Ductless AC vs. minisplit install
Hello! We are building a new construction 800 sq ft. apartment above a garage. We are planning on radiant floor heat, and using two separate ductless A/C heads for cooling the upstairs apartment. We got to thinking it might be nice to switch to a Split-mini install (still keeping the radiant floor heat) to have on demand heat during the shoulder seasons here in Colorado when the radiant floor heat isn’t on.
My concern/question is this: looking at the SEER ratings, you take a pretty big hit going from a dedicated AC to a split-mini/heat pump set-up. Is it worth the deceases from ~25 SEER to ~17 just for the few times a year we will use the heat? Thoughts? Thank you in advance!
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It may be helpful if you provide specific links to the products you're comparing. Also, the terms "ductless" and "mini split" are often used interchangeably; the real variable is whether it's cooling only or a heat pump.
There are many options of varying sizes, efficiencies, and manufacturers. It's possible you just haven't found the right high efficiency unit for your application.
I think you are asking "should we use a mini-split heat pump(s) vs a mini-split AC"?
Probably - what heats your radiant floor? Heat pumps are available with a SEER more than 25. Two units in 800 sq feet sounds excessive - have you done a load calculation?
The Fujitsu 9RLS3 is 33 SEER. So that dispenses with the concern over decrease in SEER for a heat pump. Honestly, just ditch the in floor heat. This is from someone who just built a house with in floor heat as the primary (and by the original design, only) heat source.
Hi, yes.. mini-spilt heat pump vs. a mini-split AC (cooling only). Yes, we have done a load calculation, and yes, two heads might be excessive.. but we run the bedroom head only at night, and then the other one during the day. One of the benefits I see using the ductless setup is you only cool/heat what zone you need. We don't need to cool the whole unit all the time. Also, not sure we would get adequate cooling from just one larger BTU head throughout the entire unit?
If we go with a mini-split heat pump, you can run two heads with one condenser unit outside.
You can use a single outdoor unit, but those are less efficient, and have some operational drawbacks. For one, the outdoor unit determines the turndown ratio, so that means that if you run just one head, it may cycle on and off, undermining comfort, efficiency and humidity removal effectiveness. Also, you now have a single point of failure for your two indoor units, where if you use 1-1 units, one can fail, and the other will still work just fine.
Unless having two outdoor units is terribly objectionable to you, I would suggest using completely separate units for each zone. It'll be more efficient, more comfortable, and be more flexible in use. It may also be cheaper, though you'll have to get quotes to find out for sure.
> it may cycle on and off, undermining comfort, efficiency and humidity removal effectiveness.
But don't count on it - cycling may improve comfort, efficiency and humidity removal effectiveness.
Open interior doors will move about 1000 Btu/hr (depends on the delta-T that can be tolerated).
In high dry Colorado LATENT COOLING LOADS ARE NEGATIVE, rendering humidity removal a non-issue in this instance.
Two ductless heads for an 800' apartment is likely to be sub-optimally oversized for the loads. Run the Manual-J, but the peak cooling load is likely to come in under one ton for the whole apartment. The heating load of new code-min construction might make two of them oversized for heating as well.
The industry-smallest half-ton SEER 33+ Mitsubishi FH06NA can deliver 3/4 ton of cooling at AHRI test conditions, and heat at 6430 BTU/hr @ -13F, 8700 BTU/hr @ +5F.
https://nonul.mylinkdrive.com/files/MSZ-FH06NA_MUZ-FH06NA_ProductDataSheet.pdf
https://ashp.neep.org/#!/product/25907
A pair of those could probably handle the entire 99% design heat load (if not, a pair of FH09s would) , not just the shoulder season load, but will still be a bit oversized for the cooling load.
http://meus1.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NAH_ProductDataSheet.pdf
https://ashp.neep.org/#!/product/26100
A pair of SEER 27+ 3/4 ton LG LAN/LAU090HYV1 would do it too, with a lower and slightly more efficient minimum modulation at +47F than the FH09:
https://ashp.neep.org/#!/product/25817
https://ashp.neep.org/#!/product/26100
It's important to derate for altitude as well. At 7000' the output is only ~75% of the labeled capacity at sea level, at 10,000' it's only about 70%.
https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2016/09/08060512/001%20Altitude%20Capacity%20Correction.pdf
So while a pair of 3/4 ton cold climate mini-split will still be able to carry the full heating load even Pb-ville, and would still be oversized for the cooling load, it's not quite as oversized as it would be for us low-land swamp dwellers.
Thank you all for the responses... and excellent points Joshua. Thanks again for the insights.