Cold climate mini splits double the electricity consumption in severe cold?
I have a 12k Gree Sapphire and a 12 K Midea Premier. We finally got some decent single digit cold weather last week and these units performed awesome. Its been really mild here most of the winter and they both spent most of their time modulating between 200-1200 watts. Never saw them go much higher than 1200 watts.
The other morning when I woke up it was in the single digits and I cranked the gree up really high in the morning to see how it would perform. It ramped down for a few mins then shot up to 2.2 kwhs pretty quickly. The gree says it has a 2nd stage to extend capacity in ultra low temperatures. Seems the 2nd stage doubles the electrical consumption from a high of 1.2 up to 2.4. So are these cold climate splits mostly just running at 50 percent compressor capcity in mild temperatures? I assume they are just doubling the compressor speed in really cold conditions. Is that what the 2nd stage is?
I attached an electrical usage graph. I also attached last night’s electrical consumption when it was in the mid to upper 20s as a comparison. You can see it stays below 1.2 kwh. The blips are defrost cycles.
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No one more qualified has answered, so I'll give it a try.
From the looks of it, the Gree unit is variable compressor speed, so what really happened is that instead of modulating, you bumped up the set point, and it figured it better run full tilt to try and meet that for you.
The two stage they're talking about appears to be two stages of compression, rather than two speeds.
Basically yes, your system was running along happily at 50% speed, then you made it grumpy, so it ran flat out. :)
I don’t think it’s just a big jump in compressor speed here. It would be inefficient for the design of the unit to only rarely run the compressor over 50%. Much more likely is that below some particular temperature, the unit is kicking in some electric resistance “auxiliary heat” to help boost the output air temperature from the unit. The manual should discuss this.
If it’s actually doing a huge change in the operation of the compressor, you should be able to hear the change just by listening. You’ll hear something similar to the change in sound from your car engine when gears drop while going uphill, a big step change in the load and resulting change in sound from the engine.
Bill
Well, most minisplits run part load most of the time, and that actually makes them more efficient...
There is usually a “sweet spot” for variable speed stuff though, and actual energy efficiency in terms of how much heating/cooling you get per watt of input electrical energy is usually highest at somewhere near full load. Obviously the units will have to modulate depending on the actual temperature conditions outdoors and in the house, but from a design point you want to try to design the unit to be most efficient at whatever set of operating conditions you expect the unit to see most often.
If the unit were to be running over 50% only occasionally, during periods of extreme cold, then the average efficiency of the unit overall would likely be somewhat lower than it could be. I’d also expect it to not show a big step increase in load, I’d expect a more gradual ramp up with a modulating system. A step load would be more indicative of something in the logic doing something like “below this temperature, active this heating element”.
This is all me making some educated guesses about the design of the unit though. I do NOT have any specific knowledge of this particular unit.
Bill
Hey Bill,
I think the reason for the step change was the change in set point. The user bumped up the thermostat a few degrees. Most systems I've played with will not ramp nicely as a response to set point changes, but treat a set point change as more of a "Master is cold / Hot, and we must acquiesce immediately"
As for efficiency vs part load, most minisplits I've come across do much better at very low loads, and no where near maxed out. Here's one such graph:
https://www.researchgate.net/publication/329850416/figure/fig1/AS:706448438870021@1545441841991/Variation-of-the-cooling-COP-ratio-as-a-function-of-part-load-ratio-and-outdoor-dry-bulb.png
It's for cooling instead of heating, but it's the same idea. Max efficiency is somewhere around 40% load, and it declines linearly to full load.
I've not seen any minisplits that are most efficient anywhere near full load. The whole point of inverter drive is to run as slowly as possible for longer, because this is most efficient.
I had a heat pump installed in January last year and I find the outdoor unit noisy and the fan indoors runs much more than it used to do with just the oil-fired burner. The oil-fired burner would kick in whenever I raised the temperature more than one degree C and when the outdoor temperature dropped below 15 C. It felt suddenly warmer in the house and the blower was on only about half the time when it was -15C out. The blower blew warm when the oil burner was on and cool or barely lukewarm when the heat pump was providing the heat. Since there is much cooler air circulating when the heat pump is on we tend to jack up the temperature in the house about 2 C. When are we going to get variable speed blowers so we don't have to jack up the temp?
Most minisplits have variable speed blowers...
For those who wish to retrofit a heat pump into an existing forced air furnace with a single speed blower the inconvenience of the noise and the cool breezes and the cost do not add up to a worthwhile improvement. I must say that the heat pump we installed in the shop (with a three speed fan, not a variable speed fan) is much quieter and one quarter of the cost of the central unit in the house. I think for my new house I'll invest in insulation and and windows and ERV.s or HRVs.
For what its worth, I did this retrofit a few years ago and have been mostly happy with the results (although I wouldnt necessarily have done it if not also desiring a central AC in the home). The breezes in heating mode arent really "cool" except for when the ducts are cold and need some time to warm up. The single stage HP runs loud but it isnt obnoxious to us, and the long run times definitely do a better job of minimizing room-to-room temperature variations compared to the propane furnace. Since the furnace only runs below ~20F now it gets longer run times too.
I will say my initial calculations of a three year payback for the incremental cost of going from a central AC to a central HP have been largely ruined by rising electricity costs and falling propane costs over the past 24 months in my area. Electricity is now around $0.18/kWh and our propane tank was just filled for $1.84/gallon last month. At these numbers the HP is only 5-10% cheaper per BTU depending on COP assumptions on average, and probably below 5% or even more expensive as the balance point is approached.
Do you mean you're using the old blower from the previous furnace? Our system uses the old ducts but a new variable-speed blower. Both the outside unit and the inside one are extremely quiet -- much quieter than the furnace was.
Found a Gree video they posted to youtube. They are calling it a 2 stage enthalpy added inverter compresor. I think it is also called enchanced vapor injection. It looks like under very low temperatures some kind of valve opens redirecting refrigerant and you get a 2nd stage.
So at temps below 17 degrees or so it has a 2nd stage that runs between 1200-2400 watts. At temps above 17 or so it only goes up to 1200 watts.
This appears to greatly extend low ambient output. According to the capacity table this unit can put out its full rated heat to almost -20.
Any rough estimates as to heat put out? Keep in mind that if you had a 2.24 kw space heater it would be putting out 7500 BTU's. I suspect it's close to 12k btu output so you can see the COP decline.
The submittal list the heating capacity as 3071-18766 btu. Its a 12k unit so I guess it can output up to 18k maybe in milder weather. I have been really impressed with the performance. This thing can put out some serious heat.
It looks like LG is using this technology as well.
I believe the capacities in submittal sheets are based on their output at 47º F. So definitely milder weather, but most good cold weather units can still put out a lot of heat even when the temperature drops.
I've never seen data for the Gree units, but I know the Fujitsu 12RLS3Y (which is nominally a 12K unit) can still put out 15K BTUs at -5º and nearly 17K BTUs at 5º F. It's a bit more powerful than your Gree is rated, as it can put out 22K at 47º F, but based on that capacity chart you posted, I'd be surprised if your unit's capacity wasn't well above 12K even in single-digit weather.
>"I believe the capacities in submittal sheets are based on their output at 47º F."
For an AHRI submittal a modulating (or even single speed) heat pump has to be capable of delivering at LEAST "rated" amount of heat at +17F outdoors, +70F indoors, but is tested at +47F at that "rated" output level, not running full tilt.
I'm going to be cynical for a second. The current/COP curve at higher levels of heat ouptut looks more like a 24k "non-cold-climate" heat pump.
I suspect that they make it "cold climate" by increasing the compressor size (which you can see from the spec sheet - it's about as big as a typical 24-36k compressor) - and adding some additional electronics to add lower levels of modulation.
So "2 stage" really means they made the compressor bigger and increased the ability to run at more levels of modulation. (Hence better able to match output to heat loss)
Or am I too cynical?
I think that its. Its basically an oversized compressor that runs at 50 percent capacity in mild weather. During extreme cold it utilizes the additional capacity. I found its actually rated at 18777 btus at 47 degrees drawing 1500 watts.
The NEEP data for the 12k sapphire unit is full of typos and is not correct but the 47 degree data is right. The gree literature claims up to 90 percent capacity at -22.
https://neep-ashp-prod.herokuapp.com/#!/product/25406
The 9k unit appears to have the correct information for a somewhat close comparison.
https://neep-ashp-prod.herokuapp.com/#!/product/25405
Every time I read the title of this thread I say duh.
You paid extra for the feature allowing the heat pump to work at low temps. Yes it uses more energy were you expecting a free lunch?
Would you be shocked to learn a gas furnace uses more fuel when it is cold outside?
My guess is when it gets down to -15° you will see it use 2.4 kWh for hours on end.
I would not call this a 2 stage system and the speed of operation is not the driving factor in the increased use electricity. When it is cold outside a valve is opened an some of the refrigerant is run thru the compressor twice so the compressor is working harder and uses more energy allowing it to move heat it could not otherwise.
Walta
I track gas usage sometimes in heating degree days (hdd) per therm. Mild weather or cold, the usage for gas furnaces in my experience is proportional to the Delta T or heating degrees days. This is not so with the heat pumps and I think the discussion here is how does cold weather affect the electrical usage over rated efficiency.
> I think the discussion here is how does cold weather affect the electrical usage over rated efficiency.
The HSPF rated efficiency is a seasonal average efficiency number for an ARI Zone IV climate (not IECC /DOE zone 4) not a peak load COP number, and factors-in the lower efficiency at lower temperature.
The model by which HSPF is calculated isn't perfect (none are), but if the equipment is right-sized for the loads a cold climate mini-split will pretty much hit it's seasonal average numbers even in DOE climate zone 6. After all, it's not sub-zero outside all season, or even 10% of the hours in a heating season.
It still sounds like you are looking for the free lunch.
Refrigeration becomes less efficient as the temperature differential increases. Is this a surprise?
The greater the temperature differential the more work the heat pump must do.
If you are doing more work you use more energy. Is this a surprise?
The hyper heat trick works by reducing the volume of flow and increasing the pressure.
The hyper heat models are advertised as being able to produce there rated output at a lower temp. No one claimed greater efficiencies as the temperatures decreases.
I had a few classes in refrigeration and maybe this knowledge is not as common as I assumed.
Walta
I am fully aware of how refrigeration works and am well versed in electrical consumption as I have been monitoring it for over a year on both my units.
The original question is more in regards to how a 2 stage rotary compressor works and what is occurring. The only information I can dig up online is Gree seems to be the only one using it and it appears to greatly increase efficiency. If you sort the AHRI directory by efficiency Gree mini splits with 2 stage rotary compressors are currently the top 5 listed, significantly ahead of the big names.
From my understanding of vapor injection, is that although it offers more capacity at low temperatures, it does it at the cost of lower efficiency at other times. This is why you don't want an hyper heat unit in warmer climates or if you are using it for A/C only.
By the sounds of if it, the Gree unit has a selector valve that allows it to operate as a standard compressor at milder temperatures and as a two stage vapor injection unit when cold. Sounds like a pretty nifty idea to me.
The reason you don't see the big power is that the compressor doesn't need to run as hard in the single stage mode to provide rated BTU.
Thanks. I think that's the best explanation I have heard so far.
I also noticed that non hyperheat models usually have much better COP's especially in milder weather. Seems most good cold climate units have a cop of 2 or less at 5 degrees compared to non cold climate units around 2.75-3
One of the main reasons I bought the Gree was for its impressive COP rating of 2.73 at 5 degrees. Almost eveything comparable with full heat output at 5 degrees had a COP of 2 or less. My other one is a Midea Premier Hyperheat with a COP of 2.06 at 5 degrees. The Gree is on the first floor and carries most of the heating load.
>"From my understanding of vapor injection, is that although it offers more capacity at low temperatures, it does it at the cost of lower efficiency at other times. This is why you don't want an hyper heat unit in warmer climates or if you are using it for A/C only."
The opposite is true, if designed correctly.
The efficiency of Mitsubishi's cold climate mini-splits with the vapor injection compressors are across the board better at all temperatures than with the simpler old school compressors. The SEER of an FH09 is north of 30, with an HSPF north of 13 :
https://nonul.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf
The SEER of a GL09 is under 25 with an HSPF <13:
https://nonul.mylinkdrive.com/files/MSZ-GL09NA-U1_MUZ-GL09NA-U8_Submittal.pdf
The FH has almost as much capacity a -13F that the GL musters at +5F.
Fujitsu uses vapor injection compressors across all series, but only the cold climate versions have a pan heater & associated controls for managing defrost ice build up.
I haven't check through all but for the Mitsu multisplit, the non-hyper heat units are more efficient:
https://neep-ashp-prod.herokuapp.com/#!/product/26173
vs
https://neep-ashp-prod.herokuapp.com/#!/product/29040
Mild weather, COP of the the hyper heat is 2 at max, versus 2.25 for non-hyper heat.
Cooling data is all over the place, so I think there are some data entry issues with it.
I remember seeing similar trend for LG units. Maybe Fujitsu is different.
>"Mild weather, COP of the the hyper heat is 2 at max, versus 2.25 for non-hyper heat."
Why bother looking at the MAX output COP during mild weather? That's ridiculous!
During milder weather you WON'T be running it at max (ever, unless doing deep setbacks, which would be stupid), and more likely to be cycling on/off somewhere between MIN, and RATED, so it's not legit to look ONLY at the COP at max output during milder weather.
The HSPF of the 3C30NAHZ is higher than that of the non-hyper 3C30NA, and it can modulate over a wider range.
The COP at the RATED 28,600 BTU/hr @47F for each of those is slightly HIGHER for the
hyper heating -NAHZ.
The COP at the MIN is slightly lower for the NAHZ, but it's also a significantly lower minimum, and it's difficult to factor in the additional efficiency hit for the -NA when cycling compared to the -NAHZ which will be doing a lot more modulating, less cycling.
I think this is the case of making a conclusion with incorrect specs. I did a bit more digging, the number between the hyper heat and non-hyper heat are all over the place. The Mitsu multi seems to be the one with the most issues.
After doing a bit of googling about vapor injection and COP, I'm even more confused.
It is still strange to see the consistent difference in efficiency at mild temp heating and cooling on these (LG and Gree multi splits seem to be the same). Maybe a case of multi split weirdness.
Walta,
The original question is fair, and I didn't think it was BFW577 asking for a free lunch.
Lets suspend reality, replace a gas furnance for a mini-split, but hold other (very relavent) vairables constant. Does the homeowner's heating bill go up?
This homeowner has done that. My verdict is still out.
Some parts of the question don't get fully answered with HSPF or COP or AFUE #s. Because, for one, the ratings for different types of heating fuels aren't the same, and also other variables that get held constant or trivialized. Ex: is poor installation of forced air systems vs fewer bad installations of mini-splits account for more efficient and therefore cheaper heating bills? if so, how to quantify that with data? Here's an idea: how about Grade the installations, like insulation Grades 1-3, but for HVAC systems?
One point / ding against mini-splits is temperature differential: takes more energy to transfer the heat. OK, so how does one capture that inefficieny across all heating systems? Or does this "ding" not apply to certain heating appliances?
As an unprofessional homeowner learning as I go, we laymen get overpomised with platitudes like "it's more efficient" and then get slapped with an unexpected heating bill. If the new heating appliance is sooooo efficient, why doesn't it cost less?*
* my ditching of a gas furnance is also a decision to move away from fossil fuels, so I am willing to "pay" more to get to my goal; but that is another debate for a different thread.
Its looks like the FH09 consumes almost twice as much power than the GL09 at 5 degrees. So you are almost doubling your electrical consumption for less than 3k btu at 5 degrees if I am reading it right? Thats kind of what I have been noticing. The extra capacity at low temperatures comes at almost double the electricity usage and a decent hit on COP.
FH09
10900 btu/1350 watts at 5. COP 2.37
GL09
8170 btu/850 watts at 5. COP 2.82
Why are you comparing these units at different outputs?
They should be compared at the same BTU output.
You're penalizing the hyper heat unit for being able to modulate over a wider range.
As long as it’s not too much more energy during the rest of the year, you might come out ahead over the entire year. If it uses a little extra energy all the time, but comes out WAY ahead in the cold by avoiding the need to run auxiliary electric resistance heat, you MAY come out ahead in terms of overall energy efficiency. This is what you need to look at before slamming the unit as an energy hog. In colder climates, it might actually make more sense than the unit that appears “more efficient”, but ends up running resistance heat enough in the winter to more than offset the initially good-looking energy consumption numbers.
It’s important to always look at the overall system and not just focus on any one parameter when making these kind of comparisons.
Bill
Maybe the better comparison would be the FH09 to the GL12.
Even there, the non-hyper heat beats the COP of the unit at similar BTU outputs.
https://nonul.mylinkdrive.com/files/MSZ-GL12NA-U1-MUZ-GL12NA-U1_ProductDataSheet.pdf
http://meus1.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NAH_ProductDataSheet.pdf
I wonder if the difference is the lack of pan heater and associated power consumption on the non-hyper heat unit.
If you don't need the sub 0F performance, at least on paper, a slightly over-sized non-hyper heat seems better.