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Larger mini-split using dramatically more energy than smaller one

Madcodger

Solar Wizard
Joined
Aug 17, 2022
Messages
1,072
We've used mini-splits as our primary heat source for about four years now, in three different properties, but this is the first time I've been able to directly compare units of different sizes from the same manufacturer, in the same building. What I've noticed for our recent build is that the increase in energy use between a smaller unit vs a larger one is not even close to linear, with the larger unit from the same manufacturer using a multiple more energy. For example, our 12K Mitsubishi is truly a marvel in terms of the heat it produces for the KWH it consumes, while the 18K unit, in the same product line, installed in the same week, by the same installers, uses not 50% more energy, but rather a multiple (2x-3x) to maintain the heat in the same building. Is anyone else seeing a dramatic (not linear) difference in energy use between mini-splits, based largely on just the size of the unit? It's so striking I'm thinking of having the installers check the larger unit again, although it does produce great heat. It just uses a LOT more energy than I'd expect, looking at the 12K unit.
 
How long is the line set on the 18k, more than 25’ requires additional refrigerant. Could be incorrect charge. They can also be ordered with or without a pan heater, it might have one and the 9k doesn’t.
 
Does the new, larger heat pump maybe have an auxiliary Emergency Heat module. Those are simply resistance heating elements with a COP of 1. Normally the resistance heater is only turned on in extreme cold or if the compressor will not operate there is a back up heat source.
That heat strip might also be used during defrost cycles.
 
You said it works really, really well, it might have too much refrigerant if the line set was shortened so it is pumping extra hard.
 
Assuming it's something like this, looking at the Heat its capable of screaming up to 27-30K BTU. That would sure do it, if low on refrig, or conditioning a larger space volume, higher actual temp, etc vs 12K unit... 🤔 :)

Also 18K a bit lower on SEER2 / COP if its this series or similar...
Screenshot_20250204_132339_Adobe Acrobat.jpg
 
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The other thing I would be curious about is are the outdoor units installed differently, more air blockage or air recycling though the 18k unit.

I have that 18k unit for backup heating and cooling and it will roast or freeze you out if turned up.
 
Can you post the spaces the units condition?
Possibly the larger unit is offsetting the load in some of the smaller unit space footprint.
That would lighten the load on the smaller unit, and increase the load on the larger one.
 
Do you think an ounce is causing your issues?
Yes it could be the op’s if his line set is 35’ instead of the 25 allowed he would only need couple ounces it could be more. The unit factory charge is only 3.7lbs. At this point who knows if that’s even the problem.

90EA41B8-9ACC-439D-8CC7-602E4B01FCE0.jpeg
 
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isn't the seer considerable worse on larger units? isn't 9-12k like thes best efficiency you can get, you do know the seer goes down as the units get larger right?
 
To reiterate you believe that a unit that's shy one ounce will be using massively more power than expected.
I didn’t say that I said it could come down to a few, you don’t just go adding .5 lbs at a time when your total refrigerant charge is less than 4lbs, sure way to overcharge the system and then what?
You have a refrigerant reclaimer, going to just vent it?
We don’t even know if that is the issue and your advice is to just put .5lbs of refrigerant in and see if it solves the issue?
No thanks I like to solve problems not make them worse for myself or anyone I would give advice to, you might have different ideas.
I have 36 years in the HVAC trade and 23 years as a HVAC contractor as well as being a Mitsubishi dealer, I take the advice I give seriously. I’m the guy that gets the call after you get him to overcharge his system.
 
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My mini-splits almost always are running at there lowest capacities except when the is really hot / cold. If you look at the charts the 12K is a minimum of 3700 btu vs the 18k of 5150 btu. I suspect your issue is your oversized for the btu load in hence the 18K unit, running at is minimum will not be able to match the smaller unit in performance. Since this is a new build, I assume its very low loss / load for the units.
 
We've used mini-splits as our primary heat source for about four years now, in three different properties, but this is the first time I've been able to directly compare units of different sizes from the same manufacturer, in the same building. What I've noticed for our recent build is that the increase in energy use between a smaller unit vs a larger one is not even close to linear, with the larger unit from the same manufacturer using a multiple more energy. For example, our 12K Mitsubishi is truly a marvel in terms of the heat it produces for the KWH it consumes, while the 18K unit, in the same product line, installed in the same week, by the same installers, uses not 50% more energy, but rather a multiple (2x-3x) to maintain the heat in the same building. Is anyone else seeing a dramatic (not linear) difference in energy use between mini-splits, based largely on just the size of the unit? It's so striking I'm thinking of having the installers check the larger unit again, although it does produce great heat. It just uses a LOT more energy than I'd expect, looking at the 12K unit.
Greetings Madcoger,
I was a Mitsubishi diamond contractor (sold business). I know Mitsubishi very well. Just installed a SMA36 with 8 indoor ceiling cassettes in a new house I’m building for my family in Florida. Do NOT add refrigerant to this unit and try to set with sub cooling or super heat. It will not work. Mitsubishi has a program available for free for everyone to use. You select unit and add line set length and it will calculate how much refrigeration to add. Will not have you remove any refrigeration for short line sets. Possible causes of higher energy use: low refrigeration (leak), over sized and short cycling comes to mind. Was a load calculation performed before equipment selection? Properly zoned load calculations are very important when designing for Mitsubishi ductless splits. The program I’m referring to allows one to select equipment for your area and set design parameters to size system properly. Over sizing could use significant energy start/stop. They also have a minimum capacity. Diamond system builder (program) gives you all the information.
 

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