’s because a large chunk of electricity being delivered to your house is being used to start the appliance. The same thing happens with your AC. It takes a lot of power to get started, and significantly less after it is already running.

Another reason to run constantly is the wear and tear on bearings. While a unit is running, motor shafts are actually rotating on a layer of oil. Every time it turns off, that shaft is resting on the bearing, and starting it causes them to rub together creating wear until the oil separates the shaft from the bearing.

Anonymous 0 Comments

The bigger temperature difference, the harder the machine has to work. So cooling slower for longer is easier on the machine

Anonymous 0 Comments

use dehumidifier function, not cold. We use AC on dehumidifier and 27 celsius and it feels perfect because it’s a dry heat. Huge advantage on electric bills and kids can stay on the floor in a nice temperature. If I put it on 23 cold, it would be an unhealty blanket of cold air down (small kids) and perfect for us at a higher level.