2012 was a hot one. We’ve had at least 31 days over 90F, something which hasn’t happened since 1988, and July 2012 was the second warmest July on record in the Twin Cities. This is a departure from the norm; for example the average number of 90F days is 13 – we almost tripled it this year. The 1930′s homes in our neighborhood weren’t built with AC, and retrofitting ductwork is tough. And although I obsess over our energy use like some people obsess over Kim Kardashian, we do have to be able to function on those sweltering days. Last year we struggled through the hot week with a single 8,000 BTU window unit upstairs, which was cumbersome, noisy, ineffective, and inefficient.
Enter the Mini-Split
These units come in various configurations, but simple ones like ours have an outdoor compressor/condenser similar to central AC and a single indoor air handler that hangs on the wall. Refrigerant, power, and control lines run back to the outdoor unit. Because it’s a matched set, and because there are no ducts to blow through a hot attic and leak air, they can be very efficient – ours is 25 SEER, 13.8 EER for cooling. For comparison, Energy Star standards for central AC require >=14 SEER/ >=11 EER.
We weren’t going for perfect comfort, just something to keep it tolerable, and keep the upstairs cooler at night for sleeping. One consideration with these units is that they are a single point of conditioning, and distribution can be an issue*. We installed only one unit at the top of the stairs, figuring some cold air could fall and hot air could rise. If we’d been going for more consistent whole-house comfort, at least one more head would have made sense, and we may still do that in the future.
In the end, we selected a single 12,000BTU (1 ton) unit after doing a whole-house heat gain analysis to determine the necessary cooling capacity. I contacted the fine folks at The Neighborhood Energy Connection in Saint Paul to do the analysis.
*other configurations have a small unit above/below the living space with short duct runs to different rooms to solve this issue, at the cost of some efficiency.
Effectiveness & Energy Use
Overall, it’s been pretty good. We put it in just before a perfect storm of hot: 103F days, both sides of the family visiting, and a birthday requiring use of the oven to bake a cake. That was a struggle, but it kept things reasonable – around 80F downstairs – and much drier, which makes a huge difference. Distribution was a bit of an issue; it’s hard to cool a hot kitchen with a unit on the 2nd floor.
Here might be a more representative day: Sept 11 2012 had a high of 93F. We closed the house and set the unit to 74F at about 11:00AM, and turned it back off around 7pm. The bigger spikes are likely the espresso machine. ;) Stuff later in the evening is cooking & running the dishwasher.
The graph shows indoor & outdoor temperatures, solar PV power production, and household power use. Outdoor temp peaked at 93F at 4pm, and indoor temps on the first floor peaked at about 76.5F around 1pm. On the energy side, we used 13kWh and produced about 13.5kWh. If we’d anticipated a string of hot days, we would have just left it on 24/7 to keep things cool and dried out.
In July 2011, with the window unit struggling to keep up, we used 33kWh-38kWh per day for the hot days. In July 2012, with the mini-split running 24/7 on the hottest days, we used between 19kWh and 28kWh per day – quite an improvement.
It Heats, Too
The unit can also work in reverse, as an air-source heat pump, with an HSPF of 12.0, meaning for every
BTU watt-hour it consumes, it moves 12 BTUs of heat into the house. Converted to Coefficient of Performance, this indicates that it transfers about 3.5 units of energy into the house for every unit of energy it consumes – or 3.5x more efficient than a resistance space heater. It maintains rated heating capacity down to 20F outside, and continues to function even in -5F weather.
I haven’t yet decided if we’ll use it much for heating, but it might make sense in the “shoulder seasons” when our 83 AFUE boiler would be operating infrequently and at lower efficiency. Because these electric units can heat and cool so efficiently, they are often used in the design of well-insulated net-zero or near-net-zero homes with PV installed.
Overall I’m pleased with this thing. It’s quiet, efficient, attractive, and made the hottest weeks quite tolerable. It helps that we’ve done air-sealing and insulation of our home, so the single unit is better able to meet our cooling load. I may experiment with sun control screens on the south windows next year to cut down on solar heat gain.
It’s handier, so we did run AC more days than we did last year, which is a risk if we’re trying to conserve energy. And although some studies have shown that window ACs actually save energy for that reason, in our case we used 588kWh in July 2011, and 488 kWh in July 2012, and July 2012r was hotter:
Cooling Degree Days (Base65) (Base50) Month Dept. Month Dept. Total from Total from Norm. Norm. July 2011 433 174 898 179 July 2012 475 199 940 202
Less energy + more comfort = win. We might be tempted to put one more unit on the first floor next year. I’m not sure how that will affect overall energy use, but because the home’s cooling load will be the same (and we won’t need a bunch of fans to blow the cool air downstairs) I wouldn’t expect it to rise significantly – we’ll see.