Category Archives: space conditioning

The Heat Pump debate

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The motivation to switch to a heat pump is often rooted in the desire to reduce heating costs. I get that.

High heating costs are typically associated with inefficient buildings that have a poorly insulated and drafty building envelope. Such buildings would need a rather large heat pump with a ducted or multi-zone set up, or even several heat pumps.

Our building with a subpar building envelope prior to any improvements

Anecdotes from heat pump forums, acquaintances, and friends indicate that owners of buildings with a subpar building envelope often regret switching to heat pump – mainly because their electrical bills went through the roof. If building owners assume that heat pumps are a quick fix, their conclusion that heat pumps are a waste of money, and that it is cheaper to heat with natural gas, is on fairly solid ground. But if building owners approach heat pumps as part of a broader systemic plan, heat pumps can and do generate significant cost savings. 

The system approach

We have a masonry building, built in 1902, with three apartments. Each apartment has its own airsource heat pump (single head minisplit), which is placed centrally in each apartment.

They run great! They provide the needed cooling during the summer and just about the right amount of dehumidification. They provide the needed heating during winter, even during spells of subzero temperatures. And they heat significantly more cheaply than our hydronic baseboard radiators that are powered by a high efficiency natural gas boiler.

But why?

The minisplits are only ONE component in our system approach. Preceeding and prerequisite components in our system approach were the improvements to the building envelope during our deep energy retrofit, which allowed us to substantially reduce our overall energy load: Roof insulation; wall insulation; foundation insulation; basement slab insulation; triple glazed, airtight windows; insulated exterior doors with good weather stripping; air sealing; etc. All these efforts reduced the energy load of the building by around 2/3 and catapulted us into cost-effective airsource heat pump territory.

Now that we had reduced our energy load, I could build on it with the next component: “right sizing”. I hired a competent energy auditor to run an energy model for each of the three apartments to determine the heating and cooling load.

Undersizing heat pumps is not a good idea as it will be difficult at times to keep the building warm or cool enough. Oversizing heat pumps introduces its own problems, such as short cycling, which in turn reduces efficiency and comfort, and can lead to insufficient dehumidification during summer.

The results from the energy model allowed us to go shopping for the “right sized” equipment.

We installed a 9,000 Btu cold climate Fujitsu minisplit in the garden unit (900 sf), while the 1st and 2nd floor apartments (1,500 sf each) have a 12,000 Btu cold climate Fujitsu minisplit each.

In addition to the heating and cooling loads, I paid close attention to the moisture removal capacity of the minisplits, to make sure they deliver a comfortable relative humidity level during the dog days of our Midwestern summers.

For us, the next component in our system approach was looking into a renewable energy source. Because our space conditioning load was drastically reduced, reflected in the three relatively small heat pumps, we could power them with our photovoltaic roof array. The result, thanks to our net metering agreement, is next to no heating or cooling cost.

Our electricity cost per household compared to the Illinois average. Our electricity use includes all cooling and most of the winter heating with our minisplit.

Taking a system approach, we were able to turn the paradigm on its head: Our high efficiency natural gas boiler cannot compete cost-wise against our heat pumps.

Disclaimers

But it’s not all unicorns and rainbows.

Heat pump installation requires professional expertise, attention to details, and skills, in particular when it comes to sizing and the flare connections. To avoid a never-ending loop of service calls, it is a good idea to be picky about who you let near your heat pumps.

Yes, the right heat pumps work in freezing temperatures. Our cold climate minisplits even powered us through a brief polar vortex with subzero temperatures. However, the colder it gets, the less efficiently they run. I observed that our heat pumps notably lose efficiency once we drop below 20F. They still do the job. They just draw more power.

Heat pumps are likely to run differently compared to your old heating system. If you had forced air, you turned it on and it was warm 10 minutes later. That is unlikely to happen with heat pumps. They ramp up slowly but efficiently. With heat pumps, you can enjoy more even temperatures, day and night.

Heat pumps can emit an unpleasant stale and moldy smell in the summer (see also this blog post). This happens when they run in cooling or dry mode and are turned off. The remaining condensation on the coil of the indoor unit combined with dust particles is the perfect petri dish to grow mold. This can be avoided by running them in fan mode for a few hours before turning them off. That will dry off the coil. No moisture, no mold.

And like everything else in a house, heat pumps require regular maintenance. You want to make sure to regularly clean the filters and drain lines. Also inspect the outdoor and indoor coils, and the indoor blower wheel, and have them cleaned by a professional when needed.

These are not deal breakers, at least not for us. But it’s good information to have, in case you also want to turn the paradigm on its head.

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Dissecting space conditioning

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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

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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

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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

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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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