Heat pumps are super cool and a critical part of decarbonizing the building sector. We have a ground source heat pump (AKA geothermal) for our house. It is incredibly efficient because it is exchanging heat with the ground, which stays a relatively consistent temp year round. So, in the summer when we are removing heat from the house for cooling purposes, the heat pump works much more efficiently than an A/C unit trying to push hot house air into an already hot ambient outdoor temperature. In the winter, we are using the heat underground (about 54 degrees) to warm the house. The heat pump in the basement of our house is connected to a heat exchange loop that goes 275 feet down into a borehole and then back up. The refrigerant in the loop conducts heat from the ground so that it is “naturally” heated to 54 degrees with only a tiny amount of electricity required to pump the fluid through the loop. Some additional electricity is then used to run a compressor which increases the temp from 54 to our thermostat set point (we keep our house at 63-64 in the winter months, but could run it warmer if we wanted). Anyway, this is how a ground source heat pump works, and it works very well (although is expensive to install, mainly due to the drilling costs of the borehole).
Air source heat pumps – often called “mini-splits” – work much the same way and are becoming increasingly common due to rapid advances in technology. In many climates they will never be as efficiency as a ground source heat pump for one obvious reason. The temperature gradients between indoor and outdoor air are much more extreme than between indoor air and the ground. So, they cost more to operate and also have a shorter working life than a ground source heat pump. When you install a ground source heat pump you are making a 50-year investment. With air source heat pumps you are thinking in the 10-15 year time frame. The good news is that air source heat pumps are affordable and relatively easy to install.
I decided to install one in our tiny 100sf cottage and this is how I did it.
Research
HVAC work is a bit intimidating, which is probably why most people contract for it rather than DIY. But the heat pump industry clearly recognized that there is a market for units that can be DIY installed by handy homeowners that are comfortable with all aspects of the job save for one: dealing with refrigerants. A lot of the name brand mini-splits (Mitsubishi, Daikin, etc) can only be installed by an HVAC professional because they require the unit to be charged with a refrigerant after install. This requires high pressure gauges and a vacuum pump as well as a refrigerant under pressure which is then added into the system in the proper quantity. I’m sure this can be done as a DIY job with the right tools, but no thanks. I’ll let the pros handle this.
Fortunately, at least two companies are now selling high efficiency mini-splits that come pre-charged with refrigerant which makes the installation rather DIY friendly. I researched both Mr. Cool and Pioneer. Both have solid reputations and are comparatively affordable relative to the big name brands that you are likely to see in an estimate from your local HVAC contractor if you are looking into heat pumps. Mr. Cool even offers a “DIY” product line that includes a pre-charged line set that – I believe – further simplifies installation, although you are then stuck working within the constraints of the exact line set length, which is not ideal. I ended up going with Pioneer and just completed the install. It was pretty simple and is doable in a day for someone with basic tools and patience.
The Installation Process
There are a few steps. First, you determine the location for both the indoor and outdoor units and then mount those units. The units are connected to each other by a line set, which consists of a liquid line, a gas line, a condensate drain, and a signal wire. A small wall penetration is made to pass the line set through and then the units are connected to each other. The refrigerant lines are evacuated and then the valves containing the R-410A refrigerant – which is needed to move heat through the system – are opened. The outdoor unit is then connected to a dedicated breaker in your electrical panel and you’re done.
I started by determining the locations for the mini-split units. I’m not going to go into detail on why I chose the locations I did. There are definitely best practices to follow and ultimately I had to make some minor compromises. With the locations established, I installed the wall bracket. Walls being what they are, a stud is never exactly where you want it. I was able to get screws into a stud in a few places and used wall anchors elsewhere.
Then it was time for the wall penetration. I would have preferred to go out the right side of the unit, but there was a stud exactly where I needed to be – or so I thought. The installation manual includes specifications for where your exit hole should go and I followed them exactly for my model, but it turns out they were off by about an inch and a half. The hole should actually be very close to the side of the indoor unit despite the fact that the installation manual specifies the center of the hole 2.5″ from the side. I don’t see any reason that the exit hole should be where specified since it requires an unnecessarily sharp bend in the line set. I realized this as I was feeding the line set through the wall and decided to pull everything out, widened the hole to exert less stress on the drain line while also redoing the bend in the line set to account for a hole in a different location. It all worked out, but was kind of an unnecessary pain in the butt. Incidentally, the specs for the horizontal distance of the two mounting bracket legs for the outdoor unit was also incorrect, so I suppose the lesson here is double check those measurements specified in the installation manual.
You could mount the outdoor unit at this point or work on connecting the line set to the indoor unit and then deal with the outdoor one. It doesn’t matter. The installation manual doesn’t explicitly state this and also includes a few different scenarios for how you might be connecting the two units. I think most people are going to mount the outdoor unit, then connect the line set to the indoor unit, then pass the bundled lines through the wall, mount the indoor unit on its bracket, and then connect the line set to the outdoor unit. That is how I did it any rate and it worked fine.
I bought a mounting bracket for the outdoor unit from Pioneer. It turns out there are two types they sell and I bought the wrong one, but made it work. The one I bought was for installation in a masonry wall. They also sell a version for framed walls. These units are very heavy so you definitely need to make sure your install is solid. I fastened two PT 2x4s to the exterior wall using Timberlok screws into the studs and then mounted the wall bracket to the 2x4s, again using Timberloks. If I had purchased the correct mounting bracket from Pioneer, I would have been able to avoid using the 2x4s. Oh well. Then I had a friend help me lift the unit onto the mounting bracket. The supplied bolts hold the unit to the bracket.
Then it was time to go back inside and connect the line set to the indoor unit. I started with the signal wire. You open up a cover on the front panel of the unit, pop off a panel on the back, pass the cable through the back of the unit, and then connect the wires to the terminal block. Other than the green ground wire, it doesn’t matter what goes where so long as you do the exact same thing on the terminal block on the outdoor unit.
I then needed to move the drain hose on the indoor unit to the other side, since the default exit hole is right side, but I was going out the left. There are drains on both side of the unit and the inactive one has a little rubber bung that plugs it up. That pops off and the drain hose is moved onto that drain and the bung is inserted into the one on the other side where the drain hose was previously connected. Easy enough.
Next, I connected the two refrigerant lines on the indoor unit to the two cooper lines on the line set. The installation manual advises you to use a torque wrench. I actually have a torque wrench for use on bolt heads, but I would have needed to purchase an open wrench and I didn’t want to buy one since it seemed unnecessary. I suppose there is some small risk in over tightening, but I torqued those flare nuts as hard as I could and – later – when I leak tested all seemed well so I’m not going to worry about it.
With the drain hose, copper tubes, and signal wire all connected, I then wrapped everything tightly in the supplied protective tape and attempted to pass the bundled line set through the wall penetration. This is when I discovered that my hole was not optimally placed, so I widened the wall hole a little bit. Pioneer supplies you with a little plastic protective insert that is clearly designed for the typical depth of a stud wall. This has a little flange on the end of it so it nests onto the wall. It is a very tight fit to pass the line set through this 2.5″ diameter piece. I widened the hole to 3″, slid a little 3″ PVC sleeve that I cut from drain pipe I had around from making a chicken auto feeder, and then I was able to easily pass the line set through without having to worry about kinking the drain hose or copper tubing.
At this point I was ready to connect the line set to the outdoor unit, but first I needed to practice flaring copper pipe. In theory you can avoid having to do this if you locate the outdoor unit’s refrigerant line connections at exactly the right spot so that the line set doesn’t need to be cut. It does come flared. I had about two feet more length in the line set than I needed. So, I cut out a short length of pipe (maybe 6″) to practice flaring. I had purchased an inexpensive flare tool from Amazon just for this project. The installation manual specifies the tolerances for your flares based on the diameter of the pipe. This is measured in millimeters, so some precision is necessary here. One thing that threw me off is that the factory flares on the pipe are double flared. They looks really professional (because they are). But I hadn’t expected this and I became concerned. I did some research on how to do a double flare and you need an extra set of inserts or something along those lines to create the flare. Not sure if I needed to double flare, I texted a friend that is a capable plumber and his response was something along the lines of “what is a double flare.” His father owns one of the biggest HVAC businesses in the area and he told me that the plumbers on his crews don’t double flare. I did some additional research and it looks like double flares are used for mission critical high pressure situations, like the brake lines on a car. My understanding is that they are additional insurance. I felt fine with my single flare approach and proceeded to practice making a few more flares before doing the “real thing” or so I thought. The first one I did I underestimated how much pipe I need to trim off. So, after I cut the pipe and flared it, I attempted to maneuver the pipe into place and tighten it onto the fitting, but I was finding that I needed to exert a of force on the pipe to be able to get it into place. Worried that I would bend kink the pipe, I decided to just cut off an extra inch and redo the flare.
This is what the pipe looked like after being cut and before flaring.
And then after flaring.
I deburred the pipe before flaring but I found that I wasn’t able to get a nice clean edge without using some metal sandpaper to clean things up. I don’t know if that is a normal step or maybe my flaring technique or tool, just isn’t very good. Ultimately, I was able to make the flare connections quite easily. Here is pic of the flare connection. I later went back and fully torqued the flare nut so no threads are exposed.
My work wasn’t perfect. One of the consequences of trying to bend the line set a bit too aggressively to get it into place with the stem on the outdoor unit resulted in this slight kink. I’m hoping that this is no big deal. I’m sure that a pro would never let this sort of thing happen.
Other than the electrical connection to the panel, the final step is to evacuate the lines and then introduce the refrigerant into the system. Pioneer sells a product that allows you to evacuate the lines without a vacuum pump. I read through the instructions a few times to make sure I understood the process. It works well enough, but there are a few things I learned.
First, definitely wear gloves. As the gas is released the canister will become extremely cold. I put on some very light weight gloves because I wanted to preserve some dexterity in my fingers and I wish I had gone with winter gloves. I also used eye protection.
Second, the refrigerant in the line flushing kit is Difluoroethane. This is different from the R-410A refrigerant in the heat pump, which is a very commonly used refrigerant. While R-410A is not an ozone depleting gas it does have a very high GWP (Global Warming Potential). Refrigerant management is a hugely important topic and refrigerant leaks are very high on the list of contributors to global warming. The Difluoroethane that is used to evacuate the lines has a very low GWP. This is good to know since you are releasing it into the atmosphere as a result of this purging activity. Fortunately, my installation was free of leaks so I don’t need to worry about the R-410A until it the end of life of the unit, at which point it will need to be handled by someone who can safely capture the refrigerant and recycle it.
Third, the instructions tell you to connect the flushing kit to the service port on the gas line of the unit. The liquid line should be disconnected. This all makes sense if you understand what the intent of the flushing kit is. What is not clear is the very ambiguous quantity of Difluoroethane that should be used to flush the lines. The instructions simply say to do 5 second bursts until you have used up 2/3 of the can. How on earth are you supposed to know when you have used up 2/3? I was worried that I would use the whole can and then not have enough left to pressurize the system and leak test. I did five cycles of five seconds, at which point I connected the liquid line of the line set, tightened the connection, and then released the remainder into through the service port. At this point I was worried I had a leak because I heard the sound of leaking gas. But it was back flow through the nozzle of the canister, which I believe was due to the fact that the lines were full and couldn’t handle any more gas. This would have been nice to know. With the lines pressurized, I used soapy water to check for leaks on the outdoor units connection. I didn’t both checking the indoor unit connections since they were basically buried in the wall. I’m really not sure how one would actually do a leak test on them. The instructions say to then remove the flushing kit from the service port and essentially open it up to fully release any remaining gas, presumably before disposing of the canister. Mine continued to hiss gas out the top for another 20-30 minutes which gives a real indication of how much gas I didn’t use when evacuating and pressure testing. My line set was very short, only about 8 feet, so I’m sure that has something to do with it. If you have really a long line set you might need to use the entire contents of the canister to properly evacuate and pressure test. Perhaps some better instructions would help here.
Fourth, the final step in the flushing kit manual tells you to return to the installation manual and continue with the remaining steps outlined in the section that calls for use of vacuum pump. This also could have been more clear since it really isn’t intuitive to someone (like me) who is basically following directions without any real understanding of the process. I inferred that the only logical thing remaining was to open the valve stems on the outdoor unit to release the R-410A refrigerant into the system. So that’s what I did.
All that remained was the electrical connection to my sub-panel. Full disclosure: I am not an electrician and electrical work is super dangerous if you don’t know what you’re doing.
I purchased an appliance whip to make the connection to the panel. Since my sub-panel is literally underneath the unit, I didn’t need to install an AC disconnect, but that is what would need to be done to install to code in most cases. The heat pump should be on its own breaker, too.
The wiring of the terminal block inside the outdoor unit is very straightforward.
And this is what the outdoor unit looks like once installed.
And the indoor unit looks pretty nice. It is whisper quiet. You don’t even know it’s running. So far I’m very happy with it.
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