The most recognizable and commonly used mini-split system is the wall-mounted ductless system. They’re seen all over the world and are starting to become noticed and used here in North America, particularly in homes with small heating and cooling loads. Typically, they are thought to serve only small spaces like bonus rooms, garage apartments, etc. They can actually serve a whole house, and even a whole commercial building. One of the things keeping these systems, with inverter compressor technology, from exploding as whole-house heating and cooling systems is their appearance. Not many homeowners are keen on having these systems “exposed”. That’s what an HVAC closet is for, right?

ductless-mini-split-installed-lg-squared-inc-chris-laumer-giddens (1)

Here is a picture of me installing one of the two ductless fan coils in our Atlanta Condo. This was a “dry-run”, since we painted the walls prior to final installation.

In response to the demand to “conceal” these “mini” air handlers, or make them disappear, manufacturers started making ducted mini-split systems, where the indoor fan coil is concealed in a dropped-ceiling, attic, or crawlspace, and uses ductwork to distribute the air throughout the zone that it serves. Each zone has multiple rooms, all served by a single fan coil.

The unit shown below is a concealed ducted unit by Mitsubishi Electric as it was being prepared to be installed in the encapsulated attic of a home in Grant Park, Atlanta, GA. As you can see by the person’s foot and leg next to it, it’s not very big. The dimensions for this 15,000 btu/h fan coil is 39″ wide x 28″ deep x 8″ tall.

ducted-mini-split-installed-lg-squared-inc (29)

One of the many questions I get about the ducted mini-splits systems I design for single- and multi-family homes is, “what do the air handlers look like during the construction phase when they’re installed?“, and, “can you have long duct runs with their low static pressures?” (this question comes from the construction industry, mostly HVAC contractors, other home energy raters and builders).

In response to the first question, I’ve included a sampling below of a few local projects we’ve designed to show how and where they can be installed. Every home is different, so the size, location and configuration should be determined individually through a design process.

In response to the second question, definitely! While many of the ducted fan coils come with very low available static (e.g. 0.14 – 0.30 iwc), there are several that are available with 0.60 iwc (inches of water column) and slightly higher. For the systems with low-static fan coils, the Duct Designs (Manual D) we do calculate and verify that duct runs can perform well at as long as 30 feet or more. We will also test these systems (air flow, balance, static pressure, etc.) after they have been installed to confirm that they perform as designed and as expected. The few times we discovered poor performance, we found the installation did not follow the design or manufacturers recommendations.

Project 1 (Grant Park ResidenceAtlanta, GA): Mitsubishi MR SLIM – Outdoor Unit: MXZ-4B36NA – Indoor Units: (2) SEZ-KD15NA4

Indoor Unit 1 in Encapsulated Attic




Indoor Unit 2 in Encapsulated Crawlspace

mitsubishi-ducted-mini-split-installed-lg-squared-inc-sez-kd15na4 (39)


ducted-mini-split-installed-lg-squared-inc (39)

Outdoor Unit (a.k.a. the Heat Pump) in a security cage on the South side of the home

outdoor-unit-heat-pump-inverter-compressor-mini-split-installed-lg-squared-inc (39)

Project 2: Mitsubishi MR SLIM – Outdoor Units (2): MXZ-8B48NA – Indoor Units: SEZ-KD12NA4 (1), SEZ-KD15NA4 (1), SEZ-KD18NA4 (1), PEAD-A24AA (1)

This is one of three fan coils installed in the encapsulated attic to serve the East wing of the home.





Here is the Optional Filter Box (FBM and FBL series) on all Fan Coils in the Accessible Encapsulated Attic Areas


Here is the largest of the fan coils. It serves the entire second floor of the home, and is installed in the encapsulated attic above. This is one of the high-static units that has a range of available static pressure between 0.14 – 0.60 iwc, and can be easily adjusted at the controller. The duct design determines the appropriate static pressure.




This is the fan coils serving the basement. It was suspended from the HVAC Closet ceiling, leaving the floor area available for storage and other equipment.


The basement unit was installed so that it return side of it backed up directly in to an opening the same size as the opening in the fan coil. This eliminated all return plenum ductwork, and transfer grilles (Return Air Pathways, Tamarack) were installed to provide a return pathway from all rooms in the basement.


Project 3 (Proud Green Home at Serenbe)LG Electronics HVAC Division – Outdoor Unit: ARUN036GS2 (1) - Indoor Units: ARNU093BHA2 (2), ARNU123BHA2 (1)

Here are a few shots of this system at the Proud Green Home. We have a more extensive post on the entire mechanical system at this home, including the Zehnder ERV.





Here I am having a little fun. I’m “bangin’ metal” at the Proud Green Home at Serenbe, helping make one of the return plenums

HVAC Design, Manual J, Manual S, Manual T, Manual D, Chris Laumer-Giddens, Proud Green Home

Hope you enjoyed today’s post!

Views: 54024

Tags: design, ducted, hvac, installation, mini-split


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Comment by Mihaita on July 14, 2017 at 6:57am

Also, Am I the only one that awaits the Samsung galaxy s9?

Comment by Mihaita on June 28, 2017 at 9:58am

Amazing, i bought my AC here.

Comment by H. Robinson on September 24, 2014 at 2:33pm
Brilliant post, and great photos. I'm currently planning an install using the same Mitsubishi sez kd15na unit. I've never done one of these and have a few questions.

It looks like all your installs have a secondary drain pan. Would you recommend using an internal drain pan sensor in the unit (mitsu make one that auto shuts off if the primary drain pan starts to fill). They claim you can do away with secondary pan if you use this. Or would you recommend a standard secondary pan with its own shut off switch? (Or both)

Second question: the unit has a fitting for a 1-1/4 drain line. This seems huge... Any reason the such a large drain tube is needed?

Third question: any problems with noise on the install where the return is right into the unit - manuf. guidelines say you should site the return grill at least 3 feet from the fan unit.

Thanks very much for posting these photos - they've given me loads of inspiration for this particular install.

Comment by Chris Laumer-Giddens on August 12, 2013 at 7:34am


Kinda, sorta, yes and no.

The inverter compressor allows the system to "dial up" and "dial down" based on the demand. This means that if a zone only calls for, say, 3,000 - 4,000 btu/h, it will only deliver that much. It will also only use the amount of electricity it needs to do that. It's why residential systems (outdoor plus indoor units) normally run under 10 amps, but can ramp up to 18-20 amps (total) in maximum capacity situations (which is rare).

I tend to size mini-splits based on the highest load, whether it's heating or cooling. This allows us to avoid supplemental (strip) heat. Like all systems, mini-splits de-rate as the outdoor temp drops (yes, they can operate well below 0 degrees). So, I find what the capacity is at the design temperature wherever the home is, and then select the system that has enough capacity for even lower than that for those really cold days.

Finally, the system will adjust to meet whatever the load is at any given time. If I specify a 3-ton system, and it only needs to be a 1-ton system, it will dial down to be that, which is one of the main reasons they are more efficient than most equipment. Installation plays a big roll, but not as much as the technology.

Comment by Bob Blanchette on August 12, 2013 at 7:08am

Chris could some of the 30-50% improvement over conventional systems be that they aren't grossly oversized and they are properly installed? Conventional systems are rarely sized and installed correctly.

Comment by Chris Laumer-Giddens on August 12, 2013 at 7:01am

Thanks, ken Neuhauser!

Question for you, did the contractor follow a duct design that followed Manual D protocol?

Every system we've tested did, and while there was some variation from the design pressures (less than 10%) and flows (less than 5%), the systems meet and exceed performance and comfort expectations. 

One of the strategies we employ is oversize the return grille and plenum to make sure we have plenty of available static, especially on the low- to medium-static units.

Most systems that do have poor results are "designed" and installed on-site, and follow the "the-way-we've-been-doing-it-for-30-years" protocol. That is why manufacturers recommend no more than 10'-0' of ductwork from the fan coils. Most systems are NOT designed.

We've measured energy consumption with energy monitors (and looking at utility bills), and have found them all to be at least 30%-50% or more efficient than conventional heat pumps and air conditioners. If the systems are "choked" they will need to work harder to meet setpoint. Assuming the system wasn't "properly" designed and installed, this could be why your seeing poor numbers.

We have builders and contractors coming back for more of these systems because of how well they perform and save energy.

Comment by Chris Laumer-Giddens on August 12, 2013 at 6:42am

Thanks, Ted! 

A project I designed over a year and a half ago had been having trouble getting to set point no matter what time of year. We had the manufacturer go out several times, and they did find multiple issues with the installation (holes in the line-sets, poorly installed ductwork, etc.), but none of the fixes made it work correctly. They still couldn't get to setpoint. We also confirmed the house was built the way we modeled it (it was even better than modeled)

About a month ago the homeowner left for two weeks and setback all the systems and came back to a Sauna.

When we designed the system, it was a good idea to exhaust the bathrooms through the ERV, and connect them to return plenums.

To play it safe, and because I can't predict occupant behavior, I don't recommend either anymore. I go with the simple route and take stale air from a central location, and put fresh air supply near return grilles. Completely disconnect them from the central system, mini-split or not.

As soon as the contractor disconnected the ERV from the fan coil and re-routed the supply to a central location, all the systems were worked normally with all zones reaching and maintaining setpoint.

I think for the ERV-Mini-Split units to work, there needs to be a way to provide humidification and/or additional dehumidification beyond the capabilities of the mini-splits. 

Comment by Bob Blanchette on August 5, 2013 at 9:36pm

Fan coils in ceilings have been popular for years in apartments and some commercial applications, it seems ducted mini-splits are basically an extension of the same idea. Keeping ducts short with low static pressure seems to be the key to quiet systems, regardless of fan coil design.

Comment by Adam brown on August 5, 2013 at 2:23pm

Great post!  Makes for a interesting read!

These are very popular over here in the UK, we always try and install the high static ducted units when placed in the loft void, so we can obtain the adequate airflow for rooms further away!

Not to mention the reduced noise levels, soundproof ducting, etc

If you have any time, check out our website

Comment by James Jackson on August 1, 2013 at 4:19pm


Great job with the post and the projects !

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