MRCOOL Multi-Zone Planning: Matching Air Handlers to a Condenser

A MRCOOL multi-zone system runs two to six indoor air handlers from one outdoor condenser. Here is how to pick the condenser and match air handlers that MRCOOL supports.

The Short Answer

Size each room first, then add up the air handlers and pick the condenser whose matchup chart includes that exact combination. The combined air handler capacity can run somewhat above the condenser's rating, because rooms rarely all call for full output at once.

For most homes, the 27K covers two to four bedrooms and an office, and the 36K covers a small home plus a bonus room. The 48K and 55K are for larger homes or homes with a big open living area that needs a 24K or 36K head.

MRCOOL DIY Multi-Zone Condensers Compared

Every DIY multi-zone condenser runs on 208-230V and is ENERGY STAR Cold Climate certified per MRCOOL's catalog.

Step 1: Size Every Room

Size each room on its own, using the square footage, sun, ceiling height and use of that room. Our mini split sizing guide has the chart and adjustments. Then round each room to an available head size.

Step 2: Add Up the Heads and Pick a Condenser

Add the head sizes together. Choose a condenser that lists your exact combination on MRCOOL's matchup chart and that keeps you inside two limits:

• The two-thirds floor. MRCOOL support recommends connected air handlers total at least about two-thirds of the condenser's rating. The chart lists some smaller pairings, such as 6K + 6K on the 27K. Follow the two-thirds guidance anyway; it is MRCOOL support's recommendation for how the system should be matched.
• The chart ceiling. The chart allows more connected capacity than the condenser's rating. MRCOOL notes that if every room runs at once, combined output will not exceed the condenser's rating.

That second point is why a 36K condenser can serve 45,000 BTU of heads in a home where the bedrooms and living room peak at different times of day. It is also why you should not count on that extra capacity in a home where every room needs full cooling on the same afternoon.

Three Worked Layouts

Always check your exact combination against MRCOOL's current matchup chart before you buy. Some sizes are allowed on one condenser and not another. For example, a 24K head is not on the 18K condenser's chart, and 36K heads appear only on the 48K and 55K charts.

Step 3: Wall Units, Ceiling Cassettes or Ducted

MRCOOL's chart counts cassettes and wall units the same way, so you can mix them on one condenser. The OuttaSight one-way ceiling cassette fits between standard 16 in ceiling joists and keeps the wall clear. It needs attic or joist-bay access above the room.

MRCOOL also makes a DIY hybrid ducted air handler that can serve a main living space on the 27K and larger condensers, with wall units or cassettes in the other rooms. Wild Oak Trail does not currently list the ducted unit; contact us if your layout needs one.

Step 4: Plan the Line Runs and Electrical

Each indoor unit gets its own refrigerant line set back to the condenser. MRCOOL DIY multi-zone systems use the same pre-charged Quick Connect sets as single-zone units, made in 11.5, 16, 25, 35 and 50 ft lengths.

• Check each air handler listing for whether a line set is included, and buy extra lengths to reach distant rooms.
• The condenser needs a dedicated 208-230V circuit, from 20A on the 18K to 45A on the 48K and 55K. MRCOOL recommends a licensed electrician for this work.
• Mount the condenser where every run stays under 73 ft. A central location on the outside wall usually works best.

Who Should Go Multi-Zone, and Who Should Not

Multi-zone makes sense if

• You are conditioning three or more rooms and want one outdoor unit instead of three.
• Each room should have its own temperature, such as a cool bedroom and a warm office.
• Your outside wall space is limited.

Look at single-zone systems instead if

• You only need one or two rooms. Two single-zone units can cost about the same and keep working if one fails.
• The rooms are far apart, at opposite ends of a long house, and runs would exceed 73 ft.
• You need 115V. Every multi-zone condenser and head is 230V. See the 115V options in our MRCOOL line comparison.

Running the home from batteries or solar? Multi-zone condensers draw more current than one small single-zone unit. Read running a mini split on solar or an EcoFlow first.

Frequently Asked Questions

How many zones can a MRCOOL multi-zone condenser run?

The MRCOOL DIY 5th Gen 18K condenser runs up to 3 zones, the 27K up to 4, the 36K up to 5, and the 48K and 55K up to 6. Every combination on MRCOOL's matchup chart uses at least two air handlers.

Can I connect more BTU of air handlers than the condenser is rated for?

Yes, within the combinations on MRCOOL's matchup chart. For example, a 36K condenser accepts combinations up to 51,000 BTU of heads. If every room calls for full output at the same time, combined output will not exceed the condenser's rating.

What is the minimum number of air handlers on a MRCOOL multi-zone?

Plan on at least two. Every combination on MRCOOL's chart has two or more indoor units, and MRCOOL support says connected air handlers should total at least about two-thirds of the condenser's capacity.

Can I mix ceiling cassettes and wall-mounted air handlers?

Yes. MRCOOL's matchup chart applies to ceiling cassettes and wall-mounted air handlers alike, and the OuttaSight cassettes in 6K, 9K, 12K and 18K work with all five DIY multi-zone condensers.

Can I use a 115V air handler on a MRCOOL multi-zone system?

No. MRCOOL's DIY multi-zone condensers and the air handlers listed for them are all 208-230V. The 115V 9K and 12K air handlers pair only with their matching 115V single-zone condensers.

How long can the line sets be on a MRCOOL multi-zone?

MRCOOL's service manual allows a longest single run of 73 ft, with total line length across all rooms of 123 ft on the 18K, 172 ft on the 27K and 221 ft on the 36K, 48K and 55K condensers.