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Minisplit efficiency - multiple single zone vs single multi-zone

Hello all,

I am strongly leaning toward installation of a ductless minisplit system to replace an archaic air handler used for heating and cooling. I've done a lot of research on mini-splits and would definitely prefer one of the ~-15* low-temp heating options offered by Fujitsu or Mitsubishi (this just makes sense as I'm in Chicago burbs). Currently from what I can tell these models are only available in single zone models. However, for our home I'm certain the sqf and layout of our home would mandate more than one interior unit. (2700 sqf ranch with bedrooms that are pretty well shut off from the living space).

I have several questions related to these products:
1) Are there any known/concrete plans to offer low-temp heating/ high SEER units within the multi-zone product set? In other words, would it be wise to wait another year or two to make this investment?
2) If there are no short-term plans by these providers to make these units available in the multi-zone platform, is there any objective way to evaluate whether it makes sense to go with a lower efficiency multi-zone vs multiple single zone units (meaning I would have up to three outdoor units)?
3) Separate but related question. We have a detached garage that gets good sunlight during the day. I'm curious whether it would be advantageous to install the exterior units so that they draw air from the garage (while still exhausting outdoors)? I've not actually tested this theory but I suspect the temperature inside the garage is always a few degrees higher than outside (because of solar gain & whatever heat comes up from the ground). The flip side is this would add ~20 feet of tubing to each run.

We live in Chicago so these units would be used extensively for heating. Even with a low-temp heating unit I know we will need supplemental heat. I do have a plan for this.

Lastly, I have an HVAC contractor with minisplit experience coming next week to bid this. My goal is to be as educated as possible prior to that meeting.

Asked by Brian Gray
Posted Wed, 07/02/2014 - 09:21

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2 Answers

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Efficiency is relative, but multi-splits are automatically at a slight disadvantage since the compressor has to serve multiple masters, making it harder to optimize. A 2.5 ton multi-split serving a single half-ton head when only one head is calling for heatng/cooling at a fraction of the head's capacticycan never be as well matched & optimized as when the outdoor & indoor units are matched. When all zones are calling for service at the same time, or the combined loads of the heads calling is a sufficiently high fraction of the compressor's capacity it can be pretty good.

Mitsubishi M-series H2i multi-splits already have a rated output @ -25C/-13F, just like the single-head "FH" mini-splits.:

http://www.mitsubishicomfort.com/en/consumer/product-solutions/product-s...

http://www.mehvac.com/media/214733/mxz_2b20_3b24-30_4b36na_service_obh56...

These multi-splits deliver the full-capacity rating down to +5F, and something between 70-75% of the full capacity @ -13F.

In general three singles will have higher average efficiency than a 3- head multi-split (like the 2.5 ton MXZ-3B30NA pr 2-ton MXZ-3B24NA :http://usa.mylinkdrive.com/uploads/documents/2976/document/MXZ_2B20_3B24-30_4B36NA_Service_OBH560E_6-11_%282-11,_21-32%29.pdf http://usa.mylinkdrive.com/uploads/documents/2212/document/MXZ-3B24NA_Su... ) at the same tonnage, provided the design temp loads are at least 2/3 of the rated output of the mini-splits. The smallest mini-split is rated 9000 BTU cooling, but there are 6000 BTU options for multi-splits. If a zone has a design load of 4000 BTU you'll probably get better efficiency (and definitely better comfort) out of the multi-split with a 6000 BTU/hr cassette than a 3/4 ton mini-split.

It's important to do the room-by-room load numbers to figure all this out. Depending on the floor plan possible to split the output of a mini-duct cassette to cover 2-3 adjacent rooms with short duct runs without taking a huge efficiency hit from the ducts. Splitting the output with short ducts makes it easier to better match the cassette output rating to the load to keep it modulating at part-load rather than short-cycling range. A 6000BTU head or 9000 BTU mini-split serving a 1500 BTU peak-load is probably going to be quite a bit less efficient than a 9000BTU mini-duct cassette serving three 1500 BTU loads via duct runs, even if the latter isn't a great match either- it's the difference between being 2x oversized for the load vs. 4x or 6x oversized. The ideal would be no more than 1.5x oversizing.

Putting the outdoor unit in the garage will not improve the heating season efficiency, and if it restricts air flow AT ALL, it can cut into heating season efficiency. A the air volumes the outdoor units are pushing it's literally dozens, maybe even hundreds of air exchanges per hour for a normal sized garage. (If your garage is the size of the Houston Astrodome it might help though. :-) ) What you are proposing is to use the garage as a solar collector, and as a solar collector it's both way too small and way too inefficient to be of any benefit.

A reasonably insulated older 2700' ranch with reasonable air-tightness and U0.34 windows can easily have a design heat load @ 0F (Chicago's 99% outside design temp) as low as ~30,000 BTU/hr, or lower, which is the +5F output of the -3B24NA. If your design heat load at 0F is EXACTLY 30,000 BTU/hr, the -3B30NA (capacity= 36,000 BTU/hr @ +5F) would pretty cover your extreme cold-snaps:

http://usa.mylinkdrive.com/uploads/documents/2212/document/MXZ-3B24NA_Su...

http://usa.mylinkdrive.com/uploads/documents/2209/document/MXZ-3B30NA-1_...
http://usa.mylinkdrive.com/uploads/documents/2213/document/MXZ-3B30NA_Su...

The economics of waiting until there is a big step-function on multi-split efficiency probably aren't there (but I'm not an industry insider.) Big steps like the efficiency jump from Mitsubishi's "FE" cold climate units to the recently released "FH" mini-splits are pretty rare, but if you're burning propane or heating oil while waiting, putting it off a year or two would not have been cost-effective on a lifecycle basis.

Answered by Dana Dorsett
Posted Wed, 07/02/2014 - 12:07

2.
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Dana,

Thank you so much for your response. I had a well-respected HVAC contractor come to my house yesterday. While I could tell he was knowledgeable about minisplits (something that seems rare in Chicago), I was pretty dismayed at the premium his firm is asking on these systems. After talking for an hour, showing him the house, and having him take measurements of walls & windows, he informed me that I should be prepared to pay ~$40K for this sort of system. I know what these systems cost. He's essentially tacking on $30K+ in labor and margin which I find difficult to fathom for a 1-2 weeks project. In my opinion and to sound a little jaded, the real premium here is coming from the fact that in Chicago, the only people installing multi-split systems in their homes are the uber-rich who are willing to pay any premium to boast about their state of the art, eco-friendly, green HVAC system (while they make a Latte run in their $120K Tesla). Just to reinforce this belief, the contractor then showed me photos of the $100K project they just completed to install an 8-zone minisplit system plus hydronic radiant heat in a DER house in Winnetka (where homes start at $2M). I'm just floored. Only rich people can make energy efficiency seem so damn wasteful.

I am now reaching a tipping point of feeling I should do this work myself. I don't really have a question for you but I certainly would appreciate a sanity check if this is way beyond a seasoned DIY'ers head. I already work on my house dawn to dusk on weekends (thank god my wife doesn't mind this obsession). I certainly don't need another project but it sure seems worth the effort if I can shave $30K+ off the cost of this installation. I'd much rather put that money toward retirement or my kid's college.

Thank you,
Brian

Answered by Brian Gray
Posted Thu, 07/10/2014 - 09:38

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