Category Archives: space conditioning

Minisplit winter operation

Our minisplit kept us cool during the dogs days of summer this year, and without having our electrical bill going through the roof.

But the minisplit can do more! Because it’s an air-source heat pump, it also can heat the building during the cold season. And that was something I wanted to put to the test when it recently was really cold outside.

Like I mentioned in the video, I simplified my explanation about how the system works. If you would like to read a more comprehensive and accurate description, you can find it in a previous blog post with the title “Mini what?

The morning after I took the video, our outdoor temperature had dropped to -5F, the specified minimum operating temperature. I turned the minisplit on and indeed, it still was putting out heat.

While the outdoor unit was almost inaudible during the summer when we ran the minisplit in cooling mode, it was humming away pretty good in the heating mode, as you can hear in the video. I assume that the compressor has to work harder at these cold temperatures, thus the increased noise. Not that it matters. All windows are firmly shut anyway, keeping the noise out.

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Minisplit start up

The most exciting moment is when you get to start up a new gadget – like our minisplit.

The vacuum pump had been pulling air and moisture out of the cooling lines. Once we had an acceptable vacuum pressure, James Pruyn, our installer, disconnected the pump and opened the refrigerant valve on the outdoor unit. That allowed the lines and indoor unit to be charged with the R410A refrigerant.

We were able to power up the system, and after tinkering a minute with the remote control, we got the indoor unit to spit out cool air. I had to go back and fix the leak on the condensation line, but other then that, everything ran smoothly.



I like the summer sounds of cicadas and crickets chirping, but I dislike the ever annoying humming of air conditioners. I had been very concerned about the noise levels of our minisplit–not only of the indoor unit, but also the outdoor unit.

The indoor unit runs very quietly, even at full fan speed in cooling mode. When on low speed or “dry” mode, it makes no audible noise. I literally have to put my hand up to the unit to check that there is airflow because I can’t hear it.

The next check was the noise level on the outdoor unit. I saw the cooling fan running but could not hear anything. I had to climb up next to the unit to confirm that the compressor was cranking. I could hear my neighbor’s small window AC unit, but not our minisplit.

That was welcome news. It meant that even if we have a bedroom window open, we would not have to deal with the annoying humming that you typically would expect from an AC compressor.

A couple of weeks after the installation, James Pruyn called to asked how the system was running.

During the dog days of summer, I had the minisplit sometimes running during the day, but mostly at night, and mostly in “dry” mode. It turns the fan speed to low and slowly moves the indoor air over the cold heat exchanger coil of the indoor unit. This maximizes the moisture removal and at the same time keeps the indoor air temperature steady. On the few occasions when I had to lower the indoor air temperature, I switched to minisplit to “cool” mode at high fan speed for an hour or two. After that, the “dry” mode was able to maintain the desired temperature. To help with the distribution of the conditioned air, we used a small energy efficient pedestal fan to blow the air into the north or south part of the building.

In short, the minisplit was able to maintain a comfortable temperature and comfortable humidity levels (below 60% relative humidity) on the “low” setting at most times. James called it a perfectly sized system. He was right. And I should give credit to Lindsey Elton at the Eco Achievers, because she ran the energy model to determine what our cooling load would be and what size minisplit we should install. Thank you Lindsey!

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Minisplit installation – indoor unit

While the vacuum pump is cranking away, let’s take a look at the indoor unit installation.

The indoor unit is also referred to as the evaporator, because of the phase change that takes place in the refrigerant when in cooling mode. It is a box (compact wall unit) measuring roughly 36 by 12 by 12 inches.

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The location of the compact wall unit had been determined during the rough-in. We placed it centrally located on the south wall in the dining room.


The central location is key as there is no duct system that distributes the cool air (thus the term ductless minisplit). But with the cool air discharged from the compact wall unit in the dining room, it also reaches the living room, bedrooms and kitchen. That said, we may need to use a fan to facilitate the distribution of the cool air across the rooms.


Our installer, James Pruyn, started by mounting the wall hook bracket onto which the compact wall unit will hang.

The recommended clearance for the compact wall unit is two and a half inches or greater from the ceiling and six feet or greater from the floor. We executed the rough-in of the cooling lines and condensate drain accordingly and have a ten inch clearance from the ceiling and eight foot two inch clearance from the floor.

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During the installation, we ran into a couple of flaws in the compact wall unit that are worthwhile mentioning.

The screws on the electrical terminals for the wire connections stripped. Thanks to James’ improv skills, we managed to get the wires safely connected after all.

At the end of the installation, James advised me to keep a very close eye on the condensation line once I turned the minisplit on. He connected the line from the wall unit to the line I had roughed in. But he pointed out that the provided barbed connection was very flimsy and suspected that it may leak.

And he was spot on. The connection was not watertight. Our solution was to fit a barbed coupling into the flimsy barbed connection that came with the unit. We got it watertight with a little bit of teflon tape and a screw clamp.

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Minisplit installation – outdoor unit

How do I prevent the minisplit outdoor unit from multiplying overnight?

Usually the outdoor unit is mounted on the building near ground level, but above the snow line, where it is easy to access and service. While visiting Sweden this summer, I saw a number of cottages with minisplits installed just that way.

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I am inspired by this but won’t attempt to duplicate it, because I am in Chicago. Some people may label a minisplit “green” because of its efficiency, and others may just see green when looking at it – green like Benjamin Franklins. I may be left with just the minisplit shell in the best case scenario, or with an outdoor unit that grew legs overnight and walked away.

This baby has to be mounted higher on the building, off the ground where it is difficult to reach…and fairly difficult to install, unless you have a small scaffold.

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Fortunately, there are convenient wall mounting brackets for the minisplit, which we anchored into the exterior brick wall. Our installer, James Pruyn, fixed the outdoor unit on the bracket arms and connected the supply and return cooling lines we had roughed-in a couple of years back.

And then there was the electrical connection. Our 12,000 Btu minisplit runs on 220 volt, not the typical 110 volt. When I roughed-in the cooling lines, I also made sure to run an electrical conduit from the circuit breaker panel to the building exterior. I also had set aside two slots in the circuit breaker that would give us the needed 220 volts. All that was left to do was to install an emergency disconnect on the outside, pull the electrical cables through the empty conduit, and connect the cables to the minisplit.

The outdoor unit comes pre-charged with R410A refrigerant. But before we could charge the whole system (cooling return and supply lines and indoor unit), we needed to remove any remaining air and moisture. James connected a vacuum pump to the minisplit, and from then on it was a waiting game, as we waited until there was sufficient vacuum pressure.

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Well then – lets wait…

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Picking a minisplit

Talking to a lot of people and reading the latest accounts and reports of minisplit models prompted me to eyeball the high efficiency, Energy Star rated Fujitsu models.


Their cooling operation range extends to 115 degrees Fahrenheit, while their heating operating range goes as low as five degree Fahrenheit. Think about this: At an outdoor temperature of five degrees Fahrenheit, this unit can still extract enough thermal energy out of the air to heat the inside of the building. The magic of physics!

The big question, however, revolved around cooling rather than heating. I still had to determine the actual cooling load of our first floor. The Fujitsu models came at a cooling capacity of 9,000, 12,000 and 14,500 Btu. With the help of Eco Achievers, we re-ran the energy model for the building to determined an actual cooling load of 12,000 Btu for the 1st floor.

This made the 12,000 Btu minisplit (12RLS3) a perfect match. It has a SEER rating of 29.3 and a moisture removal capacity of up to 2.7 pints per hour. I would have loved to get away with the 9,000 Btu model at a SEER of 33, but that bar was set too high for us.

I was also told that these units run extremely quietly. That was very important to me. I despise the sound of an air conditioning unit humming away during the summer. Plus, our outdoor unit will be mounted on the east wall of our building where it is fairly close to some windows.

It also comes with other bells and whistles which probably deserve their own blog post. But I first want the minisplit installed and running so that I can report back on its performance.

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