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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Energy Fair 2023

June is the month of the Midwest Renewable Energy Association’s (MREA) annual Energy Fair! This year, the event will take place Friday, June 23rd through Sunday, June 25th. The event location is, as always, at the MREA headquarters in rural Custer, Wisconsin.

If you are following this blog, you are probably looking to solve problems around the energy efficiency of your house, have green building or building science questions, or want to make your home more sustainable. I hope the blog will answer some of your questions. But I am sure that a day at the Energy Fair will answer a lot of your questions.

The Fair is a wonderful mix of educational sessions, entertainment, and exhibitors and vendors, many of whom show products that may become handy in making your home more energy efficient.

The educational sessions take place in large tents on the fairgrounds. There are eight to twelve tents (depending on the year) with sessions running parallel to each other all day long. Most sessions are an hour long.

The subjects covered include building electrification, energy efficiency, electrical vehicles and clean transportation, going solar, policy and community organizing, project finance, sustainable farms and food production, etc. You can take a look through the workshop schedule here.

Cathy and I attended the workshop the first time back in 2007. We were there just for the big day, which is always Saturday. It was a very long day, but worth every second. We came back home with so much useful information and so many of our questions answered. And even better, we came home with so many more questions generated. It was the jump off for our deep dive into our deep energy retrofit project.

The next time we attended, we opted to go for the full three days of the Energy Fair to take home as much information as we could, for the pure joy and fun of the event, to meet up with old friends, and to network and make new friends. And we’ve attended all three days just about every year since then, except when they paused the Fair during Covid.

Attending the event is surprisingly inexpensive. A one day pass at the gate cost you $20. A weekend pass at the gate costs a mere $45. I can’t remember a conference or fair I attended that only charged $45 for three days, and offered this amount and quality of workshops, exhibitors and entertainment!

If you would like to dip your feet into the MREA Energy Fair world to see if this is something for you, I would recommend driving up to Custer for Saturday, like we did back in 2007. I would not be surprised if we will see you again the following years on Friday, Saturday, and Sunday. Because this is an event one starts looking forward to starting in January!

And no, I have not been asked by MREA to write this post or promote the Energy Fair, nor am I paid compensated by MREA. This is a wonderful and fun resource that I believe is worthwhile sharing. I have, however, been honored to present some educational sessions over the past few years – and I’m presenting two sessions this year.

Hope to see you in Custer the weekend of June 23rd!

Hunting for replacement ERVs

It was time to research replacement ERVs (Energy Recovery Ventilator) after two of our units broke down in 2020. A couple of key aspects re-emerged in that process.

  1. The heat recovery efficiency of our Recoupaerator 200 DX was unmatched based on HVI (Home Ventilation Institute) data, which was confirmed with my own testing.
  2. It looked like the Recoupaerator 200 DX was the only residential ERV on the U.S. market that used an enthalpy wheel for the heat and moisture transfer.

Other residential ERVs use a static core heat exchanger, or core in short. Unlike an enthalpy wheel, the core functions on the principle of cross flow.  The exhaust and fresh stream flow across each other in the core without mixing. In the process thermal energy is transferred through the core’s membranes. And if we want to get all technical, we are talking about sensible energy (heat) and latent energy (moisture).

The cores come in two shapes: square and hexagonal. And this apparently small difference has a big impact when it comes to energy recovery. A hexagonal core has more surface area and thus provides more opportunity for sensible and latent energy transfer.

A quick review of product specifications led me to conclude that an ERV with a square core would be out of the question for us, because of the rather poor energy recovery rates.

Looking at products with a hexagonal core, I was left with three available options:

  • Zehnder CAQ350 ERV
  • Panasonic FV-20VEC1, and
  • Broan ERV200 ECM (also sold under the Venmar brand name).

To help in the decision making process, I pulled the HVI performance data for each product so that I got a good comparison.

The three hexagonal core options appeared to be all in close range of each other. Zehnder seems to be a little bit of an outlier on the net air flow side for the test data, while the Panasonic and Broan are in close range.

Looking at the power consumed in watts, it was a close race, where Broan emerged with the least power consumption.

As for the energy recovery rate – or if you prefer the technical term, the Adjusted Sensible Recovery Efficiency (ASRE), we have another close race with Zehnder squeezing into first place, closely followed by Panasonic and Broan.

These were useful data to have, but I still was left without a clear preference between the three options. So I began to look at cost. And remember, these were pre-inflation prices. Zehnder came in just above $3,000, Broan landed just under $3,000, and Panasonic just under $2,000.

It looked like I was left with two favorites in this horse race: Panasonic and Broan.

Between the two, the Broan ERV seemed to be the most compatible. It has similar dimensions to the old Recoupaerator 200 DX and as such would fit nicely into the ventilation closet. It also had very similar controls and low voltage auxiliaries, just like the Recoupaerator 200 DX.

The Panasonic also had very similar dimensions to the old Recoupaerator 200 DX and as such would fit nicely into the existing ventilation closet. But its controls seemed rather primitive, and the auxiliary controls for some reasons all required line voltage. And it was unclear what advertised auxiliary controls would actually be available.

Based on my past experience, I was not in the mood to bet all my money on one horse. Diversifying my investment seemed to be a safer path to take. So I ended up ordering one Panasonic FV-20VEC1 and one Broan ERV200 ECM to replace the two failed Recoupaerator 200 DX.

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Failure of Recoupaerator 200 DX

The pandemic brought a kind of perfect storm for many of us. In our case, it was ventilation related. Two of our ERV’s broke down – right when we really needed to rely on good ventilation. Although, considering what other folks had to endure, ours wasn’t really a perfect storm but more of an inconvenient breeze.

We had three Recoupaerators 200 DX Energy Recovery Ventilators by UltimateAir in our building, one for each apartment. And as mentioned in the past, these ERV’s were one of my favorite green building gadgets.

Six months into the pandemic, the ERV for the garden unit broke down, followed by the one on the 1st floor. You may be asking: “What is the big deal? Just get replacement parts, or a new unit”.

I tried. I contacted UltimateAir, and got crickets chirping. I eventually found out that UltimateAir went out of business at the beginning of the pandemic.

I was now sitting on three ERVs with no tech support or good way of obtaining parts or even a replacement unit. That this was a disappointing experience is an understatement. I loved the setup, effectiveness and efficiency of these ERVs from UltimateAir.

But it looked like it was time to let go and research alternative products that I could use as replacements. And that had its own challenges because of the supply chain issues that reigned during that time.

We did the only thing that we could do: Ventilated the old fashioned way by opening windows, tried not to think too much about the associated energy penalty, and remained hopeful that the supply chain would eventually unclog.

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How close to net-zero are we?

To answer this question, let’s look at the data for the solar year 2020 when our electrical use included space conditioning.

Our annual use totaled 13,428 kWh that year, while our annual production amounted to 11,390 kWh. The solar array produced enough electricity to cover 85% of our annual consumption.

To reach net-zero, we would need to be at 100% or above. So we are around 15% short of net zero and had some more homework ahead of us.

The moving goal post…

When we embarked on the project in 2009, all-electric homes were not a thing yet, heat pumps were hard to find, and solar arrays were uncommon.

At the time my focus was on using a solar hot water system to heat the building and for domestic hot water, and a photovoltaic array to cover our electrical needs. But I always found myself on thin ice when attempting to cover space heating and domestic hot water with a solar hot water system alone. In other words, getting away without a natural gas connection seemed impossible, which made the net zero goal hard to reach.

I pivoted my focus into significantly reducing the overall energy load of our building. If I had to use natural gas as an energy source, I wanted to use as little as possible. That put us on the path of our deep energy retrofit.

And it paid off.

Interim results in 2012 showed that our improvements reduced our electrical consumption by an estimated 57% and preliminary results from 2016 showed that we reduced our heating needs by an estimated 80%.

A lot has happened since 2009. Green building technologies that once were only known from excotic places like Europe or Asia suddenly made an appearance in the U.S. market, such as heat pumps. And with that, my focus on solar hot water fell away, because heat pumps emerged as a more economic option that I still could use, even if the sun was not shining.

Reducing the general electrical load of a building also has become easier since 2009 with increasingly efficient energy star appliances, LED lighting, etc.

What is standing in the way of net zero?

Yet we are not net zero, to my chagrin. What is standing in the way are two key factors:

  1. That we still rely on natural gas for cooking, domestic hot water, and occasional heating. And we still have a gas dryer.
  2. That our solar array is not large enough to cover 100% of our energy needs should we go all electric.

The second point should be reasonably easy to solve. Because we have reduced the energy load of our building significantly, we have enough room to expand our solar array to cover 100% of our energy use. And we plan on doing so – eventually – once technology catches up.

Regarding the first point – our natural gas connection – it helps to know how much natural gas goes towards what source in our building.

Analyzing our utility bills over the past seven years revealed that about 700 therms (70%) went towards space heating, with only 300 therms (30%) going towards domestic hot water, ranges, and the dryer.

The 300 therms seemed to be easy to solve. We can replace our gas dryer with a condensing dryer. The gas ranges can be replaced with induction stoves. And the heat pump water heater technologies have improved to the point where we could say goodbye to a gas fired water heater too.

As for the 700 therms going into space heating, one could argue that we solved that problem already with the addition of our minisplits. We used them to heat our building during the solar year 2020, and it worked.

But there is a problem: we drank the kool aid.

We originally, and occasionally still rely on our hydronic heating system, powered by a high efficiency boiler. The steel baseboard radiators and radiant floors deliver a comfort during the heating season that is unmatched.

The good news is that we potentially could replace our boiler with an air-to-water heat pump that used CO2 (R744) as a refrigerant. These units are slowly making an appearance in the U.S. market and are able to deliver 130F water even at very low exterior temperatures. 130F would be a suitable temperature for our hydronic heating system and domestic hot water.

Not only that, but an air-to–water heat pump would be two to three times more efficient than our high efficiency boiler. In other words, it would only require half or one third of the energy input to produce the equivalent of 700 therms heating output.

I am hopeful to eventually replace our boiler with an air-to-water heat pump and solve the 700 therms that were needed for space heating.  We subsequently could cut our natural gas supply to the building, and yet still enjoy the comfort of our hydronic heating system.

That must be expensive!

In the big picture, what is the cost of doing nothing?

And on a project basis, if it is expensive depends on one’s mindset.

Most of our system decisions, such as the heating system, were not solely based on the economics of the day, or “what is the cheapest system I can get.” We were comfortable investing in systems with a longer payback period as long as they came with:

  1. a high level of energy efficiency,
  2. some level of resiliency and longevity,
  3. improved indoor comfort and health without an energy penalty, and
  4. systems that were somewhat future-proof so that they could adapt to technology upgrades.

This required a lot of research and careful planning at the onset of our project. And it required a lot of luck, as we were gazing into the future trying to predict the path green building technologies would take.

And in practical terms?

It appears that our utility room layout could accommodate the switch from boiler to air-to-water heat pump without revamping the whole hydronic heating or domestic hot water layout.

And because we were mindful when we installed an all new electrical metallic tubing (EMT) based electrical system, providing 240V for the induction stoves and potentially the condensing dryer should just be a matter of simple rewiring.

The one item that wasn’t even remotely on the radar in 2009, and that I still have to wrap my head around, is how best to integrate and accommodate EV charging stations.

In summary: We are not net-zero yet. We are fairly close, and we know the path that will take us there.

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