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One Multi- or Two Single Mitsubishi heat pump outdoor units


I've been reading the plethora of articles on the mini-splits and I'm going to go with this system for my whole house remodel. It's a nice rectangle-shaped two story beach house (Newburyport MA) and I did go a bit crazy with a lot of windows and sliders. But lots of closed cell foam should keep the house pretty air tight and warm (a lot better than it was). Since the first floor is an open plan, I was thinking of a 15K BTU floor unit downstairs and a 9K BTU upstairs hallway unit. I do like the combined multi-zone unit for 24K BTU outdoor unit (such as the MXZ-3C24NAHZ2). I'm working on my own manual J using Load-Calc.net but I'll admit I'm probably making a lot of mistakes as I putz with it.

I read on this site that the inefficiency in these combos can be pretty high. Should I consider 2 outdoor units instead? I'm assuming the upstairs unit is mainly for cooling and the downstairs for heating. I can't really put ceiling fans upstairs so I hope the hallway unit or the sea breeze will keep it cool in July.

Thoughts on the one or two outdoor units?



BTW my general contractor loves the ducted air handler system but I hate the whole concept of ducting so I'm ignoring his ideas!

Asked by david flinn
Posted Feb 14, 2018 12:59 PM ET


8 Answers

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"... lots of close cell foam..." is a pretty un-green way to air seal & insulate a home, buying less than R1 in "whole-wall R" when installed in 2x4 studs, less than R2 in 2x6 studs. See:


Open cell foam will air seal as-well or better than closed cell, and a full-fill of half-pound open cell in a 2x4 wall uses less polymer than 1" of 2lb closed cell foam, and it's blown with water instead of the industry standard HFC245fa, which is a very powerful greenhouse gas (on the order of 1000x CO2.)

You don't mention how big this place is, but a 1.25 ton cold climate mini-split could cover fully half the whole house load of my 2400' 2x4 framed antique + 1600' of conditioned basement at your 99% outside design temp of ~ +5F. Are you sure the first floor has 18,000 BTU/hr of load?


The heating capacity @ +5F of the MXZ-3C24NAHZ2 isn't dramatically higher than just the FH15, and would be under the capacity of an FH09 plus a pair of FH06s, with a minimum modulation greater than a 3-fer of FH09s or FH06s:




Answered by Dana Dorsett
Posted Feb 14, 2018 3:09 PM ET



I do understand the closed vs open discussion for my 1150 sq ft house. I'll continue to consider the wall option. My very rough initial calcs show a 18K load on the whole house.

Regardless of the exact capacity of a unit, It seems the tricky part of all this is calculating the choice of two outside units with two inside versus the use of one multi-zone unit.

Answered by david flinn
Posted Feb 14, 2018 11:01 PM ET


A load of 18K @ +5F for an 1150' house is relealistic for a 2 x 4 /R13 house with U0.32-ish windows.

If it's basically a 1.5 story house you can probably get there with an FH06 upstairs and an FH09 downstairs, or a pair of FH09s, for less money and higher efficiency than a 3C24.. The minimum modulated output of the 3C24 at +47F for the 3C24 is 7200 BTU/hr, whereas your whole house load @ +47F is about 5000-5500 BTU/hr.

A pair of FH09s can modulate down to 3200 BTU/hr @ +47F (for the pair, 1600 BTU/hr each.) The 3C24 would be cycling the compressor on/off any time it's over ~40F, whereas with the FH09s that wouldn't happen until it's in the 50s, and you could probably turn one of them off to let the other one modulate at high efficiency to even higher outdoor temps.

The max capacity @ +5F of a pair of FH09s is 21,800 BTU/hr, only 3,200 BTU/hr less than the 3C24, and 3,800 BTU/hr more than your 18,000 BTU/hr whole house load.

Uless you really need 3 zones the 3C24 would be a lousy choice compared to a pair of FH09s.

Answered by Dana Dorsett
Posted Feb 15, 2018 8:37 AM ET


Hi Dana,

Awesome. So that answers my question: it appears 2 separate units would be more efficient than 1 multi-zone.

I'll go with the 2 9K units, one a floor unit and the other a wall that you mentioned:


Now to figure out how to find the space under the deck for two of the these!

Thank you!


Answered by david flinn
Posted Feb 15, 2018 9:09 AM ET


If they're going under the deck be sure they have plenty of access to the outdoor air- give it some margins over the minimum side , top and front spacing to obstructions, and don't snug 'um up together like conjoined twins-keep them at least 20" apart side to side, and don't have them blowing at each other.

Also pay attention to drifting snow issues. I once spotted a 2- ton multi-split compressor under a first floor deck of a condo at a ski area in VT that had but a tiny window of direct air space available at the side of the deck, and with 2' of snow on the deck it wasn't getting anything between the deck board spacing. It was probably still working, but it sorely needed to be dug out for greater access to the great outdoors.

Answered by Dana Dorsett
Posted Feb 15, 2018 3:38 PM ET


Just my 2c as a homeowner:

I have a pretty square footprint with my colonial and decent sized center staircase, with a 12k on each floor. @ 35 degrees right now, the bottom unit is pretty much running nonstop, the top of stairway unit cycles on a couple times an hour, and the attic unit runs even less.


I was reading this article that suggested that close to 75% of your heating load can be first floor only depending the layout of the house. I can attest to this as my first floor unit is a little undersized for heating in my particular situation even though my whole house BTU is more than plenty. The inverse would be true in the summer in AC mode. It's a tough balance between heating and cooling loads and not being oversized to remove humidity in the summer.

I'd recommend units that have low min modulation like Dana mentioned, and have found a wired thermostat essential for floor wide comfort. I think Mitsu has temp sensing in their remotes and can hook up to 24v Ecobee's for example. Pair that with their wireless sensors and you would have great control.

Answered by kjmass1
Posted Feb 17, 2018 3:10 PM ET


Thanks for the food for thought. Perhaps it makes sense to think of the heat down and cool upstairs. I love the article you shared. I just might boost my downstairs unit up then. But certainly I will do a calculation based on just the downstairs. I appreciate your note.

Answered by david flinn
Posted Feb 17, 2018 4:49 PM ET


With only one unit per floor the bedrooms will drop a couple degrees if you keep the doors closed. Nothing major but don't expect 100% uniformity per floor.

I'd go with separate high efficiency units if you can.

Answered by kjmass1
Posted Feb 17, 2018 8:23 PM ET

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