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Number of zones and system efficiency in Northwest climate

bloodywanker | Posted in Mechanicals on

I am setting up a 48,000 btu Mitsubishi outdoor unit for a ~2000 sf home in the Seattle area.  I am considering either (2) 24k MSZ-GL indoor units, or (4) smaller units.  The floor plan is quite open, and with some added ventilators, I believe can easily distribute the air and/or use remote thermostats so that cold spots will not be an issue with 2 zones.  However I am wondering if the minimum capacity will still be too large on these 24k units.  Our climate is pretty mild, and I want them to be able to run continuously with minimal compressor short cycling, keeping the house comfortable in the mid-seasons whatever the outside air temp is, 40s-50s.  Am I better off with a number of lower capacity units spread around?  Is “turndown” performance better on the smaller units, allowing better range of usage throughout the year?  I notice that the 24k units on low setting is still so much higher, cfm and everything.. How does my outdoor unit react and use power differently on a system of (2) at 24k, vs. say, (4) at 12K or (3) at 9k plus one at 18K?  Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    You can't design a heating system without performing a load calculation first. Here are links to relevant articles:

    "Saving Energy With Manual J and Manual D"

    "How to Perform a Heat-Loss Calculation — Part 1"

    "How to Perform a Heat-Loss Calculation — Part 2"

    "Who Can Perform My Load Calculations?"

  2. bloodywanker | | #2

    Already did this, the heating load came to 47,500. The question has more to do with how the system is split up, and the behavior of the large vs small indoor units. I'm not terribly concerned about power usage, whether it ends up being a little more one way or the other. I just want to make sure I don't make a mistake of getting the large units if they are not as smooth-running, or would give more wear and tear to the system. I can imagine a couple 9k's purring along at low setting continuously, whereas even one of the big units coming on at low - full blast compared to the small units, might heat up the space more rapidly than it should, then cycle. So question basically comes down to this: (assuming even distribution of heat) In mild cool temps, are smaller units better for keeping house steady temp? And is my 48NA capable of running with very low output to a couple of these lower btu units without cycling? Thanks again for advice anyone may have.

  3. Expert Member
    Dana Dorsett | | #3

    Check your math. A heating load of 47,500 BTU/hr @ ~25F for a 2000' house is a SHOCKINGLY high number. I've seen nearly UNINSULATED ~2000' houses in New England that came in at about that number, but at design temps 20-25F colder than yours.

    Most 2000' houses (even a rancher with an unheated 2000' basement) would come in under 30,000 BTU/hr @ 25F, quite a few would be under 25K. to hit 47-48K would require an extreme amount of sub-code window area or something.

    Getting the load numbers right and sizing correctly is critical for getting the best efficiency & comfort out of heat pumps.

    A sketch of the floor plan, with the room by room loads would be useful for making recommendations here too.

  4. bloodywanker | | #4

    It's a mid-century home with vaulted ceilings and poor insulation, and a wall of 1st generation dual glazing. See attached plan and section. The slab-on-grade basement is mirror image of the upstairs plan. I used the calculator at, gave me 19,700 for basement and 27,800 for upper level. Anyway I already have the 48k outdoor unit, but I could look in to replacing it if it's overkill. Haven't acquired the indoor units yet. Thanks again for taking a look.

  5. Expert Member
    Dana Dorsett | | #5

    The ecomfort "Mini Split and HVAC Sizing Calculator is a ridiculous piece of junk with no truly detailed construction inputs- it's not even as good as an experienced professional's WAG after merely eyeballing the house. I'd never use it for anything, so I can't tell you how much oversizing factor it's output is likely to deliver, but at least 1.5x or more is all but assured, and the error bars are large.

    Try running the room by room numbers using, and assume ZERO infiltration, and the most optimistic insulation values that could be reasonably expected for the vintage- eg: If it's batt insulated 2x4 assume it's R13, not R11, etc. Even then loadcalc regularly overshoots reality by a double digit percentage, but not by 2x. Using aggressive assumptions on everything loadcalc would usually be under 1.25x the numbers that a properly aggressive Manual-J using a professional tool would deliver.

    Are all those windows single-pane with no storms?

    If you have a wintertime heating history on the place you can validate the load calculations using a fuel-use based load calculation (winter fuel-use only):

  6. bloodywanker | | #6

    Alright, see attached for a calc from the builditsolar website. Let me know if you see anything odd. Figured 2.0 on the R-value for all these windows, which is probably generous. A bit of single pane glazing, and a few solid wood doors. Pretty mild on the design air temp, and it's a somewhat leaky house as well, yet I decided not to give a 1.0 value for air changes per hour. Heat loss out roof is less than I expected with the entire roof at R-13. Doesn't seem right that walls and ceilings are 1:1 in terms of heat loss per area..?

    Back to the system and the original question - the manual says I need at least two indoor units totaling a minimum 12k rated btu. Though the max capacity isn't probably ever needed, it seems like it will work for my needs--and I don't have to max it out, right? There's obviously cost savings upfront associated with fewer larger units. But if they cannot ramp down as nicely as the smaller units, is my main concern. As I said before, I'm not too worried about separate zone control or inadequate air circulation throughout the house.

  7. Expert Member
    Dana Dorsett | | #7

    The BuildItSolar load calculator is an I=B=R type spreadsheet type, which usually oversizes by 25-35% of reality, and the actual U-factors used there seem off, so who knows? It looks like Gary Reysa just takes 1/R of the insulation layer and just ignores the rest. If I had to guess (and I think I do) using that tool your roof losses are overstated (possibly by quite a bit) and your wall losses understated by a bit. A typical 2x4/R13 wall with 16" o.c. studs comes in at about U0.09, and Gary is using 1/R13=0.0769, which is going to understate the wall losses by about 17%.

    But the roof losses might be overstated, more than making up for the likely undershoot on calculated wall loss. Attics & roofs have a lower framing fraction than walls, and would come in a bit closer to 1/13, but with the extra air films of attics & vented roofs it'll be less than that. Without better descriptions of the structure it's hard to better estimate U-factors, but Gary Reysa's BuildItSolar heat loss calculator isn't very accurate to say the least. Even though it's WAY better than that eComfort disaster, it's still highly questionable, and I wouldn't use it for anything. A more widely accepted U-factor calculation would use parallel path methods, not center-cavity R, ignoring the rest.

    The infiltration loss of 12K out of a total of 41K is also a bit high- if your house is really that leaky it's worth fixing that. Note, without zero air leakage it would be only 29K using Gary's tool.

    Your real load is probably around 30-32K, maybe 35K if the air leaks are really atrocious.

    A pair of separate 3/4 ton AOU/ARU 9RLS3 Fujistu would more than cover the load at higher efficiency (and probably lower cost) than a pair of 1-ton heads on a MXZ-8C48NA multi-split solution. (If it's not too late and it's still in the crate, maybe you take it back?)

    The -9RLS3 was tested at 12,000 BTU/hr for it's "nominal" or "rated" AHRI capacity, but can still deliver 16,000 BTU/hr @ +17F at max speed, and even more than that at +27F. A pair of them could deliver 32,000 BTU/hr @ +17F, and (without looking it up I'd hazard) more than 35K @ +27F.

    That's considerably more capacity than Mitsubishi's MUZ/MSZ FH12NA, cold climate 1-ton, which was tested at 12,000 BTU/hr @ +47F for HSPF efficiency, but maxes out at 13,600 BTU/hr @ +17F.

    Bumping up to the 1-ton 12RLS3 buys you 17,500 BTU/hr (max) each at +17F, and would probably cover the likely excessive 41K number @ 27F.

    The minimum modulated output of the -9RLS3 is 3100 BTU/hr @ +47F, or 6200 BTU/hr total. That's less than the 7200 BTU/hr of the MXZ-8C, and better yet, they modulate. When the loads are really light you might even turn one off.

    If heat distribution were to be an issue with just two heads, a 9RLFCD or 12RLFCD modulating mini-ducted unit might cover a zone with 2-5 doored-off rooms.

    With the MXZ 8C it won't modulate the output of the individual heads, which will run in a fairly narrow range around their nominal output numbers. Setting the blower speed up/down will change the output somewhat, but it'll always be just changing the overall duty cycle with load. With modulating mini-splits they'll ramp up/down with the load, running almost continuously, with very stable room temperatures. If you're stuck with Mitsubishi but will give you nearly full trade in credit on the MXC, a pair of FH12s or an FH15 upstairs and an FH09 or FH12(or maybe an FH18) might be the better bet.

    Run the heat load numbers separately per floor to get the head sizing right. The upper floor has the roof heat loss, the lower floor does not. And seriously, use a better tool such as (or build your own IBR spreadsheet and calculate the U-factors more accurately.)

  8. jameshowison | | #8

    I'm just some homeowner, but I think the new web-based version of Cool Calc at is a great option, free and comprehensive. Made with modern web tech and works on mobile. Does room by room. You only pay for downloaded certified reports, not needed for design but probably for permits. You can share reports with contractors.

  9. Expert Member
    Dana Dorsett | | #9

    Coolcalc tends to overestimate a bit (they all do), but it's far more consistent than the BuildItSolar IBR tool.

    I recently saw a project where CoolCalc came in at ~140KBTU and LoadCalc came in at ~101K, quite a difference!) but as yet nobody has applied a "real" tool such as Wrightsoft. That was on an uninsulated double wythe brick house, so the underlying U-factor assumptions between the two tools could be off by quite a bit. For a mid-century 2x4 framed house those two tools would normally be within 15-20% of each other, but both would tend to shoot high.

  10. Lita_ | | #10

    In selecting a mini-split, how do you determine an optimal minimum modulated output for your heat load and climate zone (in this case 4C)? For example, if you have a pair of modulating mini-splits, each on its own condenser, would it be worthwhile going with Mitsubishi's MUZ/MSZ FH09NA (min-max heating 1,600 - 18,000 at 47F) to get the very low minimum modulated output, rather than Fujitsu's AOU/ARU 9RLS3 that was suggested (min-max heating 3,100 - 22,000 at 47F), which has almost double the minimum output per head but also a significantly higher maximum output?

    Or is the difference between 1,600 vs. 3,100 minimum modulated output not all that noticeable in terms of comfort & efficiency, especially as you can turn one of the mini-splits off if not needed? How does the equation change if you just have one modulating minisplit head for the house?

    1. Expert Member
      Dana Dorsett | | #11

      The short answer is "yes", lower is always better, as long as it still covers your design load. (I'm a big fan of the FH09 and her younger sister, the FH06.)

      Ideally your heat load at +47F would be above the minimum modulated output of the ductless, so that it can modulate most of the time even during the shoulder seasons rather than cycle on/off. Even though these things are very efficient at their minimum speed, the cycling takes a toll on efficiency. When modulating most decent mini-splits will have a COP of 4 or better when it's in the 40sF or higher outdoors, but if it's doing a lot of cycling that can drop down to the mid to high 2s.

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