# Central air vs mini split



## supers05 (May 23, 2015)

So that's not how you calculate power. It's 12A for the outdoor unit, period. You didn't mention the voltage for the indoor unit. (I guessed 120v) That puts your unit running at 19 instantaneous EER, which is impressive, assuming that it was actually moving it's rated capacity. (they usually don't.) You also didn't mention outdoor/indoor temps. Indoor humidity. 

For the mini splits, they get their efficiency running between 50-80% capacity. The wattage rating is for the worst case scenario, during a real hot day. They typically only manage 10ish EER during this short period. The rest of the time they do much better. (more then double that)

I'm curious why you'd want to replace a perfectly working unit. 

Cheers!


----------



## Bob Sanders (Nov 10, 2013)

You for got TIME in your efficiency calculations and that's where minsplits (claim to) beat out central systems.

Somewhat more difficult to calculate but the general claim with the mini splits is that you don't need to run them for as long as a central air unit. Because you're not heating/cooling a bunch of duct work and because you are only heating/cooling individual areas with mini splits (as opposed to the whole house) then the mini's aren't running for as long.


----------



## supers05 (May 23, 2015)

Bob Sanders said:


> You for got TIME in your efficiency calculations and that's where minsplits (claim to) beat out central systems.
> 
> Somewhat more difficult to calculate but the general claim with the mini splits is that you don't need to run them for as long as a central air unit. Because you're not heating/cooling a bunch of duct work and because you are only heating/cooling individual areas with mini splits (as opposed to the whole house) then the mini's aren't running for as long.


SEER calculations don't really take ductwork into consideration, just the minimal amount that the mfg demands. 

If sized right, minisplit units should almost never shut off. They will modulate. This is where they get their efficiency. At full capacity, the coils are undersized. However, since the amount of time that those units are pushed right to the limits, only account for a few percent of the SEER calculations, it doesn't show up. 

A/C units also don't remove a whole lot of heat for the first few minutes of every cycle, while consuming quite a bit of power at start up. Continuously running mitigates this problem. 

For example, a conventional York split unit running in second stage still be 14EER or less for the majority of the time. On first stage which is 90% capacity, they are +20EER. Mini-splits use the same principle. 

Cheers!


----------



## Justin_99 (Jan 23, 2017)

Mini splits can cool just a little, a little more, half cooling, and all the way to full cooling and full power consumption. So not going balls-out if just a little cooling is needed.

Also they manufacture their own electricity which runs the motors. This makes them very efficient. Called "inverter technology".

And because there is an inside unit in each room of the house, you can choose to only heat/cool the rooms of the house you are using. Say you have 2 spare bedrooms you don't use. You can leave the A/C off in those rooms and save even more on the energy bill.


----------



## Bob Sanders (Nov 10, 2013)

Justin_99 said:


> Also they manufacture their own electricity which runs the motors. This makes them very efficient. Called "inverter technology".


Huh??
Inverter technology is about speed control.

Incoming AC is converted to DC and then modulated up or down for fine speed control. It doesn't "manufacture" electricity.


----------



## Justin_99 (Jan 23, 2017)

Bob Sanders said:


> Huh??
> Inverter technology is about speed control.
> 
> Incoming AC is converted to DC and then modulated up or down for fine speed control. It doesn't "manufacture" electricity.


I am getting amused with people challenging what other's say lately... And not doing ANY research first...

You might want to look up the words "electrical inverter" and learn what that is.

Additionally you may want to learn about 3 phase electricity and how electric motors run more efficiently when using that type of electricity (as opposed to single phase).


----------



## Windows on Wash (Aug 30, 2011)

Cat fight before @beenthere steps in...


----------



## Bob Sanders (Nov 10, 2013)

Justin_99 said:


> I am getting amused with people challenging what other's say lately... And not doing ANY research first...
> 
> You might want to look up the words "electrical inverter" and learn what that is.


Ummm...
I'm not trying to be difficult or challenging.
You simply have it wrong, and I am correcting you. I know what an inverter is since I have a fairly substantial electronic back ground.

Inverter technology does not "manufacture" electricity! The closest thing you will get to "manufacturing" electricity is an alternator or generator... and even with that you are not "manufacturing" electricity. You are simply converting motive energy into electrical energy. 

An inverter is simply another converter of sorts and it converts between ac and dc. In A/C inverter technology Direct Current is used to control motor speed because it controls to a better degree than AC. It has a higher current ability at lower voltages than ac does.

Here it is if you don't believe me:



> *What is the Inverter technology in air conditioners?*
> 
> The Inverter technology (DC) is the latest evolution of technology concerning the electro motors of the compressors. An Inverter is used to control the speed of the compressor motor, so as to continuously regulate the temperature. The DC Inverter units have a variable-frequency drive that comprises an adjustable electrical inverter to control the speed of the electromotor, which means the compressor and the cooling / heating output. The drive *converts* the incoming AC current to DC and then through a modulation in an electrical inverter produces current of desired frequency. A microcontroller can sample each ambient air temperature and adjust accordingly the speed of the compressor. The inverter air conditioning units have increased efficiency in contraction to traditional air conditioners, extended life of their parts and the sharp fluctuations in the load are eliminated. This makes the inverter AC units quieter, with lower operating cost and with less broke downs. The inverter AC units might be more expensive than the constant speed air conditioners, but this is balanced by lower energy bills. The payback time is approximately two years depending on the usage.


https://www.inventorairconditioner.com/blog/faq/what-is-the-inverter-technology-in-air-conditioners

Absolutely NOWHERE in that description does it say inverters "manufacture" electricity. Inverters convert ac to dc and the current is then stepped up or down to control speed.

Please check your "facts" before jumping down someones throat.


----------



## Bob Sanders (Nov 10, 2013)

Justin_99 said:


> Additionally you may want to learn about 3 phase electricity and how electric motors run more efficiently when using that type of electricity (as opposed to single phase).


I deal mostly with 3 phase power on commercial boilers (blower motors 3 HP to around 100 hp) just about every day of the week. Although 3 phase can be found in many residential situations... it's not the norm. 120v/240v single phase is the norm.

3 phase is great for power. It however is not great for speed control or even high torque at lower voltages because it has the same problem as single phase 120/240.... and that is that each phase goes to 0 volts and 0 amps 60 times a second. No torque or horsepower is being supplied at those moments in time.... although 3 phase is better because the phases are out a bit relative to one another so when one is at 0V,0A the other are producing. Still, it is not a constant current, like direct current.


----------



## user_12345a (Nov 23, 2014)

I sure wish appliances manufactured their own electricity - would solve a lot of our problems. there's no free lunch and you're using the wrong terminology.

inverter/frequency drives found in mini-splits and ecm motors don't actually put out constant DC power but it does only flow in one direction, the windings are pulsed on and off. 

There are many ways to do speed control but inverter technology with DC phases being pulsed on is best.

AC frequency drive, where the controller outputs AC at the same voltage but varies the frequency is second best.

The worst is to have a longer motor winding with different taps or even a resistive circuit doing the control - makes motors very inefficient at low speeds.


----------



## supers05 (May 23, 2015)

So if you want to be very specific....

Mini-splits use motors almost identical to ECMs. They are DC Motors, not because they are more incredibly more efficient, but because the rotors are permanent magnets. Usually rare earth magnets (neodymium). However, newer designs use better geometry so they can use lower cost ferrite magnets instead. 

The side effect is a motor that MAINTAINS it's efficiency at lower RPM and loading. (at full load a 3HP, 3 phase motor losses out by about 2% or so) The second side effect is the much higher starting torque, with greatly reduced starting current compared to squirrel cage induction motors. (even when compared to 3 phase versions) 

You inherently loose efficiency when rectifying, smoothing, then inverting back to pulsating DC. (That's AC with a DC bias equal to Vpk.) Without the motor, it's a neat square waveform. The 2% net increase above accounts for this. VFDs do the exact same thing, but without the bias. 

As for the word manufacture..... It's defined as making one thing from another. So technically it isn't wrong, just not exactly the correct place. 

PS. 
- 60Hz AC = a zero crossing 120 times per second. 
- current is always lagging Voltage in motors. (not accounting for any capacitors) So voltage and current are never 0 at the same time. 
- most VFDs operate 3 phase motors. Fixed capacitance, with greatly reduced starting torque causes problems with single phase motors. 
- there are better designs for speed control, but none as cheap for medium motor HP ratings, or as maintenance free. 
- while it's not common, 6 phase has better performance capabilities, and 12 phase more so. (both in motors and distribution.) Additionally complexity, costs, and the requirement to keep the phases in order make it unattractive. 
- being defensive makes it very difficult to learn from others.... 


Reference:
http://www.appliancedesign.com/articles/92754-hvac-moves-toward-higher-efficiency-ecm-motors

http://www.nrcan.gc.ca/energy/products/reference/15214

https://www.merriam-webster.com/dictionary/manufacture

Cheers!


----------



## beenthere (Oct 11, 2008)

Gerard120 said:


> Hi,
> I am having some problems figuring out why, everything says the mini splits are more efficient than central air, but when my central a/c 5ton(60,000 btu) unit was installed i was told it would operate around 3000- 3500 watts, and when i check the 40 amp 240v circuit each hot is about 12amps each, going to the outside condenser, and for the inside fan/cooling coil, 30 amp circuit, each run at 2 amps , so thats pretty close i believe at 28 amps to the estimate i was given, i know start up amperage is much higher but briefly(reason for the large breakers 40 amp i believe but am not sure). I has two zones and the amperage stays consistent with one or two on,(fan setting on the board in nom). When i look up mini split 36,000 btu units (which say they actually produce more like 31,000 btu cooling), they say they operate at 3000-4000watts, i wanted to do two separate units for personal reasons instead of a mini split 48,000 btu dual zone. But my problem is that for half the cooling at 31,000 actual cooling capacity, the specs are saying my power usage will be the same as the 5ton central i already am using, so i'm very confused and any help would be greatly appretiated. While the 5ton unit is in operation, since my power distribution is 208/240 3 phase i am at 70-75% on each line(phase) and i guess you can see why i'm worried about half the cooling capacity at double the power. I hope someone out there can help because the manufacturers cant seem to. thanks again!


If your 5 ton condenser is 240 volt 3 phase, then it alone was using 4.98 KWs while you were checking it. If the air handler/indoor fan is also 240 volt 3 phase, it was using .83KWs. Bringing your total KWs to 5.81 KWs for your 5 ton A/C.


----------



## beenthere (Oct 11, 2008)

Bob Sanders said:


> Huh??
> Inverter technology is about speed control.
> 
> Incoming AC is converted to DC and then modulated up or down for fine speed control. It doesn't "manufacture" electricity.


This variable frequency drive includes a rectifier that converts the incoming Alternating Current (AC) to Direct Current (DC). It then utilizes Pulse Width Modulation (PWM) of the DC in the inverter *to generate AC* of desired frequency. The alternating current is used to drive an induction motor or a brushless motor.

http://www.brighthubengineering.com...ons/124042-dc-inverters-for-air-conditioners/

While it doesn't generate current from thin air. It does generate an AC. Which is sometimes misunderstood as making more electric then supplied to to it.


----------

