Cassettefrom $1,312
How do you plan a MRCOOL DIY 4 zone mini split?
Short answer: A MRCOOL DIY 4 zone mini split is a DIY 5th Gen multi-zone system: one outdoor unit (27K, 36K, 48K or 55K) feeding four indoor units, each with its own line set. Complete 4-room wall systems start from $4,836 (prices as of November 2026) with a 16 ft line set per room. Choose the outdoor unit for the rooms that run at the same time, and keep all four line sets under 221 ft combined.
Four rooms is where a lot of whole-house plans land: a living area and three bedrooms, or two bedrooms, an office and a finished basement. This MRCOOL DIY 4 zone mini split guide covers which outdoor unit suits which mix of rooms, wall units vs ceiling cassettes, line set planning, and what the job involves.
What comes in a 4-room system?
A complete 4-room wall system includes:
- The outdoor unit, sized 27K, 36K, 48K or 55K.
- Four wall-mounted air handlers in the sizes you pick, from 6K up to 24K.
- A 16 ft pre-charged Quick Connect line set for each room. Longer sets are chosen separately.
Your order is made of separate parts, so it arrives as several boxes: one outdoor unit, four indoor units and four line sets. Check every box before you sign for the delivery. If one box is damaged, refuse only that one and keep the rest.
No system includes the outdoor accessories. Plan on a disconnect switch ($59), a non-metallic whip ($72), an equipment pad ($172) or wall bracket, plus the wire and breaker from your panel.
Which outdoor unit fits which mix of rooms?
All four DIY 5th Gen outdoor units that can serve four rooms are shown below, with a sample room mix for each. Size the rooms first (6K for a small bedroom or office up to roughly 250 sq ft, 12K for a living room of about 350 to 550 sq ft), then match the total you'll run at once.
| Outdoor unit | Rooms it serves | Sample 4-room mix | Good fit for |
|---|---|---|---|
| 27K | 2 to 4 | 6K, 6K, 6K, 9K | A cabin or small house with four small rooms |
| 36K | 2 to 5 | 12K, 9K, 9K, 6K | A ranch with a living room and three bedrooms |
| 48K | 2 to 6 | 18K, 12K, 9K, 9K | A great room plus bedrooms, or four 12K rooms |
| 55K | 2 to 6 | 24K, 12K, 9K, 9K | A big open main floor, or a cold climate where all four rooms heat at once |
The indoor units can also add up to more than the outdoor unit. Put two 12K and two 9K rooms on a 36K outdoor unit and you have 42K of indoor units. When only some rooms run, each gets its full rating. When all four run hard on the hottest or coldest day, they share the 36K and the outdoor unit never goes over it, so each room gets a bit less than it asks for.
That's fine if the bedrooms rest during the day and the living room rests at night. It's a poor fit if all four rooms run full blast for weeks every winter. Our multi-zone explainer walks through the sharing with a 27K example.
This 27-second overview shows a whole-home setup, handy for picturing one outdoor unit feeding several rooms before you plan yours.
Our 4-room listings come in preset room mixes. If the mix you have in mind isn't on the 4-room systems page, call us and we'll check it for you.
Is one 4-room system better than four single zones?
Here's how the numbers compare for four small rooms with 9K units:
| Setup | Price, from | Outdoor units to mount and wire |
|---|---|---|
| Four DIY Select 9K single zones | $4,996 | 4 |
| Four DIY EasyPro 9K single zones | $5,948 | 4 |
| Four DIY 5th Gen 9K single zones | $9,316 | 4 |
| One DIY 5th Gen 4-room wall system | $4,836 | 1 |
The multi-zone puts the 5th Gen line in every room for about the price of four Select single zones, with one outdoor unit, one pad, one disconnect and one circuit to run instead of four.
There are trade-offs on both sides. Four single zones keep working one at a time if a unit fails, and each room can heat or cool on its own. On the 4-room system, every room runs in the same mode, and if the outdoor unit stops, all four rooms stop with it.
If a fifth room is on your wish list, price it now. Five-room wall systems start from $5,963, against $4,836 for four rooms, and a fifth room can change which outdoor unit you need.
Should you use wall units or ceiling cassettes?
Wall air handlers are the standard choice and the cheapest, from $457 each as separate parts. They hang high on a wall and need a 3 in hole behind them for the line set.
A one-way ceiling cassette sits nearly flush in the ceiling and starts from $1,312. It comes in 6K to 18K, and it needs space above the ceiling, usually an attic or open joist bay, for the unit, the line set and the drain.
A full cassette system costs a lot more, with 2-room cassette systems already starting from $4,633. A common middle ground is a cassette in the living room, where looks matter most, and wall units in the bedrooms. That mix is built from separate parts, so ask us to check the combination before you order.
Next: when is a ceiling cassette worth the extra cost?
How do you plan four line sets within 221 ft?
The MRCOOL 5th Gen multi-zone manual sets three limits for the 4- to 6-room outdoor units:
- Up to 75 ft of line set for any one room.
- No more than 221 ft for all line sets together.
- A height difference of 24 ft at most between indoor units.
Here's a typical plan for a two-story house, with the outdoor unit at the back:
| Room | Measured path plus slack | Line set to order |
|---|---|---|
| Living room, first floor | 14 ft | 16 ft (included) |
| Primary bedroom, second floor | 31 ft | 35 ft |
| Bedroom 2, second floor | 44 ft | 50 ft |
| Office over the garage | 60 ft | 66 ft (50 + 16 with a coupler kit) |
| Total | 167 ft, within 221 ft |
Four 50 ft sets would total 200 ft and still pass. Four 66 ft runs would total 264 ft and fail, which usually means the outdoor unit belongs closer to the middle of the house.
Watch the 24 ft rule if you mix a basement room with a second floor bedroom, and measure floor to floor before you buy. Our line set length guide shows how to measure each path, and pipe sizes for each indoor unit are on the line set page.
How much work is a 4-room DIY install?
A first-time installer usually spends about a day on the outdoor unit and the first room. Each room after that goes faster once you've done one. Two people make setting the outdoor unit much easier.
The refrigerant side needs no HVAC tech, thanks to the Quick Connect line sets. The electrical side is one dedicated circuit, breaker and disconnect for the outdoor unit, and many states and towns require a licensed electrician for it. Permit fees typically run about $50 to $300. For breaker and wire sizes, see the installation manual for your size on the multi-zone manuals page.
What if my house doesn't fit the usual plan?
What if one room needs cooling while the others heat?
A sunroom or server closet that wants cooling in January doesn't belong on the 4-room system, because all rooms share one mode. Give it a single zone of its own and put the other three rooms on a 3-room system.
What if it gets down to −20°F here?
Expect all four rooms to heat at once on those nights, so size the outdoor unit close to the indoor total. Check the heating range for your outdoor unit on the multi-zone specifications page. The 5th Gen multi-zone keeps heating down to −13°F outside; below that, keep a backup heat source and call us for cold-climate sizing.
What if I live in Hawaii or Alaska?
Shipping is free to the 48 contiguous states. For Hawaii and Alaska we ship by quote, so use the quote request page before you order.
What should you do next?
List your four rooms with their square footage, pick an indoor size for each, and note which ones run together on your hottest and coldest days. Then measure each line set path from the spot where the outdoor unit will sit.
With those numbers, the multi-zone sizing guide will point you to an outdoor unit. If you want us to look over the plan, call +1 (844) 999-9725, Monday to Friday, 9 AM to 5 PM EST, or text 302-385-6142.
