What Are Ductless Mini Split Systems

by | Last updated Jan 10, 2021 | Sustainable Design

A ductless mini split is a heat pump system which can provide heating and cooling. It has an exterior condenser which is connected to interior cassettes through a series of lines. The interior cassettes are all controlled independently allowing you to set different temperatures for different rooms, as each cassette’s temperature setting is independent of the other cassettes in the system.


I am Jorge Fontan an architect in New York and owner of NYC based architecture firm Fontan Architecture. We have been using mini splits in many of our projects, in this post we will review a few basics about mini splits and their components.


Mini Split Condenser

The condenser (or compressor) is the active mechanism in the system where the heat transfer happens. It is where heat is released for cooling or gained for heating. The condenser is installed outdoors as it needs fresh air to cool down its pipes.

The photo below is of a mini split condenser outside of a Townhouse in Brooklyn, NY where we replaced the original heating system with a new mini split heating and cooling combined system. This specific unit is the Mitsubishi City Multi system with Hyper Heat. There are many different manufacturers of mini splits, this is what we used on this project.

Mini Split Condenser

Mini Split Condenser


Mini Split Cassette

The Cassette is the air handling unit within the system. The cassette is a piece of equipment that blows warm or cool air by recirculating air and heating or cooling it. Every room will have its own cassette or multiple cassettes for larger spaces. The cassette is fed by pipes that connect to the condenser outside. Every cassette connected to the same condenser can be controlled independently, the different cassettes can be off at different times with some on and some off or they can all be set to different temperatures simultaneously.

Below is a picture of a ceiling mounted cassette for a mini split system in the same townhouse we mentioned earlier.

Ceiling Mounted Mini Split Cassette

Ceiling Mounted Mini Split Cassette


Ductless Mini Split Line Set & Pipes

As the name implies these systems have no ducts. The air which is blown by the cassette is recirculated air from within the room. The cassette and condenser are connected by a line set through which fluid flows for heating and cooling. The cassettes also have a condensate pipe to drain condensation.

Mini Split Line Set

Mini Split Line Set


Thermostat / Controls

Most mini splits are controlled by remote. you can also have them hooked up to a separate thermostat. For increased energy savings you can connect them to a smart thermostat. I have another post on the Nest Learning Thermostat if you want to learn more about smart thermostats.



Mini splits are a good option for sustainability. Controlling the temperature in different areas is normally more efficient because you can reduce your energy usage in the unused spaces.

Having a ductless system is also more energy efficient. According to the US Department of Energy up to 30% of energy usage in heating and air conditioning is lost through heat loss in a ducted system.

I have another post you can check out if you want to see 21 Ideas for a Sustainable House.



Normally a mini split system has a greater upfront cost for the equipment and installation but there will be savings over time as they typically have a lower operating cost.


Ductless Mini Splits for Heating and Cooling

As an architect, I study design and construction, but these are complicated and quite involved issues. Every project is different and must be assessed on its own unique characteristics. This post does not assume to cover every possible issue or condition, but provide a general overview of the topic.


Thank You for Reading Our Blog Post on Ductless Mini Split Systems

I hope this was helpful. If you would like to speak with an architect, you can contact us at Fontan Architecture directly.


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Jorge Fontan

Jorge Fontan

This post was written by Jorge Fontan AIA a Registered Architect and owner of New York City architecture firm Fontan Architecture. Jorge Fontan has earned 3 degrees in the study of architecture including two degrees from the City University of New York and a Masters Degree in Advanced Architectural Design from Columbia University. Jorge has a background in construction and has been practicing architecture for 20 years where he has designed renovations and new developments of various building types.