Why are all the highest SEER mini splits low BTU (6k or 9k)?

[dzl_]

I have noticed that all of the most efficient mini splits I've encountered are low 9k or below. Often there is a very large difference in efficiency (specifically SEER)


Some examples:

Carrier Infinity
9k = 42 SEER
12k = 32 SEER

Innovair Elite
9k = 38 SEER
12k = 29 SEER

Gree Sapphire
9k = 38 SEER
12k = 31 SEER

Fujitsu Halcyon
9k = 33 SEER
12k = 29 SEER

Mitsubishi M-Series
6k = 33 SEER
9k = 31 SEER
12k = 26 SEER

[CHEESE_WAGON]

High SEER comes from pairing larger indoor coil(s) with smaller compressor(s). But it requires an expansion valve to operate properly.

[cadillackid]

Originally Posted by CHEESE_WAGON

High SEER comes from pairing larger indoor coil(s) with smaller compressor(s). But it requires an expansion valve to operate properly.

all of the mini-splits with inverter compressors have electronic expansion valves in them.. the better ones use 3 sensors on each coil so you can figure out the true saturation point in the coil..



its a known fact that minisplits run most effiecient at between 30-80% rated capacity..



from what ive seen in taking units apart that many of the 9000 BTU units are exactly the same as the 12,000 BTU units.. a couple appear to have slightly smaller indoor units but 2 that I messed with were identical...


so run a computer program that limits the max capacity to 9000 BTU and the unit is running always at less than its true rated capacity and can register ultra-high SEER ratings..



the variable speed nature of minisplits means that really you can install 12,000 BTU units and even if the yare oversize they will just slow down the compressor and the fans to match the cooling load and still control humidity..



the ducted units in my house have very long run times but are constant and the watt usage is low.. even though if I ran all the units on full blast it is way oversized for the house (but gives me the quick cooldown I like because i dont run A/C in my house until its time to sleep).. even on somewhat cooler ultra humid days when I run them I never get a clammy house because my blowers and those compressors slow down to match the load..



the outdoor fans slow down to maintain enough head pressure and heat in the refrigerant so that you dont freeze across the EEV ..


-Christopher

[dzl_]

Originally Posted by cadillackid

all of the mini-splits with inverter compressors have electronic expansion valves in them.. the better ones use 3 sensors on each coil so you can figure out the true saturation point in the coil..

its a known fact that minisplits run most efficient at between 30-80% rated capacity..


from what ive seen in taking units apart that many of the 9000 BTU units are exactly the same as the 12,000 BTU units.. a couple appear to have slightly smaller indoor units but 2 that I messed with were identical...


so run a computer program that limits the max capacity to 9000 BTU and the unit is running always at less than its true rated capacity and can register ultra-high SEER ratings..

the variable speed nature of minisplits means that really you can install 12,000 BTU units and even if the yare oversize they will just slow down the compressor and the fans to match the cooling load and still control humidity..

the ducted units in my house have very long run times but are constant and the watt usage is low.. even though if I ran all the units on full blast it is way oversized for the house (but gives me the quick cooldown I like because i dont run A/C in my house until its time to sleep).. even on somewhat cooler ultra humid days when I run them I never get a clammy house because my blowers and those compressors slow down to match the load..

the outdoor fans slow down to maintain enough head pressure and heat in the refrigerant so that you dont freeze across the EEV ..

-Christopher

Thank you for the very informative response. You have so much good knowledge on HVAC.


I'm still not grasping why the small units would be higher SEER.


If like you said, the sweet spot for efficeincy is 30-80%, and the components used on 9k and 12k are similar or the same, I wonder if its possible that these units are 'tuned down' 12k's being forced to operate in their maximum efficiency zone (9k is about 75% of 12k)

[cadillackid]

thats exactly what i think they do for the 9000 is take basically the same hardware for a 12000 and run it at less than capacity limited by the computer.. I cant say for 100% sure but it definitely seems like from my experience what they do..

[kazetsukai]

Unless you have not insulated (in which case you really don't care about efficiency anyway) in your build it isn't really like you need a huge unit. My opinion.

Basis anecdote: Innovair (37.5 SEER) 9000 BTU here, it could easily make the cabin uncomfortably cold all summer and kept temperatures consistent at 70F all winter down to single digits. I built leaving the door open for a second unit, at this point in time I think that is overkill, maybe I'll change my tune if I get stuck somewhere with 110F weather, idk. I doubt it.

Anyone installing a split- do not sandwich the coolant lines together, you will destroy whatever efficiency the split offers. My lines are in separate runs of PVC conduit.

Air circulation is a far bigger problem for me than cooling/heating capacity.

[CHEESE_WAGON]

Originally Posted by cadillackid

thats exactly what i think they do for the 9000 is take basically the same hardware for a 12000 and run it at less than capacity limited by the computer.. I cant say for 100% sure but it definitely seems like from my experience what they do..

I can't say for 100% sure with a mini-split, either, as you have one compressor across multiple coils (at least I'm under that impression). But I can tell you that my old man just replaced his heat pump (he is an HVAC control technician by trade and has a rather astounding knowledge of the mechanicals as well).

The unit he got had a 6-ton indoor coil with a 5-ton outdoor unit, and an expansion valve had to be installed to achieve proper efficiency. He forewent the expansion valve initially (old-school thinking - "I don't need that junk") and quickly found the efficiency was about 10% of what it should be. Unit ran for hours and the house couldn't get below 72. I did some research and showed him the expansion valve was necessary, so he finally put it in and voila, the system ran 60% less and could keep the house at 66 all day.

Point is, the higher SEER rating in this case came from using an expansion valve to keep the compressor from flooding the larger coil it was supplying. I am not sure if this same principle applies with a mini-split, but that is how typical residential / commercial units achieve higher SEER.

[cadillackid]

only multi-split minisplits have one compressor across multiple coils.. however they operate with multiple expansion valves.. very similar to a commercial walk-in cooler.. multiple evaporaotrs each with its own TxV..



in any syste, using a Piston orifice with R410A is going to result in poor performance except under high loads.. you are always going to toss too much refrigerant in the evaporator.. a larger than capacity evaporator (5 ton condensor 6 ton evaporator) is a trick used for years in extra humid conditions.. you have to make sure you r airflow is controlled but essentially you create a colder coil but more area thus eliminating humidity..


the minisplits with the real high SEER are generally the single zone.. on indoor, one outdoor coil.. and one compressor.. they are heat-pumps and employ an accumulator so they have extra refrigerant for the heat cycle.. that accumulator also has liquid in it during cooling cycle.. its insulated so essentially its a buffer so that the EEV cant drop the suction pressure too low and freeze the coil.. brilliant really!!