Category Archives: space conditioning

Dissecting space conditioning

Our January cold spell along with new data from the 2020 Residential Energy Consumption Survey inspired me to further dissect the issue of space conditioning. When it comes to energy use in a building, space conditioning is the 900 pound gorilla in the room. We reduced our space conditioning load through three steps:

First step: The deep energy retrofit, which significantly reduced our overall energy needs through building envelope improvements among other things. This blog is packed with information on insulation, air sealing, window selection, etc. And you can find a summary post here on steps to reduce your overall energy needs.

Second step: Adding a photovoltaic array to our roof top to cover our remaining energy needs. You can search this blog for “solar” or “photovoltaic” to find detailed information on this step.

Third step: Installing heat pumps (also known as minisplits) for space conditioning. You can find more information by searching this blog for “minisplit” and “heat pump”.

The first step was the heavy lifting, and got us the biggest bang for the buck. In fact, our building’s energy consumption for space conditioning ended up below the national average.

And in monetary terms, cooling our building in 2020 was “free” because of the second step: our photovoltaic roof array which provided the needed electricity. Heating our building was almost free. It cost us $173.40 to heat our 4,500 sf building in 2020.

If you would like to know about the nuts and bolts behind those numbers, keep on reading!

Parsing out space conditioning

I used the solar year 2020 (April 1st, 2020 till March 31, 2021), because it was a twelve month stretch where all our space conditioning needs were covered by our heat pumps (single head minisplits). I separated out the general electrical consumption from the energy used for space conditioning by looking at our electrical use on a monthly basis, plus factoring in data from our home energy monitors. The building’s average monthly electrical consumption for everything but space conditioning was 700 kWh.

Our building’s energy use for the solar year 2020 totaled 13,428 kWh. Assuming the average use of 700 kWh per month, we used an estimated 8,400 kWh during the solar year 2020 without accounting for heating and cooling, which took an estimated 5,028 kWh.

solar year 2020Building (kWh)One household in our building (kWh)
Total energy use13,428 (100%)4,476
General energy use (excluding space conditioning)8,400 (62.5%)2,800
Energy use for space conditioning5,028 (37.5%)1,676

2020 data from the U.S. Energy Information Administration shows that space conditioning consumes 46% of the building’s energy use in 2-4 unit apartment buildings like ours (or 52% on average per U.S. household).

Our deep energy retrofit allowed us to reduce that number from 46% to 37.5%, an estimated 8.5% decrease during the solar year 2020.

We are not talking about how much energy is used here, but how that energy use is distributed across various categories, from space heating to refrigeration and all other.

When comparing the 2020 data to that of 2015, we see that these numbers are fairly constant. They are actually hard to change, particularly in existing buildings, because of long established construction types, materials, and methods.

The fact that we were still able to shrink the percentage of energy going towards space heating and air conditioning by a whopping 8.5% for the solar year 2020 is a testament to the success of step number one: reduction of our overall energy load through building envelope improvements. And it pays off:

In terms of heating cost…

…how did I get to $173.40 to heat our 4,500 sf building for the solar year 2020?

From April through to December we only paid for fixed costs ($12.83/month for customer and meter charges) because our photovoltaic array combined with our net-metering agreement covered our electrical needs. For the last three months of the solar year (January, February and March) we had to purchase electricity and paid a total of $289 for the 2,038 kWh we used.

TotalkWh w/o space conditioningSpace conditioning
Jan 20211,839 kWh minus700 kWh =1,139 kWh
Feb 20212,139 kWh minus700 kWh =1,439 kWh
Mar 20211,254 kW minus700 kWh =554 kWh
Total5,232 kWh (or 100%)3,132 kWh (or 60%)
Total cost$289 (or 100%)$173.40 (or 60%)

Looking at the total kWh consumed and the breakdown between kWh for space conditioning and kWh for everything else, an estimated 60% ($173.40) of that energy went towards space conditioning (heating) our 4,500 sf building with the minisplits for the three months we ran a deficit.

There is nothing mysterious about this, as long as you don’t fall into the trap by starting your project with a heat pump.

Follow the three steps, and numbers like this (or better) can become a reality:

  1. Address thermal deficits in the building envelope first to significantly reduce the overall energy load of the building.
  2. Combine those improvements with a renewable energy project, such as a photovoltaic array, that now has the potential to cover 100% or close to 100% of your energy needs. 
  3. Install an efficient heat pump system that is small and compact due to the reduced overall energy load of your building, and subsequently is largely or entirely powered by your renewable energy system.

But there was something magical about this: We ended up with a very comfortable home!

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Minisplit cooling pause

A typical summer in Chicago comes with heat and humidity that is every now and then interrupted by cooler spells with lower dew points. Those spells can be pleasant enough for us to stop running the minisplit in cooling mode and instead open the windows.

Once the heat and humidity roars back into town, we shut the windows in a hurry and power up the minisplit for that pleasant cool breeze. Except, there isn’t much pleasantness in that breeze, unless you enjoy a musty and mildew-drenched flavor.

If you abruptly stop the minisplit in cooling mode, the fins on the evaporator/condenser will still be drenched in condensate droplets. It is not easy to see in the above pictures, but believe me, the droplets are hiding in there.

And they will be sitting there for several days like a bunched up, wet towel in the corner of someone’s bathroom. If, after a few days, you dare to pick up that towel and give a sniff, you experience a similar flavor to that of the minisplit after it had been paused for a few hours or days. It is a death knell to indoor air quality (IAQ).

The good news is that this is an easy to solve problem. Rather than abruptly stopping the minisplit in cooling mode, switch it to low speed fan mode, and let it run for half a day or overnight. The fan keeps drawing air across the fins and will slowly dry them out.

It’s like taking your wet towel and hanging it up to dry. That towel definitely will smell a lot better – and so will your minisplit once you start it up again in cooling mode.

If you would also like to dry out the condensation collection pan at the bottom of the indoor unit, keep the minisplit in fan mode for a good day. This is definitely recommended at the end of the cooling season (end of summer).

And if you turn off cooling mode for a week or longer before starting it up again, you may want to consider cleaning the condensate drain line, as described in the previous post, just to be on the safe side.

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Minisplit cooling startup

Cooling season has started. Our living space has been comfortable in terms of temperature and humidity since we turned off the heating mode on our minisplit back in March. Now it is time to bring the temperature and humidity down a notch so that we can sleep comfortably at night.

The last time the minisplit ran in cooling mode was about eight or nine months ago. Since that time, dust may have accumulated in the condensate collection pan. Once that dust mixes with the first condensate from the heat exchanger coils, it may cake up and block the drain line that is supposed to safely evacuate the water to the outside.

If that is the case, you will notice water droplets on the luvers and a water puddle on the floor under the minisplit.

It’s time to turn the minisplit off and clean that condensate drain line. Or, even better, as a routine maintenance item, preemptively clean the condensate drain line at the beginning of each cooling season.

To do so, find the discharge point of your drain line, which typically would be outside the building. Take a wet/dry shop vacuum with a narrow nozzle. Fit the nozzle over the drain line and proceed to evacuate any water, dust and crud that may have accumulated in the drain line since it last ran in cooling mode. Once the vacuum doesn’t pull any more water or crud out of the drain, start up the minisplit in cooling mode and monitor whether you get any more spillover from the condensate collection plan on the indoor unit. If you do, repeat the cleaning process. If you don’t, great job, and enjoy your cool building interior!

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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.

minisplit-21

minisplit-22

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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