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How a mini split works
A mini split is a heat pump in two pieces. The outdoor unit (condenser) holds the compressor, and the indoor unit (air handler) hangs on the wall or sits in the ceiling and blows the air. A copper line set carries refrigerant between them, so there are no ducts. In summer it pulls heat out of the room and dumps it outside. In winter it runs the other way and pulls heat from the outside air into the room.
Every MRCOOL DIY system cools, heats and dehumidifies, and the inverter compressor speeds up and slows down to hold the temperature instead of switching on and off.
Single zone or multi-zone
- Single zone: one outdoor unit and one indoor unit for one room. It comes as a complete system with the line set in the box.
- Multi-zone: one outdoor unit running 2 to 6 indoor units, so each room gets its own temperature. You pick the outdoor unit, then an air handler and a line set for each room.
If you're cooling a bedroom or a garage, single zone is simpler and cheaper. For three bedrooms and a living room, one multi-zone outdoor unit is easier than four outdoor units on the side of the house.
What's new in the 5th Gen
- R-454B refrigerant. It replaces R-410A and has about 78% lower global warming potential.
- Higher efficiency. Up to 23.9 SEER2 depending on size, rated under the newer SEER2 test.
- Cold-climate heat. 5th Gen units keep heating down to −13°F outside. Where winters get colder than that, keep a backup heat source for the coldest nights.
- Smart control built in. The MRCOOL app works over Wi-Fi, with Alexa and Google voice control.
- Same DIY install. Pre-charged Quick Connect line sets mean no vacuum pump, no gauges and no refrigerant handling.
- Multi-zone up to 6 rooms. Outdoor units from 18K to 55K BTU.
What's in the box
A single zone complete system ships with:
- Outdoor unit and indoor air handler
- A 25 ft pre-charged Quick Connect line set with communication wire (sealed, and it can't be exchanged)
- Wall sleeve, drain line extension and wrap tape
- Remote control, wall mounting plate and manuals
Not in the box: the electrical disconnect, the whip from the disconnect to the condenser, a pad or wall bracket for the outdoor unit and a line set cover. We sell all of these in Accessories.
For multi-zone, the outdoor unit, each air handler and each line set are separate items, so you build the system room by room.
Sizing: BTU by square footage
BTU is how much heat a unit can move in an hour. Measure each room (length × width = square feet) and match it to an indoor unit:
| Room size | Indoor unit | Single zone voltage |
|---|---|---|
| Under 150 sq ft | 6K BTU (multi-zone only) | – |
| 150–350 sq ft | 9K BTU | 115V |
| 351–500 sq ft | 12K BTU | 115V |
| 501–750 sq ft | 18K BTU | 230V |
| 751–1,000 sq ft | 24K BTU | 230V |
| 1,001–1,500 sq ft | 36K BTU | 230V |
Go up a size if the room has high ceilings, lots of windows facing south or west, poor insulation, or it's a kitchen. A room that's mostly shaded or well insulated can sit at the low end of its range. Not sure? Call us at +1 (844) 999-9725 with your room sizes and we'll size it for you.
Building a multi-zone system
Pick an indoor unit for each room using the table above, add up the BTUs, then pick the outdoor unit that covers that total.
| Outdoor unit | Rooms | Total area |
|---|---|---|
| 18K BTU | 2–3 | Up to 750 sq ft |
| 27K BTU | 2–4 | 751–1,125 sq ft |
| 36K BTU | 2–5 | 1,126–1,500 sq ft |
| 48K BTU | 2–6 | 1,501–2,000 sq ft |
| 55K BTU | 2–6 | 2,001–2,300 sq ft |
Example: a 12K bedroom, a 9K bedroom and a 9K office add up to 30K, which is more than the 27K covers, so pick the 36K outdoor unit. Every multi-zone outdoor unit runs on 230V.
Indoor units come as wall air handlers (6K to 36K BTU) or ceiling cassettes (6K to 18K BTU). Cassettes sit flush in the ceiling and blow air in four directions, which works well in open rooms or where there's no good wall.
Line sets
Every indoor unit needs its own pre-charged line set, sized to the air handler:
| Indoor unit | Line set size |
|---|---|
| 6K, 9K, 12K | 1/4 × 3/8 |
| 18K | 1/4 × 1/2 |
| 24K, 36K | 3/8 × 5/8 |
Lengths are 16, 25, 35 and 50 ft. Pre-charged lines can't be cut, so pick the shortest length that reaches and coil the extra neatly behind the outdoor unit. For a longer run, join a 50 ft line set with a 16 ft or 25 ft one using a coupler kit, for 66 ft or up to 75 ft max.
Electrical
- 9K and 12K single zone systems run on 115V. Everything else, including every multi-zone outdoor unit, runs on 230V.
- The outdoor unit needs its own dedicated circuit, sized to the breaker rating on the unit's spec sheet.
- Code requires a disconnect within sight of the outdoor unit, and a whip runs from the disconnect to the condenser.
- The indoor unit is powered through the communication wire from the outdoor unit, so you don't need an outlet inside.
The refrigerant side is DIY. If you're not comfortable running a new 230V circuit, have an electrician do just that part. It's a much smaller job than a full HVAC install.
Installing it yourself
Tools you'll need: a stud finder, a level, a drill, a 3-1/2 in hole saw, two adjustable wrenches and a screwdriver. Most single room installs take a day.
- Pick the spots. Mount the indoor unit high on a wall with clear space around it, ideally an outside wall. The outdoor unit goes on a level pad or a wall bracket, with open space around it and within reach of your line set.
- Mount the wall plate. Screw it into studs and level it.
- Drill the wall hole. Use the 3-1/2 in hole saw, angled slightly down toward the outside so condensate drains out. Fit the wall sleeve.
- Run the lines. Feed the line set, communication wire and drain hose through the wall and hang the indoor unit on the plate.
- Set the outdoor unit. Place it on the pad or bracket and keep it level.
- Connect the Quick Connect fittings. Thread them on by hand first, then tighten with two wrenches until snug, following the manual. Don't over-tighten.
- Connect the communication wire at both units, and have the power wiring done to the disconnect.
- Test it. Turn on the power, run cooling and heating, then check every fitting for leaks with soapy water.
- Finish up. Wrap or cover the line set, and connect the app over Wi-Fi.
The full steps and torque values are in the install manual for your model, under Single zone manuals and Multi-zone manuals.
What it costs
With a DIY system you pay for the equipment and skip the HVAC installer's labor bill, which is usually the biggest part of a professionally installed mini split. Plan your budget around:
- The system itself, or for multi-zone the outdoor unit plus an air handler and line set per room
- A disconnect, a whip and a pad or wall bracket
- An electrician for the circuit, if you don't do that part yourself
- A line set cover, if you want a clean look on the outside wall
Shipping is free to the 48 contiguous states. See current prices for single zone systems and multi-zone outdoor units.
Warranty
MRCOOL DIY 5th Gen systems carry a 5 year parts and 7 year compressor warranty, and it covers systems you install yourself. MRCOOL also offers a lifetime compressor warranty option when you register the system and follow their maintenance program. Read the current terms in the MRCOOL warranty (PDF) before you register, since MRCOOL sets the rules.
Care and troubleshooting
- Clean the filters every 2 to 4 weeks during heavy use. Rinse them, let them dry and slide them back in.
- Keep the outdoor unit clear of leaves, snow and plants.
- Not cooling or heating? Check the breaker and the disconnect, confirm the mode on the remote, and clean the filters.
- Water dripping indoors? Make sure the drain hose runs downhill all the way and isn't kinked.
- Error code on the display? Look it up in your model's manual, or call MRCOOL support with the code.
Frequently asked questions
Do I need an HVAC technician?
No. The line sets are pre-charged, so there's no vacuum pump or refrigerant work. You may want an electrician for a new 230V circuit.
Does it heat in winter?
Yes. 5th Gen units keep heating down to −13°F outside. Below that, keep a backup heat source for the coldest nights.
Can I add rooms later?
Yes, if the outdoor unit has a free port and BTU to spare. Plan the outdoor unit for the rooms you'll add later.
Can I cut the line set shorter?
No. Cutting releases the refrigerant. Pick a shorter length and coil the extra.
Can I use my 4th Gen parts with a 5th Gen system?
No. The 5th Gen uses R-454B, so outdoor units, air handlers and line sets have to be 5th Gen.
Why buy from us
We're an authorized MRCOOL dealer, so every system comes with the full manufacturer warranty. Shipping is free to the 48 contiguous states, and real people answer the phone Monday to Friday, 9 AM to 5 PM EST, at +1 (844) 999-9725.