Ductless mini split systems seem to be everywhere now. These efficient, cost-effective systems make heating and cooling a breeze. No pun intended. But only if they're running well. These systems are often tailored to DIYers and people who are cost conscious. But, if you're taking on a project like this, you'll end up wondering "what size mini split do I need?" or "should I size up or down?" With advancements in technology and efficiency gains in these systems, these answers change all the time. Trying to size your mini split without the most recent and accurate information is impossible, and more than that, it can cost you big.
Mini split specifications and descriptions probably have you asking yourself, "what does BTU/h mean?" This is what we use to rate the heating and cooling capacities of AC units. It stands for British thermal units per hour. The higher this number, the higher the capacity of the unit. In other words, the more BTUs, the more square feet you can cool/heat.
How Many BTUs Do I Need?
BTUs and mini split sizing go hand in hand. The right size will make all the difference for climate comfort, efficiency, and reliability. This is why in the HVAC industry we are constantly improving our estimation techniques and methods to help answer the question "what size mini split do I need?" Before we get into how to size a system correctly, we'll look at why sizing matters.
Problems with Oversized Mini Split Systems
A ductless mini split that is too large for a space will lead to short cycling — which is when the system turns on and off too frequently. In these cases, the unit is "too good" at cooling or heating a space, causing constant restarts. Which means more stress on system components and increased energy costs. In the long run, this leads to frequent repairs and shorter system lifespans.
Problems with Undersized Mini Split Systems
Mini splits that are too small have the opposite problem. Undersized mini splits often can't heat or cool the space to the set temperature. A unit that's too small might run excessively or almost nonstop without ever getting to the set temperature. This will lead to expensive electricity bills and over time, these undersized systems will be less reliable and will have issues sooner than a properly sized unit.
How to Size Your Unit
In general, sizing your mini split depends completely on how well suited you want your system to be for your home. You can take a lot of variables into account to be as accurate as possible, or just a few to get a rough estimate. But how do these estimates work? Below is a series of questions that will help you estimate your heating and cooling needs. (For the most accurate estimates, ask one of your local experts to do a "Manual J Load Calculation," or use our calculator)
Easy Sizing Method
You Will Need to Know:
- Square feet
- Primary use (heating or cooling, if both use heating)
If primary use is cooling: square feet × 22 = required BTU capacity
If primary use is heating or both: square feet × 30 = required BTU capacity
Examples
Cooling Example: 500 sq feet × 22 = 11,000 BTU minimum
Heating Example: 500 sq feet × 30 = 15,000 BTU minimum
This is your approximate BTU requirement.
Heating is typically less efficient than cooling, so that's why heating requires more BTUs.
Your easy estimate is done, please be cautious as this is a rough estimate.
You would want to purchase a unit with a BTU capacity close to this measurement. For those that want a more accurate estimate, you can use these additional questions.
Advanced Sizing Method (Additional Questions)
You Will Need:
- Age of home
- Ceiling height
How Old Is Your Home?
- 10 years: +2.5% (to original BTU estimate)
- 20 years: +5%
- 30 years: +10%
- 40 years: +15%
- 50+ years: +20%
How High Are Your Ceilings?
- 8 ft: +0% (add in addition to newly calculated estimate)
- 10 ft: +5%
- 12 ft: +10%
Example
11,000 BTU 20 years old = 11,000 + 5% = 11,550 12 ft ceilings = 11,550 + 10% = 12,705 BTU total
This is a better estimate than most people get, but keep in mind it's not perfect. For the most precise calculation please use our free calculator, or contact a local HVAC professional.
Additional Factors That Affect Mini Split Sizing
Ceiling Height
Rooms with ceilings that are higher than 8 feet have more air volume and might need a higher-BTU unit.
Insulation
Newer homes, or homes with good insulation, can keep conditioned air inside the home better. These homes may need fewer BTUs.
Seasonal Climate
At extreme temperatures, mini split efficiency can drop, requiring more BTUs.
How Many Rooms Can a Mini Split Cool?
A single indoor air handler should be used for one room or space, but a single outdoor condenser can power many air handlers. Check out our multi-zone systems to cool multiple rooms with one outdoor unit.
