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We have a 14x15.5 family room addition on a slab. For summers we've used a window a/c. The room connects to the kitchen via a small 4.5 x 4.5 foyer. It has a doorway with the door removed. The rest of the house is central air. We're thinking about getting a mini split a/c unit. Given the space what btu should we get? What pros/cons would there be for a this type of unit for this setup? Is there a concern with leaving the door open with the both house a/c and mini split unit on?

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  • BTU per square foot heating rule of thumb is from 30 BTU to 60 BTU per square foot.
    – Traveler
    Feb 17, 2023 at 21:47
  • Interesting decision. I don't tend to think about minisplit except as whole-house systems.
    – keshlam
    Feb 17, 2023 at 22:32
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    What's the units, meters or feet?
    – user71659
    Feb 18, 2023 at 6:46
  • I manage just fine on 12.8 BTU/Hr/SqFt. Rules of thumb are a route to error and oversized, inefficient systems.
    – Ecnerwal
    Feb 18, 2023 at 12:03

2 Answers 2

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Sizing: TBD

To get an accurate estimate of the capacity you need, you'd need more information that just the size of the room. For example, how many walls are exterior vs. interior, how much glass is there, which way does the room face, and what climate are you in? Do you want cooling only, or do you also need heat in the winter?

For a general starting point, what was the BTU capacity of your old window unit and was that adequate, or did it leave you wanting?

Playing nice with Central Air

As long as you are using both units to do the same thing (i.e. you're not trying to cool the house with central air while using the heat pump to run a hot yoga studio in the addition), then both systems will complement each other just fine. The conditioned air leaking in through the open doorway from the rest of the house will lessen the cooling load on the mini split, and vice-versa. I commonly see mini-splits used for supplemental heating and cooling in additions where extending ductwork is simply impossible or cost-prohibitive. The only real downside (and really just a minor annoyance) is that the controls aren't easy to integrate. Once you get used to having separate controls for the house system and the mini split, you're fine -- just don't walk away with the remote for the addition and forget where you left it. :)

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Size of the room is a terrible method. Sure, it's what window air conditioners "guide" you with, but it's utterly inaccurate, because 3 rooms of the same size might be poorly insulated, at some "average" level of insulation, or well-insulated. And any one of those general classifications can differ in detail from another in the same classification.

So, you need to assess, or assess and address (because if you improve insulation, you can buy a smaller unit, and any unit will use less power over a long time) the insulation in the room and your cooling (and heating, unless you get a crippled unit) needs of the space after you have that information. That's as simple or difficult as adding up all the areas of all the parts built with the same level of insulation, and multiplying by the conductance (inverse resistance, which is "R-value", so 1 divided by R-Value) and multiplied by the maximum temperature differential you need to maintain, in order to get cooling and/or heat required per hour for that area and insulation. Then you add up all the heat-flows of all the parts and know the total cooling/heat required per hour at the "design temperature" which is the temperature resulting in the maximum differential you intend to meet.

If you're doing this anywhere close to right, you'll see that windows take a lot of cooling or heating.

When you don't meet the load due to an unusually hot or cold day you didn't design for, the space runs warmer in cooling season or cooler in heating season at the maximum differential from outside temperature the unit you have can maintain, if it can't get all the way to your set temperature.

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