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I'm ready to purchase a Mini Split system for the home I'm working on, but this is the first time I have to design and purchase a system from scratch. The Mini Split is the only option for conditioning the air, so please don't suggest another type of system.

I'm working with a frame, vinyl sided home,960 total sq ft. About 450 for the living/kitchen/ dinette open concept. There are 7'6" ceiling throughout.

A 340sq/ft master-bedroom with en-suite and a 120sq/ft second bedroom. The remaining 50sq/ft is second bath and laundry. The homes has R11 insulation in the walls and R13 in the ceilings with a metal roof. Windows are original 1984 single pane with most of them shaded by a carport and porch. I'm in Fla. and need to be sure the tenants will not complain about not being cool enough. Summer temps are mid 90s from May through October. I will need to drop the temp about 20-25 deg most of the year. Heat is needed only about 30 days a year.

I'm thinking I need a triple split system, but I'm unsure of the total BTUs. ( Calculations are varying between 28000 and 36000 BTU) The size of the indoor units is in question in my mind as well.

I am thinking of a unit in the LR, one in the Master and one in the 2nd BR. Is 18000 too big for the main part of the home? Will 9000 be adequate for the master suite? Please understand the walls and ceiling are closed, so adding insulation or replacing windows is off the table.

I have done calculations and the tools I used put things on the borderline, so I am looking for experienced input based on the things I have in place. I don't want to pay for a system that is larger than needed, but also don't want to put in a system too small that will not cool properly. Thanks in advance.

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    Improving the insulation & windows before sizing a system will save you money on buying the system you need, as well as long term operating costs.
    – Ecnerwal
    Nov 25, 2022 at 15:49
  • This is an investment property. I will not be able to get my investment in windows and insulation, returned in less than 10 yrs . Operating costs will be borne by the tenant.
    – RMDman
    Nov 25, 2022 at 16:00
  • If $2000 on windows and/or insulation saves $2001 on mini-splits (because you can buy fewer, or smaller due to less heat load), you get payback instantly. That math only works when contemplating buying a new system, but it does work then.
    – Ecnerwal
    Nov 25, 2022 at 16:10
  • For your comparison: 28000 BTUs + $2900 36000 BTUs + $3600 The math does not add up to $2000. As it is I did not ask for advice on calculating the profitability of rehabbing a home as a long term investment. I do appreciate your time in providing input.
    – RMDman
    Nov 25, 2022 at 16:19
  • Off the top of my head, a 3 ton (36,000 BTUs) system seems overkill for a 960 sq ft house.
    – SteveSh
    Nov 25, 2022 at 16:28

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Add up the square footage of your single-pane windows. Probably throw the door in there too unless you know it's better. Guesstimate R1 for those (1 BTU per hour per square foot per degree Farenheit)

Add up the square footage of your wall area, divide by 11 (or probably less, if it's R11 between studs that are considerably less.)

Add up the square footage of your roof/ceiling area, divide by 13 (or probably less, if it's R13 between rafters/joists that are considerably less.)

Add all those together, multiply by by the maximum temperature differential you want to maintain (so, 30 degrees if planning for 98 out and 68 in)

That gives your base load in BTU/Hr assuming no air exchange/ventilation. Ventilation, intentional or via leaks, will add to the load.

If the roof or attic runs much hotter than the outside air temperature due to insolation, the temperature differential for the roof area will be higher, so the heat flow will be larger. i.e. if the roof or attic (if any) is 120F, then the differential temperature across the R13 roof area is 52, not 30.

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  • Using your formulas, my BTU/Hr load is 14404. Allowing the temp. differential for the entire area to be 52 since the insulation is what it is. So a mini unit the Livingroom/ kitchen of 12000 BTU will work and 9000 BTU units in the bedrooms served by a 28000 BTU heatpump will work?
    – RMDman
    Nov 25, 2022 at 19:36
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    If the load for the whole house comes out 14K BTU/hr then 28-30K BTU/hr is roughly double the load, so dehumidification will suffer, reducing occupant comfort, and ability to run at partial load without stopping and starting will also suffer.
    – Ecnerwal
    Nov 25, 2022 at 20:13
  • Ok, Thanks. The info is now getting useful. Am I correct in assuming I should use 3 indoor units One for the mail living area and 1 each for the bedrooms? I'm concerned that the air will not move to the bedrooms if only 1 large unit is used in the LR.
    – RMDman
    Nov 25, 2022 at 20:49
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    You can use a single interior unit that's mini-ducted (aka slim ducted) to all three rooms (yup, it's still a mini-split) or you can use 3 smaller heads or you can use one big head and separate ducts/fans to move air. 3 smaller heads is the typical mini-split approach. One ducted head might well cost less than 3 normal heads, even with duct installation.
    – Ecnerwal
    Nov 25, 2022 at 21:08

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