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?
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.
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.
Indoor Unit 1 in Encapsulated Attic
Indoor Unit 2 in Encapsulated Crawlspace
Outdoor Unit (a.k.a. the Heat Pump) in a security cage on the South side of the home
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.
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
Hope you enjoyed today’s post!