In this general article, I explain why I don’t generally like to use mini-splits.
The Positive Sides
When people say mini-splits, in general we mean “ductless” style units. These can either be high wall, floor mounted, or ceiling mounted consoles. These units do offer really high efficiency and becuase they’re “ductless” you don’t have duct gains/losses.
These units are common in many other countries and have been around for a long time. They do are generally quick and fairly easy to install, but are mostly a pain in the tail to work on.
So, the plus side to these units are that they are really efficient, they generally have models that have a high-heat output for colder climates, although this option is becoming more prevelant in traditional unitary style heat pumps as well. You do not have heat gains or losses due to ductwork being in unconditioned spaces.
The Down Sides
Most mini-split systems do not do the greatest at humidity removal. This is partially because they use proprietary algorithms to control the blower and compressor. They do achieve long run-times, which is often good for comfort stand points, however there is generally not a good way to control / hack them to work towards achieving good IAQ.
These units offer next to no filtration, so in order to have filtration you need to utilize a stand-alone / portable filtration means. Which a lot of the portable filtration systems have problems of their own (come bundled with UV / PCO technology or other “space” technology that can lead to their own IAQ problems). It also introduces another fan, which may / may not be that efficient. In other words, when considering other items to make them more comparable to what a traditional system offers, are they really that much more efficient?
These systems also do not have a good method of offering basic fresh air control / management, so other systems, such as an ERV need to be installed to handle the fresh air requirements of the building.
The controls for these systems are often hard to understand / use. They do allow you to change fan speeds, but they control the compressor. The fans often run all the time, which is not necessarily a bad thing, but they don’t really offer many ramping profiles or adjustments to change the blower speed when the compressor is off vs. on.
These systems get dirty quickly, are generally a pain to clean properly, and are not easily repairable. Manufacturer support is often poor, the documents aren’t always translated very well, and most technicians hate to work on them.
What about ducted models?
But what about the ducted models? Well, the ducted models are a step in the right direction. You can use filters on them, although some of the manufacturer’s do not recommend installing better / improved filter cabinets (like a 4-5" media filter), however we have done that successfully many times and always include media filters as an option on our installations, but you do need to make sure that they have a very low pressure drop, as they a lot are not designed for very much static pressure.
The ducted models still generally have the same control problems, previously mentioned. If you read my introduction to the coil-bypass-system , you have a basic understanding that even most traditional systems / designers do not focus on the proper air-changes in a structure to maintain proper IAQ levels.
You can’t easily pair mini-splits with larger fan-coils because they are “communicating” style systems.
Conclusion
Don’t get me wrong, there are applications that are well suited for mini-splits. These are often applications like sun-rooms, garages, small server-rooms / network storage rooms, and many others. But in my opinion these are not at all my favorite and are generally really far down my list of options that I want to recommend to my clients.
As an industry, I feel we need to step back and refocus on the pillars of IAQ. Filtration, fresh-air, and humidity control. The manufacturers / government should put less emphasis on chasing efficiency just to shoot ourselves in the foot and loose the qualities that make traditional / unitary style systems cover more if not all of the 3 pillars of proper IAQ.
There’s plenty that I did not cover and I’m sure I missed some things, but just needed to rant for a few minutes… Thank you for making it to the end!
