Quick answer: Most basements need a 9K to 24K mini split. A 500 sq ft basement fits a 12K, which MRCOOL lists for up to 550 sq ft. An 800 sq ft space fits an 18K. Also check your wall height. MRCOOL wants at least 5.9 ft below a wall unit and 5.9 inches above it.
Basements stay cool in winter, and many have no heat at all. A mini split can heat the space in winter and cool it in summer. This guide covers the size to buy, the wall height you need, and how to handle the drain and line set below ground. For any room, our mini split sizing guide explains the full method.
What size mini split do I need for my basement?
Start with the square feet. Multiply the length of the basement by its width. A space that is 20 ft by 25 ft is 500 sq ft. Then match it to the chart below.
Our sizing guide uses the ENERGY STAR room chart. It lists 12,000 BTU for 450 to 550 sq ft and 18,000 BTU for 700 to 1,000 sq ft. MRCOOL also lists an "up to" coverage for each DIY 5th Gen size:
- 9K: up to 400 sq ft
- 12K: up to 550 sq ft
- 18K: up to 800 sq ft
- 24K: up to 1,050 sq ft
- 36K: up to 1,550 sq ft
These are best case numbers. Basements with bare concrete walls, drafty rim joists, or lots of windows can need more. For a basement that will be the main living space, run our free Manual J calculator. It looks at your climate, insulation, and windows. For a quick number for one room, try the mini split sizing calculator. Sizing a garage instead? See our garage and workshop sizing guide.
How much wall height does a basement mini split need?
This is the step most basement buyers miss. MRCOOL's 5th Gen DIY manual lists these clearances for the wall unit:
- At least 5.9 ft from the floor to the bottom of the unit
- At least 5.9 inches from the top of the unit to the ceiling
- At least 4.72 inches on each side
Add the height of the unit itself, and you get the wall height you need. Chill's product pages list the indoor unit heights. The 9K and 12K units are 11.69 inches tall. The 18K and 24K units are 13.19 inches tall. The 36K unit is 14.25 inches tall.
Here is the math for a 12K: 70.8 inches (5.9 ft) plus 11.69 inches plus 5.9 inches equals about 88.4 inches. That is about 7 ft 4.5 in of wall from the floor to the ceiling or the bottom of a beam or duct. Measure your lowest point, not just the open middle of the room.
Basement size chart: which MRCOOL DIY size fits?
Here is how square feet, BTU, and wall height line up for a basement.
| Basement size | Chart BTU (ENERGY STAR) | MRCOOL DIY size (listed coverage) | Wall height needed for a wall unit (our math) |
|---|---|---|---|
| 300 to 400 sq ft | 8,000 to 9,000 | 9K (up to 400 sq ft) | About 7 ft 4.5 in |
| 450 to 550 sq ft | 12,000 | 12K (up to 550 sq ft) | About 7 ft 4.5 in |
| 550 to 800 sq ft | 14,000 to 18,000 | 18K (up to 800 sq ft) | About 7 ft 6 in |
| 800 to 1,050 sq ft | 18,000 to 21,000 | 24K (up to 1,050 sq ft) | About 7 ft 6 in |
| 1,050 to 1,550 sq ft | 21,000 to 30,000 | 36K (up to 1,550 sq ft) | About 7 ft 7 in |
Sources: ENERGY STAR chart as shown in our sizing guide, MRCOOL coverage and unit heights from Chill product pages, and clearances from the MRCOOL 5th Gen DIY manual. Wall heights are our own math from those numbers.
What are my options for a low basement ceiling?
A low ceiling does not rule out a mini split. It changes which indoor unit fits best.
A wall unit is the simplest choice when you have the height. The MRCOOL DIY 12K 5th Gen runs on 115V and lists coverage up to 550 sq ft. The 18K 5th Gen runs on 230V and lists up to 800 sq ft.
A ceiling cassette sits up between the joists, so it does not need the 5.9 ft wall clearance. Many basements have open joists or a drop ceiling, which can make this easier. The MRCOOL DIY 12K Hyper Heat ceiling cassette system lists coverage up to 550 sq ft, and its cassette is 8.98 inches tall. MRCOOL designed the DIY cassette to fit between joists on 16 inch centers. Read our ceiling cassette fit checklist before you buy one.
Will a mini split heat a cold basement in winter?
Yes, a mini split is a heat pump, so it heats as well as cools. MRCOOL's manual lists heat mode for room temperatures from 32°F to 86°F. The standard 5th Gen heats in outdoor temperatures down to -13°F. The Hyper Heat models heat down to -22°F, per their Chill product pages.
Live where winters get very cold? Look at the MRCOOL DIY Hyper Heat systems. Our cold climate comparison shows how they stack up against the standard 5th Gen.
How do I handle the drain and line set in a basement?
Basements add two extra steps.
The drain. A wall unit drains by gravity. MRCOOL says to drill the wall hole at a slight downward angle, about 0.2 to 0.275 inches lower on the outside. The drain needs to flow down and out from there. Part of a basement wall sits below the ground outside. Place the wall hole high enough that the line set and drain exit above the ground. Where gravity will not work, you may need a condensate pump. Our condensate drain guide covers gravity drains and pumps. Chill's what comes with a MRCOOL DIY guide notes that the current 5th Gen ceiling cassette has a built in condensate pump.
The wall hole. Many basement walls are concrete or block. MRCOOL's manual calls for a 3.54 inch core drill for the wall hole. For a concrete or brick wall, drill 0.2 inch holes and use the sleeve anchors that come with the unit to hold the mounting plate.
Keep it dry. MRCOOL says not to run the unit in a room exposed to a lot of water, such as a bathroom or laundry room. Keep the indoor unit away from a basement laundry area or a wet corner.
Do I need more than one zone for a basement?
One indoor unit cannot push air into rooms behind closed doors. A basement with a bedroom, an office, or a gym behind walls may do better with a multi zone system. That means one outdoor unit with an indoor unit in each room. Browse the MRCOOL DIY multi zone systems, and see our guide on what to buy for a 2 zone DIY mini split.
Still shopping?
See every size in our MRCOOL DIY 5th Gen mini splits collection, or check the Hyper Heat and multi zone collections above. Want help matching a system to your basement? Call Chill Mini Splits at (888) 231-8920.
FAQ
What size mini split do I need for a 500 sq ft basement?
A 12K is the usual fit. The ENERGY STAR chart in our sizing guide lists 12,000 BTU for 450 to 550 sq ft. MRCOOL lists the DIY 12K 5th Gen for up to 550 sq ft. Size up for bare concrete walls or lots of windows.
What size mini split do I need for a 1,000 sq ft basement?
The chart lists about 18,000 to 21,000 BTU for that size. The MRCOOL DIY 24K lists coverage up to 1,050 sq ft. A basement split into rooms may do better with a multi zone system.
How high should a mini split be mounted in a basement?
MRCOOL's 5th Gen manual calls for at least 5.9 ft from the floor to the bottom of the wall unit. It also calls for at least 5.9 inches above the unit. For a 12K, that adds up to about 7 ft 4.5 in of wall.
Can a mini split heat a basement in winter?
Yes. A mini split is a heat pump. MRCOOL lists heat mode for room temperatures from 32°F to 86°F. The standard 5th Gen heats down to -13°F outside, and Hyper Heat models heat down to -22°F.
What if my basement ceiling is too low for a wall unit?
A ceiling cassette is one option. It sits between the joists, and the MRCOOL DIY cassette is 8.98 inches tall. It is made to fit joists on 16 inch centers. Check our cassette fit checklist first.
Do I need a condensate pump for a basement mini split?
Only where water cannot drain down and out by gravity. A wall hole that exits above the ground outside often works with gravity. The current MRCOOL DIY 5th Gen ceiling cassette has a built in pump.
Who can answer install questions about my MRCOOL unit?
MRCOOL tech support can help with install and setup questions at 270-366-0457. You can also find the full install manual on MRCOOL's site.
