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

How do I determine the size of a ductless minisplit?

Thompdon12 | Posted in General Questions on

How do I determine the size of a ductless mini-split Heating and Cooling for my new to me Cottage type home built in 1925.

It is 36 feet wide by 26 feet deep. It is an open ceiling type to the roof rafters about 12ft high at the center peak with R-19 insulation, the walls are R-13, Two inside rooms/divider walls with no ceilings (open concept).

The two side outside walks and the front wall get no sun there Is total of 9 double pain windows on these walls the back wall 30ft long has 10 single pain wood frame windows that slide open from right to left.

I would need to heat the basement which is the same size but with 7 foot ceiling 3 of the walls are underground Level the back wall is above ground at 30 ft long and all single pain windows

The basement just needs to be kept above freezing 40-50 degrees?

I am in north east Pennsylvania the winters average 20-35 degrees but dip down to 0 sometimes

I was told I need 24,000 btu for the upstairs and 9,000 for the basement dose this sound correct.

The only other heat source is a fireplace.

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. Thompdon12 | | #1

    Sorry it is 30 ft wide not 36 ft

  2. GBA Editor
    Martin Holladay | | #2

    There are two steps to this process.

    Step 1: You perform a heating and cooling load calculation. This is usually done with software, but paper-and-pencil methods work, too (especially for heating load calculations -- not so much for cooling load calculations). The most common method using software is called the Manual J method. For more information on these topics, see these four 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

    Calculating Cooling Loads

    Step 2: You need to select your equipment. This task is explained in the following article: How To Buy a Ductless Minisplit.

  3. Dana1 | | #3

    A calculation to get the numbers right are critical to getting the performance out of a mini split. That said, a typical ~1000 square foot house with clear glass storm windows and a full basement would come in at about 15,000 BTU/hr @ 0F, but could be as high as 20,000 BTU/hr. If it's over that there is usually a LOT of low hanging fruit on the building upgrades front. Most decent 18,000 BTU/hr mini-splits would cover the load for the first floor. The 10 single-pane sliders might boost it that high, but do the math. Low E storm windows over most of them (the ones you don't really need to open) is probably called for. The cooling load is unlikely to be over 12,000 BTU/hr unless it has a LOT of west facing glass.

    If the basement only needs to be kept above freezing, that can be achieved by insulating the foundation walls and adding low-E storm windows to the single-panes. That's where the basement mini-split money is better spent. The actual heating load would probably under 5000 BTU/hr, but the warmth of the conditioned space above would keep it well above freezing (as long as you keep the windows closed. :-) )

  4. Thompdon12 | | #4

    Thanks For the replies guys

    Dana are you saying something like this or us that to big. That would be 15k upstairs and 9k in basemrnt

  5. Dana1 | | #5

    That particular model might not cut it even at 10F outdoors- you really DO have to do the heat load calculations, and look at the extended capacity tables. The submittal sheet only spells out 13,750 BTU/hr for the 15K head in a 9K +15K combination with that compressor. That is probably it's maximum output @ +17F (though it could be somewhat higher), and capacity falls off with outdoor temperature. Your 99% outside design temp is probably below +10F. (Scranton's 99% temp is +7F)

    A separate -FH15NA (or -FH18NA) plus -FH09NA (- or FH06NA) would probably cut it (but seriously, do the math).

  6. Thompdon12 | | #6

    Hi Dana

    I found a chart onine that I followed its called Accu-Size heating and cooling home analysis.

    According to the chart assuming I did it correctly it was pretty easy to follow fill in the blanks with the measurements. It says I have a 35405 BTU heat loss on the first floor alone. Does that mean I need a 35000 BTU outside unit?

  7. Dana1 | | #7

    The Accu-Size Heating & Cooling Home Analysis ( ) is a very crude quick & dirty I=B=R method load tool, set up for an assumed 0F outdoor design temperature and a 70F indoor temperature. That is probably more than 5F cooler than your 99% outside design condition, and a couple of degrees north of a code-min 68F indoor temp, adding something on the order of 10% to the design load just from that single error alone.

    Looking at your Accu-Size sheet, how much of the heat loss is from windows? The U-factors they used for windows is a crazy-high ~U 1.0 for double panes, and U1.4 or single panes. The U-1.4 might be real if it's the world's crummiest uninsulated aluminum sashed aluminium framed window, but a wood-sash single pane is about U1.0, and even a pretty lousy wood-sash or vinyl clear glass (no low-E) double pane would be about U0.7 or lower. An antique wood sash single pane with clear-glass aluminum triple-track storm is about U0.5, as is most 3/4" thick clear glass double panes.

    If history is any guide the numbers you would get from that sheet would overshoot reality by 25-50%, and it could be a more That amount of error would be fine for a hot-air furnace, but is squishier than ideal for a ductless heat pump.

    Using realistic U-factors & outdoor design temperatures, and infiltration losses (which can almost always be cost-effectively reduced by quite a bit), and adjusting for internal heat sources such as live mammal occupants, (225-250 BTU/hr per, even while sleeping) and 24/7 plug loads such as refrigerators (250-300 BTU/hr for a pretty good Energy Star version 400-500 BTU/hr for a 25 year old version) , cable boxes & DVRs (50-250 BTU/hr depending on model, sometimes more) you're probably looking a true heat load under 20,000 BTU/hr, at 68F indoors, +7F outdoors, under 15,000 BTU/hr if it's reasonably air tight.

    The BTU ratings of the outdoor units refer to the rated cooling output, not it's heating output. To find out the heating output you need to at the very least refer to it's HSPF/SEER submittal sheets such as those linked to in response #5. If you take just one of them, say the FH15, that is a 15,000 BTU/hr nominal cooling compressor:

    But it's "rated" ( the output at which efficiency was tested, not it's maximum) heating output at +17F is 18,000 BTU/hr @ +17F, which is 3000 BTU/hr higher than the 15,000 BTU/hr cooling rating of that compressor.

    According to the submittal sheet it can still deliver 18,000 BTU/hr at +5F, and is probably a reasonable fit for your true 99% load, but it wouldn't be crazy to go for the 1.5 ton version in that series (which puts out over 20,000 BTU/hr @ +5F.)

  8. Thompdon12 | | #8

    Thanks Dana for your input I appreciate it.

    So if I went with the MUZ-FH18NA can I run both the 18000 BTU and the 9000 BTU indoor units on that one outdoor unit.

    If you where going buy a dual unit for yourself what manufacture and units would you get

  9. Thompdon12 | | #9

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |