GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Yet another minisplit sizing question

kevperro | Posted in Mechanicals on

I’m building a new home in Washington state (Seattle area, Marine 4C) and it should be fairly well built as I’m keeping the design simple, with rectangular dims and minimizing penetrations in roof/wall/floor. The builder claims “energy efficient” methods but they all do. Washington does require fairly progressive standards and a blower door test is standard fare. I won’t have results until the building is done and I’m planning for my mini-split now.

The building is 2800 ft^2, two story, R-21 walls, R-49 roof, R-30 floor, 0.30 windows. I’ve limited door/window area to around 280 ft^2 and I’m not installing a ducted system so there are no issues with duct leakage/pentetration into the shell. The house is on a crawl space, which I will eventually seal and run it with mechanical ventilation under a negative pressure relative to the living space. I’ll of corse mechanically vent into the outdoor heat pump. 🙂

I have considered running duct work to the bedrooms (all upstairs) and running a single panasonic inline fan to continuously circulate air from the bedrooms to downstairs (eventually into the crawl) I’ve considered this for two reasons. First, the main limitation of ASHRA 62.2 with a single point exhaust is the limitation of NOT necessarily pulling the air out of the locations where you need to dilute it. Second, I want to use one, perhaps two mini-splits on the entire structure and I want to promote even temp distribution.

By code I have to use the single zone electrical heaters, which will be my primary heat until the inspectors are gone. Once they are out of the way I’ll install a single mini-split downstairs, a second smaller capacity upstairs. I’m strongly considering the highly efficient Mitsubishi MSZ-FH (probably the 12,000 flavor). Most of our energy use is for heating but our weather is mild, rarely below freezing and when it is for short periods of time.

So…long diatribe here but I’m looking for confirmation on sizing. I think I can get by with the 1-ton downstairs as my primary heat source. The house has opening in the stairwell going up and an open area in the entrance where heat will naturally rise into the building. Upstairs I was seriously considering buying a cheaper Gree unit because it’s primary purpose will be cooling and not as mission critical as the downstairs unit which will be my primary heating source.

Any input or help is much appreciated. I’ve installed mini-splits in two different houses on my own so I’m fairly confident with the install. What I primarily need reassurance on is calculation of needed size.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. jackofalltrades777 | | #1

    With a Zone4c climate I believe a 1-ton unit servicing 2 floors and almost 3,000 sqft is NOT doable. Unless you have built it to Passive House standards and the home is utilizing passive solar heating, a single 1-ton unit will not work. I believe at a minimum you will need TWO units, one for each floor, or 2-tons total for it to provide proper heating/cooling.

    2,800 sqft (2 story) with only 12,000 btu is beyond pushing it. A general rule of thumb (yes, I know, we hate rule of thumb) is that for a super tight energy efficient home, it requires 1 ton for every 1,000 sqft of space. A PH can pull it off but your home is not a PH.

  2. GBA Editor
    Martin Holladay | | #2

    A few comments:

    1. You can't size a heating or cooling system without performing a Manual J heating load and cooling load calculation. This should be standard procedure and is required by code. If you can't do the calculations yourself, you should hire a qualified mechanical engineer or energy consultant to perform the calculation. For more information, see How to Perform a Heat-Loss Calculation.

    2. You may want to read this article: Rules of Thumb for Ductless Minisplits.

    3. Although the topic of your question is ostensibly about designing your heating and cooling system, you have included information and questions about designing your mechanical ventilation system. It seems to me as if you may be confusing these two unrelated topics. Once you have designed your heating and cooling system, you can then design your ventilation system. There is no reason that these systems should share the same ducts, and their functions are entirely separate -- so don't confuse them. For more information on ventilation, see Designing a Good Ventilation System.

  3. STEPHEN SHEEHY | | #3

    Kevin. Do the blower door test early enough in the process that you can use the results to improve the airtightness. Once the house is done, your options are limited.

  4. kevperro | | #4

    Thank you both. For clarity perhaps I should outline that the house is designed with electric zoned heating to meet the code mandated heating load. What I'm looking to do with the mini-splits is to provide the heating for 80-90% of the year when we are WELL above design temps (23-27 deg in our area). Our temps do not vary much so I feel pretty comfortable looking at heat needs above these design temps and relying upon the electric resistance heating to temper the extremes.

    There is precedence for this in the BPA studies.

  5. GBA Editor
    Martin Holladay | | #5

    You can use any outdoor temperature you want when you do your Manual J calculations. But I still recommend that you perform a heat loss calculation.

  6. Expert Member
    Dana Dorsett | | #6

    Can't really tell much about the room-by-room heat loads if you're only giving us the total window area. If there are areas on the first floor that can be doored off, it's not a big efficiency hit ton install a Fujistu RLFCD series unit for the first floor installing in the crawl space with short duct runs, if you seal & insulate the crawlspace walls (pour the foundation in insulated concrete forms!)or put the R30 floor insulation below the ducts & mini-duct cassette:

    The 1-ton unit has an HSPF of 11.5, the 3/4 ton is good for 12.2. That compares to 12.5 for the -FH12NA 1-ton:

    But if you're using the resistance heaters for temperature balance it's unlikely you'd do as well as the 11.5 HSPF you'd get with the -12RLFCD.

    You may be able to pull off something similar with another mini-duct cassette for the second floor.

    Your insulation & window parameters are essentially IRC 2012 code-minimum for zone 4C. The amount of temperature balancing you get out of the ventilation system in a code-min house is extremely limited at best- not a good strategy. If you had U0.18 windows and R30+ whole-wall (all thermal bridging accounted for, not center-cavity R) walls that strategy would have at least some benefit, but in a code-min house it's completely useless.

    If it's not too late in the design process, it's well worth going better than code-min. A good starting place can be found in table 2, page 10 here:

    Note, those recommendations are all "whole-assembly" R values, not center cavity.

    The whole-wall R of 2x6 /R21 construction is about R14 (or R15 if 24" o.c. studs with advanced framing techniques), but with an additional 2.5" of exterior EPS you'd hit the recommended R25 wall for zone 4 in Table 2 . If you poured the foundation/crawlspace walls with a 2.5" +2.5" insulated concrete form (the lowest-R for many ICF vendors) you would be above the recommended R15 for foundations, but you could position the 2x6 framing directly on the edge of the foundation, and have a continuous R10 thermal break over the whole shebang, and you can skip the R30 floor insulation.

    Then for the crawlspace floor 2" of EPS under a 6-mil poly vapor retarder and a 2" rat-slab brings the crawl fully inside of conditioned space, no need to drive air-infiltration by de-pressurizing it. A miniscule amount of exchange with conditions-space air is more than enough to avoid stagnation issues.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |