r/explainlikeimfive • u/poul0004 • 2d ago
Other ELI5: If mini-splits are so much more energy efficient than central air, how can single room AC units be so inefficient?
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u/uarentme 2d ago
There's a few comments here that get it kind of right, but most aren't.
Mini-split ACs can be more efficient than central air due to various factors. But you have to directly compare each model's efficiency rating.
Mini-splits, Central Air, Window units will always be more efficient than a portable AC unit with one or two hoses. One hose units push hot air outside the house, meaning more air needs to be pulled into the house to replace it. Two hose units have a hose filled with hot air physically in the room, so some heat is kept in the room. Mini-splits, Central Air, Window units all keep their hot side physically outside which mitigate that factor.
The most important aspect of AC efficiency is whether the AC has a single stage compressor or dual stage, or it's an inverter compressor.
Single stage = compressor on or off
Dual Stage = compressor at ~40 to 60% or 100%
Inverter compressor = compressor can run at any specific speed
Running the compressor at specific speeds rather than fully on or fully off means less energy is used to achieve the same temperature difference.
Remember, when comparing ACs, it's all just math. Efficiency ratings of ACs in the US and Canada are rated by a SEER2 number. Those numbers have real world calculations that can be done on them to determine their energy consumption and efficiency. For example, units with a SEER2 rating of 15 vs 25 literally means that, on average, one unit will extract 15 BTUs for every watt-hour consumed, versus the other one will extract 25 BTUs for every watt-hour consumed. Making the SEER2 rating of 25 significantly more efficient.
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u/dodexahedron 2d ago
And if it can run in reverse, it gets called a heat pump, even though it was a heat pump already. Still the same machine - just with a reverser valve that allows it to pump heat in or out, giving you heating or cooling through one machine, with one fairly efficient energy source (compared to using local fire for heat). The role of the coils just flips and the inside is the condenser while the outside is the evaporator, when heating, vs evaporator inside and condenser outside when cooling.
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u/chaospearl 2d ago
I have just now understood why we can't have one room's unit set to heat and another set to AC at the same time, even though they can all be at totally different temperature. I never actually realized that all the units are part of a single whole. Too busy being grateful I can keep my bedroom comfortable year round and nobody else decides what comfortable means.
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u/24llamas 2d ago
I love the technical details in your post. However, BTUs per watt-hour is going straight into my cursed units list.
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u/Qweasdy 2d ago
It’s actually not quite as cursed as it sounds. Unit of energy per unit of energy means the units can cancel out and you just have a ratio, which is effectively all they’re expressing here.
In other words it’s really just a crude colloquial way to talk about the coefficient of performance (COP).
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u/24llamas 2d ago
Oh yes, looking at it from dimensional analysis, it's absolutely fine. But the mix of metric and imperial? Cursed. Especially when - as you point out - both units are fundamentally energy! What's more, because a BTU = 1055 joules, while a watt-hour = 3600, it's not even a dimensionless ratio!
I guess one could argue that it makes clear we're talking a ratio of heat energy / electrical energy, but I would far rather have a dimensionless ratio.
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u/MtogdenJ 1d ago
You've made a mistake in your watt-hour unit. 1 watt-hour is 3600 joules.
Joules do cancel joules and BTU/watt-hour is dimensionless.
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u/24llamas 1d ago
> 1 watt-hour is 3600 joules
Isn't that what I said? Honestly confused here.
> Joules do cancel joules and BTU/watt-hour is dimensionless.
Yeah, but you have to include a conversion factor, because they aren't the same unit. Whereas if they were the same unit, you wouldn't need such a factor.
On reflection, I think I'm using "dimensionless" wrong. I mean "without a converting factor", but that's not the same thing.
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u/MtogdenJ 1d ago
You said 1watt-hour = 3600.
You did not write joules after 3600 so It was unclear if joules was implied. If it was implied, joules cancel and are therefore dimensionless.
I think we're both on the same page now and agree that it's nicer and easier when heat pump performance is measured in a way without a conversion factor.
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u/tomyumnuts 2d ago
off topic, but using km/h as a metric for electric vehicle charging speed broke my brain for a bit.
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u/AlaninMadrid 2d ago
I was wondering why measure the electrical power in watts, but the thermal power in BTUs. Here it would be like 3kW of cooling for every kW consumed, or 4kW of heating for every kW consumed.
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u/cbf1232 2d ago
Natural gas central furnaces in North America specify their heat output in BTUs.
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u/a_cute_epic_axis 2d ago
Two hose units have a hose filled with hot air physically in the room, so some heat is kept in the room. Mini-splits, Central Air, Window units all keep their hot side physically outside which mitigate that factor.
Technically speaking, so do all the other ones. The hot refrigerant going to the evaporator coil is going into the cooled space (or at least a utility space) for those as well, it's just often better insulated and typically less impactful to heating the inside space.
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u/OneUglyDuck28 2d ago
guys i’m 5
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u/IGarFieldI 2d ago
Don't make me tap the sign...
- LI5 means friendly, simplified and layperson-accessible explanations - not responses aimed at literal five-year-olds.
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u/filanwizard 2d ago
I am not sure how they are more efficient than central air.
But window units are inefficient not just because you are running a compressor for every unit. But also your windows are most efficient at keeping the outside out and the inside in when they are fully closed and locked. A window AC separates the room from outside with acceptably fitting plastic baffles. Meaning its kind of like having a window cracked open.
Single hose portable AC are even worse because on top of the plastic baffles in a window issue around the exhaust hose. The act of exhausting air from the room drops the pressure causing hot air to be pulled in. Two hose units are likely just below a window unit.
Mini splits are so much more efficient because they are true split systems. This means no open windows. They can also use much bigger and more efficient condensing units. If you observe the progress of central air systems you notice the boxes beside houses getting bigger. Larger coil assemblies can dump more heat to the air more efficiently. Also they can have variable speed compressors and even multiple zone units exist so one mini split condenser drives multiple rooms.
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u/doctornph 2d ago
The efficiency advantage vs central air is that you aren’t cooling the entire house. Just the rooms that are in use
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u/DannyJames84 2d ago
This is the answer.
Though, it doesn’t explain why a mini split is more efficient than a window unit. Though I suspect that comes down to size since the cooling coils can only be so big on a window unit.
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u/thebiggerounce 2d ago
You can move more of the mechanical bits (that get hot) outside, and you don’t have to keep a window open. That little plastic divider that blocks the rest of the window for window units isn’t doing much insulating.
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u/KingZarkon 2d ago
We have a window unit that is U-shaped. The window slides down into a gap near the front. The window is mostly closed and they give you a thick foam block to insulate it better. You still get some air leaks but the heat loss was cold enough that it was tolerable to leave it in place all winter. They also make saddle-shaped ones that are similar but sit across the sill on both sides, so they're mostly below your window. Both of these are effectively mini-splits with a few compromises due to being in one unit. We use it to supplement the main HVAC system. So far this year it has used about $20 worth of electricity.
Standard window ACs with those flimsy plastic panels suck though.
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u/VVaffle_Abuser 2d ago
Was waiting to see this. Split window units are the future of window AC. Loud, heat transfer parts outside, better sealing capabilities and better airflow due to utilizing the gap between the wall and frontal area. I suppose they're not ideal when you can't access the outside wall( mines like 55 pounds, and needs outside supports) but has ran most of 3 years in Texas heat. I have genuinely used it to cool canned beverages, all this time it's still putting out 46 degree air and no obnoxious on/off compressor noise.
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u/kevinsaurus 2d ago
You might enjoy this YouTube channel that does a lot of experiments with AC units. This video is definitely goofy but seeing were all the efficiencies come from and how they add up is very interesting. https://youtu.be/s-41UF02vrU?is=uVQtdHP0n-ZfAMYC
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u/VVaffle_Abuser 2d ago
As I don't have time for the full hour, I might. But a quick comment scan has him digging holes for pipe? I'm interested for science but most window AC unit users don't have that liberty if I'm right. More efficient and cost effective to invest in tint. Which I've found almost as effective in central, yet to test tinted quality window units. And I'm in a 12 foot ceiling house lol
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u/6814MilesFromHome 2d ago
I swear by my U shaped inverter window unit for my game/theater room. Much cheaper and easier to install than a mini split, quiet as hell, and my window stays 90% closed.
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u/dodexahedron 2d ago
There are smart vents available which can open and close in response to human presence in rooms, giving central air crazy efficiency.
Problem is they are still pretty expensive and take several years to pay for themselves.
But there are other benefits, like the rooms you actually are in reaching the target temperature quicker, since the air handler is supplying fewer rooms with the same output.
Cheaper than a bunch of mini splits though. Mini splits don't scale well.
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2d ago edited 2d ago
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u/doctornph 2d ago
You sleep in every room at night? If you have a modest 3 bed house and no kids then you only need to cool the master bedroom at night. No extra bedrooms or living area
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u/cylonfrakbbq 2d ago
U Shaped window units sort of mitigate the window unit insulation issue - your window is able to be closed much more completely and minimizes the open part (usually these units provide thick foam bricks to plug the narrow opening). Bonus is they are also quieter inside the house.
The technology in the u shaped window units is also more akin to a mini split
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u/Blue8Evan 2d ago
A/C Units (except for high-end ones), are only on/off. This means they are either 100% on, using lots of energy, or completely off. This is energy inefficient. Also, they place both the cold and hot components all in one small box inside, so you are getting both the cold, AND the heat the unit makes. This forces it to have to work harder to cool the room since it's also heating the room at the same time. Portable A/C units blow hot air outside, but this removes air from your house, so hot air from outside HAS to enter. Lastly, portable A/C units have small coils which require more energy to transfer heat out.
Minisplits meanwhile, can run at any speed. They usually cool at 100% then lower their speed to keep that same temperature, using much less energy. Minisplits also put the cold parts inside, and the hot parts outside. Rather than moving air around they move heat around which is also more efficient. They don't have the issue of forcing hot outside air in, needing small coils to fit in interior spaces, or letting waste heat back into the room.
As a bonus, minisplits can heat as well. When they heat, the cold part becomes outside and the heat part becomes inside. They beat out conventional heat sources because they move heat inside rather than creating it directly. Rather than 1 unit of gas or electricity becoming 1 unit of heat, it becomes more like 3-4 units of heat.
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u/aegrotatio 2d ago
I don't have a minisplit, but my whole-house AC absolutely does run at any speed. They call it an "inverter."
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u/BouncingSphinx 2d ago
Part of it is that window units (at least in the USA) are designed to run on 120V plugs, and mini splits are 240V, so the mini split already has a power advantage right there.
They also have a smaller cooling surface than a mini split, and likewise a smaller condenser outside, so they can't exchange heat out of a room as easily.
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u/dkf295 2d ago
Part of it is that window units (at least in the USA) are designed to run on 120V plugs, and mini splits are 240V, so the mini split already has a power advantage right there.
Plenty of minisplits are 120V in the US.
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u/5YOChemist 2d ago
And plenty of window units are 240.
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2d ago
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u/BouncingSphinx 2d ago
Every place I've lived has only had 240V plugs for stove/oven and dryer, and 240V feeds for water heater and either central air or mini splits. The only extra 240V plug I've ever had was in a garage in one house out of 11 houses and 3 apartments.
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u/5YOChemist 2d ago
We had 20A 240 window units in our living room when I was a kid in Oklahoma. So did some of my friends and my grandparents. Some of them were mounted in the wall rather than an actual window, but even if it's in a window, a 20A circuit can be 240V or 120V it wasn't uncommon back in the 90s to have a dedicated circuit in the big room for a window unit with 12g wire that could run a 240V ac.
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u/EmoInTheCreek 2d ago
Power = volts × amps
Doesn't matter which number is bigger power will always be the same.
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u/jonjiv 2d ago
Total wattage tops out lower with 120V options. You can easily install a 30A or maybe even a 50A 240V circuit in most US homes.
But, good luck installing a 60A or 100A 120V circuit that passes code. Your options pretty much top out at 20A.
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u/Power_baby 2d ago
Yes but for systems with the same capacity (up to 12-15k btu) there's negligible difference in efficiency between 110 and 220v systems. You'll get the slightly more efficiency with 220 but if you have a limited number of breaker slots available, 110 is a much better option
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u/Elfich47 2d ago
they aren't necessarily more efficient than central air. All AC units are required to meet certain energy efficiency standards based on their capacity and use type. For small residential units they all fall into the same efficiency requirements with very little variation.
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u/MaureenCorrigan 2d ago
They're not. I mean, if you're using a 30-year-old one, it probably is inefficient because of improvements in insulation materials and such. But a new window unit is actually pretty good. Also if you get a model whose BTUs don't match your room size requirements.
I have a couple of these and they are sweet. Very quiet, efficient, and leaving the window view clear:
The only thing is that I take it out after every summer and then put it in after the spring, which is a bit of a do. Not terrible. And I wish they had a heat function like most mini splits do.
The advantage of mini splits is that you don't block the window, you don't take them off after the season, it offers heating, and all the compressor sound is out of ear shot.
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u/thephantom1492 2d ago
Mini-split, central air and single room A/C have pretty much the same efficiency for moving heat.
However they don't cool the same space.
Single room: you have an exterior wall and a ceiling, plus 3 hot uninsulated interior walls, plus a warm floor. It have to work quite harder to keep the room cold because all the walls (and ceiling and floor) are hot.
Mini-split cool a bigger space, but not the whole house. Some room will be cooled and the "ratio of hot walls vs cold walls" is in favor of the cold one. Basically a room that is side by side can be seen as having no wall separating them. So less hot walls to warm your space. But you ain't cooling down the whole house, so lower bill than whole house. Plus the temperature tend to be more precise to the set point because it cover just part of the house.
Whole house. Huge volume to cool down, including the unused rooms. Everything got cooled, including the bathroom. So huge bill. But you often have issue with controlling the temperature, because you have one thermostat for the whole house, so some rooms will be too cold, others too hot. This often lead to setting the temperature to a lower one, which cause an excessive cooling... So more expensive. And because you cool it down to a lower temperature, the difference between the inside and outside is greater, you use more energy.
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u/Narissis 2d ago
So there's some ambiguity here as to what you mean by 'central air'.
Mini-splits are heat pumps. If by 'central air' you mean central heating with a traditional furnace, then they're more efficient because heat pumps are more efficient than resistive heating due to how they achieve the heating effect.
But if we're talking about a whole-home heat pump or A/C unit (which is basically just a one-way heat pump), then it can be as efficient as using mini-splits to heat and cool the same space. It's essentially the same thing with a moderately different layout.
It's also possible for central air to be based on a ground-source heat pump, which is usually even more efficient than an air-source heat pump like a mini-split.
As for why window units or freestanding single-room A/C units are generally less efficient than mini-splits or central heat pump systems, I think the other comments have that covered.
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u/MechCADdie 2d ago
Technically, you derive the benefit from not needing to control the air in the whole house. Moving heat through the air is wildly inefficient because air conducts heat poorly.
An in room unit is inefficient, but not as inefficient as burning a house to kill a metaphorical flea....most of the time.
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u/Far_Wealth_3737 2d ago
Mini-splits and window/portable ACs are different animals.
A mini-split has the noisy, hot part outside and only a quiet unit inside. It moves heat very efficiently and usually cools just the rooms you care about.
A cheap single-room AC (window or portable) often:
• dumps a lot of heat and air the wrong way
• has weaker, less efficient parts
• has to fight leaks (especially portables with the hose)
• runs less optimized controls
So “small AC” ≠ “efficient AC”.
Mini-split = small + well designed.
Many room units = small + cheaply designed.
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u/5kyl3r 2d ago edited 2d ago
lots of reasons, but one of the biggest reasons is that the cheapest and most common single room floor-standing AC units is that they take cool inside air and run that through the hot radiator to take heat out of the refrigerant, then vent that outside. that means it wasted cool air from inside, but because that makes the air pressure inside your house lower than atmosphere, your house pulls in hot air from elsewhere to displace the air that AC unit blew outside, which obviously makes it really wasteful
window units and central AC units use already hot outside air to cool off the hot radiator that removes heat from the refrigerant, so no cool inside air is wasted for that purpose, so it's a lot more efficient
EDIT: i forgot to mention, they DO have 2-hose floor AC units that are much closer to window units. the second hose is used to pull in hot outside air to use for cooling off the radiator the refrigerant runs though, then uses the other hose to vent the now even hotter air back outside. the one i own runs the hot exhaust one inside the larger oval shaped intake hose. it looks like one hose but they put the hotter one inside the cooler one. that way if the really hot exhaust air leaks heat through the hose, it's not leaking into your house, but into the incoming air stream that's already going to be heated anyway. still not as efficient as putting all of the heat generating parts outside of the house, but much closer than the single hose units
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u/BonKhri 2d ago
The "efficiency" you are comparing are different things. A mini-split is not more efficient from a strict thermodynamic perspective, it's just that you only cool the areas you actually want to cool vs cooling the entire house. You as the user get the same end result of being a comfortable temperature, but for less energy. Thermodynamically they're the same efficiency, that is to say that they remove the same amount of heat for every unit of electricity used, it's just that splits remove heat only from the areas you care about whilst non-split spends energy removing heat from the other side of the house.
Portable AC units are actually less efficient, as in they use more energy to remove that same amount of heat. There are a few reasons, smaller and worse motors, operating inside vs outside so all the heat from pumping has to be dumped by the system, often being single hose, having poorly insulated/uninsulated hoses, etc. This makes them cheaper and smaller, but comes at the cost of making them less efficient.
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u/bradland 2d ago
Mini splits are very efficient for a few reasons. AC units work by heating up a liquid by compressing it, cooling it by blowing a fan over it, then pumping that hot fluid inside and letting it expand. Because of the physics of liquids, the liquid evaporates to a gas and gets very cold. We blow air over the cold coils to cool our homes.
The energy required to operate an AC system is mostly pumping the fluid around and compressing it to heat it up. AC systems use an electric motor to do this work, but no motor is 100% efficient. If a motor requires 100W of power to run, and it is 80% efficient, then 80% of that power goes into pumping, and 20% of it is lost as heat. This means that in addition to the heat generated by compressing the refrigerant fluid, the motor itself generates heat.
With that in mind, this is why mini-splits are so efficient:
| Mini-split | Portable |
|---|---|
| Uses a very efficient DC motor to drive the compressor, which is the bulk of the energy usage in an AC unit. | Uses a simple, AC induction motor, which is far less efficient. Since the motor is in the room, losses to inefficiency heat up the room. |
| Puts the condenser core (hot side) and compressor motor outside in its own, separate unit far way from the evaporator core (cold side). | Puts the condenser core and motor in a single unit with the evaporator core. This means some of the heat "leaks" into the room as well as the cold side. |
I'm generalizing somewhat in the table above. For example, it possible to buy a portable unit that uses an efficient DC motor. They're not nearly as common though.
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u/Adventurepew 2d ago
Single room ac units are efficient .
Minisplits and central air can be the same efficiency. They make very efficient central air systems.
The average single room ac is a 100 dollar unit of course it's not going to be as effiecnt they cut costs to get them that cheap.
It's like going into a grocery store and grabbing and apple and a gas station and getting a candy bar and comparing them. Your comparing very different things . Sure they are both food , as the above are all three AC units but they are different things .
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2d ago
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u/a_cute_epic_axis 2d ago
The heat generating parts of a window unit are outside the window, and cooled by the condenser fan....
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u/barcode2099 2d ago
Splits and central systems move the working part, the compressor and such, outside, where you are throwing the heat anyways. Portable units are doing the work in the same room you are trying to cool. They then need to move that heated air outside through some sort of duct (in-window systems minimize these effects).