How to assess efficiency of 2+ mini splits vs. single ducted central system?
I’m located in Central NC and manual J is about 21,000 for both heating and cooling, though I don’t think that info is particularly necessary for the question that I have.
I am about to collect bids on replacing an aging, inefficient traditional split-system ducted heat pump. I’ve narrowed my choices down to a few options. All work well even at the lowest temperatures that my area typically sees.
1) 2-Ton Carrier 5-stage 18VS (25VNA8) heat pump (~18 SEER, 11 HSPF). Modulates down to ~25% of nominal capacity. I believe minimum cooling capacity is somewhere in the ballpark of 8,500 BTU.
2) 2-Ton Trane XV18 variable speed heat pump (~18 SEER, 10 HSPF). Also modulates down to ~25% of nominal capacity but is essentially continuously variable (something like 750-stages) up to full capacity. Minimum cooling capacity is somewhere around 6,000 BTU.
3) Three Mitsubishi MUZ-FH09NA mini splits + matched indoor unit (~30 SEER, 13 HSPF). Continuously variable from about 1,700 BTU all the way up to rated capacity. Three separate units would be necessary to adequately get heating and cooling to my entire floor plan. Thus, total minimum cooling capacity would be 1,700 x 3 = 5,100 (though I suppose I could turn one or two off to get it even lower, if I wanted to).
Based purely on SEER, HSPF, and modulating capabilities, the Mitsubishi mini splits obviously win. However, as noted above, three separate systems would be required to adequately heat and cool my floor plan. This is fine from a capacity stand point because each system has such great modulating capabilities (that is why I am not interested in a multi-split — they don’t modulate as well as the single-split systems).
My question is this: Does the fact that the Mitsubishi setup would have three separate compressors, three separate evaporators, three separate EVERYTHING, somehow in the real world make it less efficient than the single Trane or Carrier would be?
Or to put it more broadly: How does one go about comparing multiple mini splits vs. a single central ducted system when trying to determine which will ultimately be most efficient and cheapest to operate?
Does it ultimately come down to how many BTU is the system(s) providing, and what is the net efficiency of the system(s)?
So, to give just one example, would it be correct to conclude that if the Trane were providing 21,000 BTU of cooling at a given point in time, and the THREE Mitsubishi systems were collectively providing 21,000 BTU (i.e., 7k each), that the three Mitsubishi’s COLLECTIVELY are still more energy efficient than the single Trane, because the Mitsubishis are providing the cooling at ~30 SEER, whereas the Trane is providing the cooling at 18 SEER?
Or does the fact that the Mitsubishi requires the use of three separate systems somehow factor into this equation such that the single Trane may actually be the more efficient choice because only a single compressor and air handler are involved?
Any input appreciated!
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Replies
I do not think you will get a true answer to your question just too many variables.
In general mechanical systems become more efficient as they get bigger.
Will 100% of the duct work and equipment be located inside the conditioned space? If not the minis will be the winner because of the duct losses.
Will all the living spaces in the house all be on the same level? If not the minis will be the winner because you get built in zoning.
Will you be able mount the mini heads on the wall as low as the instruction recommend? Most do not and pay efficiency penalty.
Walta
Thanks for the response. I get that real world answers are tough, so maybe I should eliminate variables and ask from a theoretical perspective.
System "A" -- the three Mitsubishi 30-SEER mini splits described above
System "B" -- Trane XV18 described above
Would System A, operating at 21k total (7k each), be more or less efficient than System B operating at 21,000 BTU, in the exact same space under the exact same conditions?
I think the bottom line of the question is this: Is the fact that the Mitsubishis are 30-SEER and the Trane is 18-SEER the answer? And 21,000 BTU at 30-SEER is more efficient than 21,000 BTU at 18-SEER, period--full stop--therefore the Mitsubishis win, even though there are 3 of them?
Or does the fact that there are 3 separate Mitsubishi units, vs a single Trane, make a difference?
Another way of wording this to try to clarify my question:
If it were a single 30-SEER Mitsubishi mini split operating at 21k BTU versus a single 18-SEER Trane operating at 21k BTU, then obviously the single 30-SEER Mitsubishi is at least theoretically the more efficient option.
Are three 30-SEER Mitsubishi mini splits that are operating at 21k BTU (7k each) less efficient than one 30-SEER Mitsubishi mini split operating at 21k BTU, notwithstanding the fact that both are 30-SEER, simply b/c operating three separate compressors/evaporators/etc. takes extra energy?
It's a simple question in my mind but I'm not sure it's coming across in writing. Hah.
It sounds like you are looking for facts. I do not think anyone can answer your questin as a fact that one is a better choice you will need to settle for someone’s opinion.
I ask you for 2 basic questions about your house I would think less of anyone that would give you an opinion without knowing the answers. If you do not understand the questions please ask for clarification.
1 Are 100% of the duct work and equipment located inside the conditioned space?
2 Are all the living spaces in the house on the same level?
Walta
Aun,
I agree with Walter. There are a lot of variables. But if you want to know the efficiency of the entire heating and cooling system, you need to consider the ductwork, not just the heating and cooling equipment.
If any ducts are located outside of the home's conditioned envelope -- in an unconditioned attic or an unconditioned crawlspace -- Bingo! You've got your answer. The ductless system wins every time.
Even if 100% of your ducts are inside your home's conditioned envelope, I suspect that the Mitsubishi equipment will be more efficient.
Oh, apologies Walter, I thought your questions were just things to consider rather than you actually wanting me to answer. All ducts are in a conditioned crawl space. And, the answer to your second question is, I think, "effectively, yes." -- There are two stories, but the second story is served by separate systems. So this Trane vs. Mitsubishi question is for first floor only.
Second story is served by a completely separate traditional ducted split system...
Edit: And thanks to Martin for the reply as well. I guess the take-away message I am getting is that while, it might be easy to say a SINGLE 30-SEER mini split would be expected to be more efficient than a SINGLE 18-SEER ducted system, that the question becomes more difficult when you split your single mini split into three separate units.
Generally crawls spaces are more outside the conditioned space than inside. Does the crawlspace have vents to the outdoors? Are exterior wall of the crawlspace uninsulated? If answer is yes I would say the crawlspace is outside the conditioned space.
Having your ducts in a crawlspace would push me toward the minis, knowing it will lower your energy usage but understand you are making a big change in the way your house works that carry risks. One risk if the crawlspace was at all damp before it could turn into a mold forest without the heating and drying provided by your old leaky ducts. Another risk is without the leaky ducts the crawlspace will be colder in the winter and notice the colder floors frozen pipes maybe more likely.
It has been my experience that the quality of the installation of HVAC equipment is a bigger factor in performance than the quality of the equipment selected. Please try to hear what your HVAC contractor is saying without using words. Most today only want to bid minis and do so with the “I do not want this job price tag” pushing you to buy a ducted system. You will need to find the contactor that believes in mini splits. Not the ones that bid them for effect.
Walta
How about:
4) Fujitsu ARU/AOU18RLFCD
http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/18RLFCD.pdf
Max cooling is 20,100 BTU/hr, so it might come up just a bit short in summer, but it should cover the wintertime load with a bit of capacity to spare. Since it modulates down to 3100 BTU/hr (both cooling and heating at relevant temps) it would modulate more than a 3x -FH09 solution.
The wide modulation of the range and lack of duct losses the 3x -FH09 solution would probably beat the Carrier Greenspeed and Trane XV18 solutions, especially if one or more can be turned off most of the time.
Dana, what type of evaporator is that in the Fujitsu you linked? Is it some sort of a ducted vent? The specs of the system you posted are promising but I'm not sure I understand the system...
Edit: just watched some YouTube videos. So is that basically Fujitsu version of a ducted air handler that can connect to my existing duct work? If so that looks pretty awesome. Looks to have much better modulating capability than Mitsubishi ducted air handler...
The xxRLFD cassettes are very small air handlers with a modulating blower & refrigerant coil. Technically it's only an evaporator when in cooling mode- it's a condenser when in heating mode.
Whether it will work with pre-existing ducts would require some analysis of your ducts. It's cfm specs are at lower backpressure water-inches than most full-sized air handlers are specified at, but more oomph than most competitors' mini-duct cassettes. If your ducts are oversized, designed for bigger than needed air handler cfm there is at least a good chance it can work.
i have a similar query. But it is two years later so the answer may be different now. I have a completely unheeded summer house at the seashore built in 1970 with minimum insulation. I want to winterize heat it and also have the option to air condition sometimes in the summer. I do not want to air-conditioning all the time because I like natural air and breeze. There is a crawlspace under the entire house except for a 10 x 10 full basement. House footprint is 40 x 20’. The ceiling of the central room is 22 feet high. Next to it are a dining alcove and a quasi-open kitchen it’s 10 ft.². separated by a cedar wall is a 10 x 10‘ bedroom and there is an 8 x 10‘ bathroom and just 3 feet of hallway. It is a very compact design. There is very little actual wall space and a lot of glass. The Rear of the living room consists of two sliding glass doors —recently installed top of the line insulation wise. The front of the LR has two large 6’ high windows, a door-sized fix glass pane, and the door. Dining area west facing has an 8 foot casement window. So does the east facing bedroom.
Two years ago I had Mitsubishi engineers look at the place and they said there was not enough wall space to install their units. This year I had a general HVAC contractor come and he said I should have a ducted system in the crawlspace with floor vents. He said I would have to cover the air vents in the walls of the crawlspace in the winter but they are very valuable because they have kept the house from getting moldy over the 50 years I have had it. The Bosch system he proposed is very expensive with a quote of $17,400 excluding thermostats, electricity and new wall and crawl,space ceiling insulation. I am having another contractor come next week to give me his opinion. What should I ask him and how do I choose?