What Size Ductless Mini Split AC Do I Need?

July 09, 2018

Unlike central air conditioners, which are sized to cool an entire home, ductless ACs are sized to cool only one room/area.

But here's the deal:  Without a technician looking at the room you want to cool, it's impossible to to know exactly what size ductless mini split system you need.

You see, a professional needs to perform a “load calculation” to determine mini split AC size. A load calculation takes into account many factors like...

  • Square footage of the room

  • Insulation of the room

  • Orientation of the room

  • And much more (we'll cover that later on)

But you still probably have lots of questions about ductless mini split size.

In this article, we'll explain:

1) How ductless units are sized

2) Why correct size matters

3) What a “load calculation” is


Need a professional to size your ductless AC for you?


How ductless units are sized

Ductless units are sized according to how much heat they can remove from a room in an hour. This amount of heat is measured in BTUs (British Thermal Units). One BTU is roughly equivalent to the amount of heat produced by burning through one match.

So you’ll often see ductless units referred to by their “BTU output” (i.e. “a 15,000 BTU-output ductless unit”). Ductless BTU outputs can range anywhere from 9,000 to 48,000 BTU.

Now, before we talk about how you can determine the BTU output you need, let’s answer the question we get all the time from Denver homeowners: Is it really that bad if I get a unit that’s too big or too small?

Does correct size really matter for a ductless AC?

Yes. An undersized or oversized ductless mini split means lower comfort and lots of money wasted.

We’ll explain…

A ductless unit that’s too small for the room…

...will run non-stop trying to cool a room that’s too big for the unit to handle.

And a unit that runs non-stop will:

  • Skyrocket your energy bills

  • Need frequent repairs

  • Die sooner than expected

A ductless unit that’s too big for the room…

...will the cool the room quickly then shut off.

This quick ON/OFF pattern (called short-cycling) will cause:

  • High humidity levels in the home

  • Hot/cold spots throughout the room

  • High energy bills

  • Frequent repairs

  • Lowered lifespan

So now that we know correct size matters, let’s look at how a professional load calculation works and how it ensures that you get the precise ductless size you need.

How a “load calculation” works

To determine the ductless AC size you need, a professional should always perform a load calculation instead of only looking at the square footage of your room.

During this load calculation, a professional will measure the amount of heat that enters a room based on factors like:

  • How many windows does the room have?

  • How many doors does the room have?

  • Is the room a kitchen space or close to the kitchen?

  • What’s the climate of the area?

  • What’s the square footage of the room, not including closets?

  • Does the room get direct sunlight? If so, how much and for how long?

  • How well insulated is the room?

  • What height are the ceilings?

  • Are there any additional heat sources in the room?

  • How many lightbulbs are in the room? What kind of lightbulbs are they?

  • What is the usual number of occupants in this room?

  • What kind of flooring does the room have: carpet or hardwood?

This precise calculation allows a professional to determine the exact BTU output you need from your ductless mini split—no more, no less.

Most quality professionals will include a load calculation in the initial installation estimate.

That said, if you’re in the middle of choosing a professional to install your ductless unit, stay away from any tech that sizes your ductless unit over the phone (using only square footage). Only looking at square footage often results in the wrong ductless AC size.

For example:

Let’s say that you have a 900-square-foot room.

If you were to only use square footage to size your ductless unit, you’d end up with a unit that has a 16,000 to 17,000 BTU output right?

Now let’s take a closer look at that 900-square-foot room. Let’s say that the ceilings are 10 feet high and about 90% of the east-facing wall is covered in poorly-insulated windows.

After taking into account those additional factors, a professional would likely increase the needed BTU-output by about 50%, which leaves you with a unit that has a 24,000 to 25,000 BTU output.

(And that’s a big difference.)

Need a load calculation for your ductless unit? Ask our Denver techs

We offer FREE load calculations and upfront, honest pricing.

Get your ductless unit sized for FREE

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