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Small House Air Conditioner – Using a Mini-split Heat Pump

We live in the southwest corner of New Mexico at about 6,000 feet elevation. Our lower latitude combined with higher elevation creates “four gentle seasons”, a fact the town tourism website loves to promote. For that reason, it is not uncommon to find homes here that do not use air conditioning and who just tough it out for the couple of weeks when the heat gets intense. We, on the other hand, decided during our design stage that creating an energy efficient and stable climate controlled environment was important to us. That led us to our solution for a small house air conditioner.

With the high temperatures we have recently been experiencing (nearly 100 degrees for several days in a row), I thought it would be a good time to expand upon the heating/cooling system we chose for our house.

[We go back and forth in calling our home a tiny house, but considering it contains two dogs, two humans, a piano and a parrot; it is a tiny house for us. We use ‘tiny’ and ‘small’ interchangeably.]

Wikipedia’s very outdated Silver City, NM  climate chart.

Minimalist Design for Tiny Homes on Wheels

Building a tiny home on wheels typically means working with a limited amount of space – both wall and floor – and within a limited width (to fit on the road). After storage is built in and furniture is added, the choices for adding extensive ventilation or heating/cooling systems are few. During our research stage, we found that many tiny homes used a small marine propane heater (kind of similar to what RVs use). For cooling, we noticed that many tiny home dwellers viewed air conditioning as kind of a luxury item. And in their minimalist design, most passed on installing a tiny house air conditioner.

In more recent times, I have seen some tiny home designers choose to go in a different direction. Instead of relying on open windows to cool down their space, they install a mini-split heat pump. For a number of reasons that we will list below, this is also what we did.

Tiny house air conditioner

By the time we were ready to haul over the three large boxes containing the mini-split, we had already gotten rid of our second vehicle. Brian transported everything from the apartment to the house using our electric cargo bike. Here he is bringing over the condenser.

Using a Mini-Split as the Small House Air Conditioner and Heater

The mini-split (heat pump) is a device that transfers energy from the outdoor environment to a an indoor environment.  The split system has two main components – an outdoor compressor/condenser and an indoor air-handling unit (evaporator). The term ‘split heat pump’ is often used to describe a system that has a single large outdoor condenser and multiple indoor units. The term ‘mini split’ has been more recently used to describe smaller systems that have a single outdoor unit and a single indoor unit. (For the rest of the blog post, we will just say ‘mini-split’ when talking about our system.)

Our primary drivers in choosing a mini-split for our small house air conditioner were that it was incredibly energy efficient, it required minimal infrastructure and wall penetrations, and it could also act as the heater.

Currently, the #1 Best Seller on Amazon, in Split-System Air Conditioners, is this 12,000 BTU model by Pioneer. 

From our readings and conversations, we have learned that, relative to older technologies, few people seem to be aware of the awe-inspiring power of the mini-split. Often times, we would find that someone would move into a newer home, want better heating or cooling than their previous home, and therefore request a bigger heater/cooler of the same type as they had before. They were compensating in a misguided way. The larger factor in creating a pleasant internal climate is eliminating the stratification created by drafty building practices. With a tighter house, one can create precise climate control and have excellent efficiency by using a mini-split.

A mini-split is magic in that it operates ‘above-unity’. That means that its output effects actually exceed its input requirements by a factor of three. Another way to say this is that we get three units of energy out for every one unit of energy consumed. In contrast, our Stanley space heater generates one unit of heat for every one unit of energy it uses.

The incredible feat is possible because of the way the machine works. Rather than turning energy directly into heat, it uses a relatively small amount of energy to pump refrigerant around in a clever way that transfers environmental heat out of the house or into the house, depending on the setting.  Hence the term ‘ heat pump’.

Past Experiences that we found Lacking

  • While living in CT, we used a large window air conditioner. Each year, we would wait until we could not no longer bear the humid heat. Then we would go get the monster down from above the garage and carefully install it in one of the downstairs windows. It did not cool the second floor. Our sleeping cooling regimen was to turn the downstairs AC off, open a couple of windows upstairs and shoot a powerful Vornado fan out a downstairs window. The suction effect would pull a stiff breeze through the upstairs.  While not incredibly effective, this system was inexpensive and quiet (because the fan was far away).
  • While living in CT, we had previously heated with oil (and water-filled wall radiators) – the typical heating setup of the Northeast. The wall radiators created a constraint in where we could put furniture, looked dated and were not quick in their ability to warm a room.
  • In our NM rental apartment, we had a wood-stove-sized propane furnace. It took up a fairly large amount of space, was loud and only heated (efficiently) the space around it.
  • In our NM rental apartment, which had no windows in the directions where wind actually moved (west to east, generally),  we were without AC. Those summer nights were the worst sleeping experiences I have ever had.

This is our old living room – we would install the window AC unit in the far right window to keep it out of our way when using the couch.

AC as viewed from the outside. (My, what a lovely terraced garden area! ….after I moved in, I weeded and beautified the area. The blackberries took over again shortly thereafter and this area once again became impassable.)

Radiators along the wall in our old bedroom.

As an alternative to a mini-split or a window AC, some homes use Central Air. When we were initially working through our floor plan designs (and planning to build super insulated and air-tight), we had tried to fit in the ducting for a ventilation system. After realizing how difficult this was with a tiny space without internal rooms, and upon discovering an ingenious ventilation system that required no ducts (Lunos e2), we scrapped the idea of incorporating ducts altogether. The same difficulty applies for Central Air. The extensive ductwork makes it a poor fit for a tiny/small house air conditioner.

Our Energy Efficient System for Heating and Cooling

For most people, running one’s air conditioner or heater all the time is not energy efficient. This poor efficiency results in high electric bills, making air conditioning more of a luxury item. Our summer electric bill in CT ranged from $270 – $300/mo. That being said, there is a way to build a home with both comfort and efficiency in mind.

The following approach was what we used when building our little house. It works for homes of all sizes (including tiny homes) but is easiest for new construction where one can be attentive to detail from the get-go.

Creating a Tight Building Envelope

In order to use a small house air conditioner all the time (i.e. have it set to go on when a certain temperature threshold has been reached) your home should be sealed up. Otherwise, you would be sharing your climate control efforts with the outside world.

So, step one is to build a house with good insulation that is as air-sealed as possible. You can also retrofit older homes by upgrading your insulation and improving your sealing efforts around windows, doors and other wall penetrations. While it was common building practice in times past to deliberately (or otherwise) not seal up one’s home, more recent energy-efficient practices have been implemented to create homes with a tighter ‘envelope’.  We chose to seal up cracks and holes (such as around wall penetrations, in framing members, etc.) in order to air-seal our home.

If you followed along during our build process, you often witnessed us blowing in spray-foam, applying caulk and shoving in foam closures – all measures to block the movement of air between the inside and outside. Our efforts also doubled as preventive measures against bug or rodent invasions.

Spray foam at the joint between the end cap and the arched wall.

Spray foam where wires come out of the arched wall and enter the bathroom.

Caulking the seams in the underlayment.

Installing plywood under the house to secure our recently blown-in cellulose.

Duct Seal jammed into the gaps of outlet and light boxes.

Providing Ventilation

Once you have an air-tight home, the next step is to install a mechanical ventilation system so you’re breathing fresh air.  As mentioned above, we eliminated a ducted system when we found our ductless solution. We installed the Lunos e² HRV system to continuously ventilate at 22 CFM with 90% thermal efficiency.  As prominent ASHRAE fellow Joe Lstiburek has said since 2005: “Build Tight & Ventilate Right”.  We couldn’t be happier with this system.

Our HRV system also does a great job of breaking up any indoor air layers and thus whatever heating or cooling effect is being generated by the mini-split. Despite having 17 foot ceilings, our house has no perceptible air stratification.

If you are not building the home yourself, then a quick note about contractors may be helpful. Unless a customer makes particular requests for construction details or energy certifications, contractors will generally do only what is required by building code. The building code sets a very low bar (R-13 walls anyone?) and makes no provision for evaluating a structure’s final energy efficiency. For more info about building to higher standards check out GreenbuildingAdvisor and the 475 building supply blog.

So, after building an air-tight structure and incorporating a ventilation system, the remaining piece of the puzzle is choosing a heating and cooling system that enables an installation with minimal interruptions to the air-sealed envelope. We knew we wanted a small house air conditioner and heater in one unit so it was not difficult to decide. Enter the mini-split.

One of the Lunos e2 HRV components on the north end cap.

Blueridge Mini-Split

White-label manufactured by the company Gree, we chose our Blueridge unit (9,000 BTC, 0.75 ton) primarily because it was the simplest and most reliable machine (according to Alpine Home Air). We liked its simple approach (i.e. the remote controller did not have a futuristic control screen) and It was also the least expensive we encountered at the time. Note that ours is as basic as it gets and there are machines on the market that perform with even greater efficiency.

The energy efficiency of mini splits is measured by a Seasonal Energy Efficiency Ratio (SEER). The higher the SEER, the more efficient the unit. Currently, federal requirements state that mini splits must have a 13.0 SEER rating to be used in the United States. Our unit has a SEER rating of 15. I have seen some available with a SEER rating of 31.

The model that is selling well on Amazon has a SEER rating of 17.2.

You can learn more about buying a mini-split over at GreenBuildingAdvisor. Read carefully the section about calculating your load. If we had gone with the size suggestion found here, a site that sells the products, we would have ended up with a 12,000-14,000 BTU unit.

Based on our calculations and things we read on internet forums, we had the sense that our 9,000 BTC unit would *probably* work for our house. It was recommended for a home no larger than 450 square feet.

[We ordered the same model for our extremely drafty business building (which is about 225 sq ft) and found it to be too powerful for our space. It worked GREAT but was sometimes difficult to establish a setting where we were not getting blown away by ice-winds.]

We purchased our unit (the Blueridge BMKH09-15YN4GA) for $529 from Alpine Home Air. Separately, we purchased the Alpine 40680150B3B6 line sets for $80. Total cost was $609. The shipping was free.

In case you are new to our blog, you can check out the installation of it here.

Downsides to the Mini-split as a Small House Air Conditioner

Most people will quote the high initial cost of the mini-split as the biggest downside. So if you are building as small as possible and/or using recycled/repurposed materials, the mini-split may not be your first choice. You may have also decided to go with no small house air conditioner and are using the windows open approach; in which case, perhaps a small propane heater would be all you would need.

We, however, understood how amazingly energy efficient the unit would be and were looking long-term. The $609 did not bother us. We were also planning to build the perfect small house and were therefore not concerned about allocating the wall space to the indoor evaporator component. We knew we would have plenty of high-up wall space.

For us, the biggest downside was needing to hire an HVAC technician to either install the unit or ‘commission’ it. This may not be a challenge in a large city, but here in Silver City, there was only one guy who could do it. And he waited until the last possible moment (a couple of hours before our plumbing/mechanical inspector arrived) to do the work. Including the permit the inspector requested that he pull on our behalf, this service cost us about $150. Note that we installed the unit ourselves; he just came to nitrogen flush the lines and establish a strong vacuum before allowing refrigerant into the lines.

The outdoor unit (the condenser) with linesets connected. We built the platform using concrete blocks. The lines enter the house right above the window through a 3″ wall penetration. We sealed up the opening using a combination of butyl tape and duct seal to air-seal around the opening.

Energy Bill with a Mini-split

We moved into our house last August but did not finish weatherizing (spray foaming around the windows, adding indoor window casings, etc.) until January. Both for our human comfort and the tuning stabilization of the piano (pianos are sensitive to swings in humidity and temperature), we opted for a consistent internal climate. Since moving in, we have kept the house at 70-76 degrees and 30-40% humidity. The mini-split automatically cycles on whenever a temperature threshold is exceeded. If we are particularly hot or cold, we may switch it to cooling or heat mode, respectively.

As a result of our approach (tight building envelope, mechanical ventilation with the HRV, a mini-split for heating and cooling and no windows left open), our electric bill has been quite reasonable. It includes the use of our on-demand hot water heater (‘tankless’), our mini-split, our induction cooktop and all other normal home appliances.

January    $107
February  $76
March       $57
April          $52
May           $51

With our particular temperature settings, we can walk around in shorts all year long. You’ll notice we pay a little more for this luxury in January. But hey, it’s all for the piano!

The weather started warming up near the end of February so we were not running the heat as much. For March through April’s end, we had it set to auto almost all the time; with the exception of heat mode in the early mornings and evenings.

In May, we stopped using the heat setting altogether. We would set it to auto when we went to work and when we slept. Afternoons and evenings were usually on cooling mode.

I’m curious to see June’s statement. That is when we really started using our small house air conditioner. Last month, we left the mini split on cool mode all the time. We would leave it at 76 when we left for work. And we would turn it down to 74 when we arrived home in the early afternoon.

Mini-split heat pump that acts as a small house air conditioner and heater

Mini-split setting when we arrived home on July 3rd.

What the piano monitor was reading at the same time. The only downside to using a small house air conditioner (or any other air conditioner for that matter) is that it can decrease your humidity beyond what is desirable. The HRV helps stabilize humidity.  We try to keep the piano at 40% +/- 10%. It works for the most part. The largest variation is in the drier direction. It tends to happen only during cooling mode when people or dogs have been out of the house for an extended period of time.  Dog breath really helps with humidity.

Summary

Some owner-builders may not want a small house air conditioner; especially if they live in a temperate area or they move their house around with the seasons. Given our desire to stabilize the environment for the piano, to have an energy efficient home and to live and sleep comfortably, the mini-split + HRV has been a fantastic option. We would do nothing differently.

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