Category:

Happy Equinox

3/20/2024

The Vernal (Spring) Equinox arrived last night at 11:06 PM EDT. So, for some of us yesterday was the first day or spring while it's today for the rest of y'all.

Living in a passive solar home makes us appreciate the Equinox. At sunset, the sun shines directly down the hallway and we have an "Indiana Jones" moment.

We had a good freeze Sunday night and near freeze last night. The daffodils are about done but the hostas are sprouting. Time to start planting!

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2018 Energy Report

1/24/2019

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We received our final electric bill for 2018 last week so it's time to summarize the 2018 energy use. The total energy use was 16,160 kWh (1,897 more than last year) at a cost of $1856 ( $140 more than last year).

The following chart shows the overall use by category.

We used a total of 4980 kWh for HVAC (compared to 3882 last year). This was for a total of 4107 Heating Degree Days (HDD) compared to 3548 last year. In other words, the heating seasons were 16% colder but we used 28% more energy for heating and de-humidification.

The next chart shows the actual HDD and CDD for 2018 versus the historical model. February was much warmer while January, March and November were colder than average.

The next chart illustrates the energy consumed for heating. As in previous reports, the dashed grey line shows the theoretical energy used per Heating Degree Day (HDD) with no sun (i.e. a non-solar house). The dashed yellow line shows the theoretical energy use taking passive solar gain into account. In general we expect to receive about 30% of our heating load from the sun.

The solid orange line is the most important and shows the actual performance over the year. If the orange line is below the yellow line then the house is performing better than expected (i.e. it is using less energy than predicted).

The house did not perform as well as last year. I have no explanation for the particularly poor performance in October. In January and February I was using the wood stove to supplement the heat and the energy use was substantially less than the model. I used the stove very little in the other months because of the moderate temperatures.

Historical Perspective

To be noteworthy (or should I say "Magazine-worthy") a project needs to achieve net-zero which means that the house puts as much (or more) energy back into the grid as it consumes.

Georgia is a solar-unfriendly state. Net-zero would be economically impractical here so I decided that designing neoTerra to only consume 20% as much energy as the Chicago house would be a worthwhile and achievable goal.

Both houses have approximately the same amount of conditioned space so a direct comparison is fair. Note that Chicago is a 6000 HDD climate while North Georgia is only 4000 HDD. This means that roughly 33% of the savings in heat was achieved by merely by moving to a better climate.

That leaves about a 50% reduction that must be achieved through better design and construction.

The following chart shows energy use for a 4-year period in Chicago (2011-2014) followed by the subsequent 4-year period here at neoTerra. Note that Chicago utilized natural gas for heating, hot water and cooking so I have converted the electric consumption to equivalent BTUs.

The average energy use in Chicago was 2078 Therms/year while the average for neoTerra is only 480. In short, the energy we consume here is only 23% of what we previously consumed. That's pretty close to the original goal of 20%.

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January 2018 Energy Report

2/20/2018

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I'm getting away from doing monthly energy reports and will only be writing seasonal and annual reports instead. However, in the 2017 Annual Summary report last month I promised that I would review the results from using the wood stove to supplement the heat pumps. I will cover that in this post.

Energy Report

At 952 Heating Degree Days (HDD) this past January was the coldest month in the three years we've lived here. With the help of the wood stove, we only used 1012 kWh for heating (more on that later in this post).

We used a total of 2065 kWh so about 49% was for heating. The heating cost was about $91 (1012 kWh at about 9 cents per). All other items were pretty typical for winter use.

The Economics of Heating with Wood - Part 2

In December and January I burned roughly 1 face cord of mixed wood each month. As you might expect, both months showed a reduction in electric use. With two months of data I can now compare the results against past months that had similar heating loads.

First, I'll explain a bit about the two month experiment. If you're heating with propane (for example) the comparison is easy. You can just compare the cost of propane to the cost of an equivalent amount of firewood.

Liquid propane produces 91,500 BTU per gallon. If propane costs $1.60 then one therm (i.e. 100,000 BTUs) will cost about $2.19. Mixed firewood produces about 24,000,000 (yes million) BTUs per cord. If you can buy the firewood for $250.00 per cord that comes to about about $1.25 per therm. At these example prices the firewood is clearly less expensive than propane.

Note that firewood, propane, natural gas, etc will always produce a consistent amount of heat regardless of the outside temperature. In other words, a gallon of propane always delivers 91,500 BTU no matter how warm or cold it is outside.

Heat pumps, however, are more efficient at warmer temperatures and less efficient at colder temperatures. In simple terms, this means that the colder it gets, the more expensive it is to run. The following chart shows the efficiency of the Diakin mini-splits taken from the Diakin Engineering Manual.

The chart is a little hard to read but it is showing efficiency from 5 degrees F (on the left) up to 60 degrees on the right. The solid orange bars show the actual efficiency at the Diakin engineering test points of 5, 14, 23, 32, 43, 50 and 59 degrees. The efficiency is about 1.5 at 5 degrees and is an amazing 2.82 at 59 degrees.

The hollow orange bars are from my computer model. I have decreased the efficiency by about 10% in a rough attempt to account for defrost cycles.

The next chart shows the cost per therm at outside temperatures from 5 degrees to 60 degrees. This chart assumes that electricity costs about 9 cents per kWh and firewood costs $250 per cord.

The orange firewood line is flat and costs $1.25/Therm regardless of outside temperature. The Diakin (yellow line) ranges from $1.73/Therm at 5 degrees down to only $0.85/Therm at 60 degrees.

At 25 degrees they are equal and at temperatures above 25 it is cheaper to run the Diakins. I should also point out that even at 5 degrees the Diakins are less expensive to operate than burning propane!

Results

Based on the theoretical performance outlined above I decided I would use the stove only when the temperature is below 30 degrees. I used about .75 face cord in December and about 1 face cord in January (which was colder). We have a small pile left for February.

By comparing to previous months it appears that this saved between 30-40%.

For example, in January 2018 we had 952 HDD and used 1012 kWh. The closest previous month was January 2016 at 912 HDD and 1507 kWh. January 2018 was 5% colder but used 1/3 less energy. The wood stove saved abut 550 kWh.

December of 2017 showed similar results. December 2017 had 785 HDD and used 799 kWh compared to February 2016 where we had 747 HDD and consumed 1139 kWh. Here again the savings is about 1/3.

Although the results are good, it shows that we are only saving $45-50 per month from an amount of firewood that would cost about $80 if I had to buy it. I suspect that my free firewood is pretty low quality and that all oak or hickory would do better. In any case, it's a free resource so I will continue to use the woodstove in the future.

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2017 Energy Report

1/21/2018

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We received our final electric bill for 2017 a few days ago so it's time to summarize the 2017 Energy Report. The total energy use for the year was 14,738 kWh at a total cost of $1716.21. This was about 1,000 kWh more than last year.

Of this, $1476.21 (approximately 10 cents per kWh, as expected) was for energy and the remaining $240.00 was for connection/metering fees.

The following pie chart shows the overall energy use.

The well, water heater, refrigerator and dryer were consistent with previous years.

We used a total of 3882 kWh for HVAC, which is approximately $382.00. We did not run any AC but we did consume about 430 kWh dehumidifying the lower level. Overall, we used a lower percentage of energy on HVAC but a much higher percentage of "not measured" than 2016.

The following chart shows that January through April were warmer than historical average while October through December were close to average.

The next chart shows the energy consumed for heating. As in previous reports, the dashed grey line shows the theoretical energy used per Heating Degree Day (HDD) with no sun (i.e. a non-solar house). The dashed yellow line shows the theoretical energy use taking passive solar gain into account. In general we expect to receive about 30% of our heating load from the sun.

The solid orange line is the most important and shows the actual performance over the year. The house performed better than the model in every month except January.

December is particularly important. This is the first month where I started an experiment to use the wood stove whenever the temperature dropped below 30 degrees. Although this past December was 10% colder than average, the house performed about 30% better than the model.

I can't jump to conclusions since there are other variables that can affect the performance (number of sunny days, more windy vs less windy). However, the results look promising and I will continue the experiment through January.

In the January report I'll review the results over the two month period and provide a further discussion on the economics of wood heat.

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Fall 2017 Energy Report

12/14/2017

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Winter will officially start in a few days on the Winter Solstice, December 21st, so this is a good time to review the first two months of the 2017 heating season.

Fall 2017 Energy Use

The first chart shows the Heating Degree Days this year compared to the historical average. October was almost perfectly average and November was slightly warmer than the historical model.

The second chart shows the actual energy use compared to last year. As mentioned in the previous report, we are using about 100 kWh ($10.00) more per month this year. Some of this increase is due to running dehumidification on the lower level and some is due to additional appliances, equipment and lighting.

The third chart is the most important as it summarizes the heating energy efficiency.

As in previous reports, the dashed grey line indicates the theoretical efficiency with no solar energy based on the historical average temperatures for each month. The dashed yellow line shows the theoretical efficiency with solar gain. Recall that lower on the chart indicates better efficiency (i.e. less electricity used).

This month I have added a new element to the chart. The light orange line shows the theoretical efficiency (i.e. the computer model) based on the actual temperatures recorded for the month (rather than the historical average temperatures). In the future I may just remove the dashed yellow line since it isn't as useful as actual data.

The darker orange line shows the actual efficiency recorded for the month. Except for January, the house is performing better than the model. The heat pumps are probably operating a bit more efficiently than the values I used in the model. In March, April and October we were not using the wood stove so I plan to adjust the model to line up better with the actual results.

The Economics of Heating with Wood

We installed a wood stove last December but didn't use it much. This winter I plan to burn two face cords of seasoned wood to get an idea of how much energy we can save.

A cord of wood is a stack that measures 8 feet wide by 4 feet high by 4 feet deep. Thus, it is approximately 128 cubic feet of wood.

Firewood is typically cut 16"-18" long so it will fit into a fireplace. A face cord is 8 feet wide by 4 feet tall but only one log deep. In other words, a face cord is 1/3 of a full cord (about 43 cubic feet).

The energy content of mixed hardwood is about 25,000,000 BTU per cord (i.e. 250 Therms). A high efficiency stove is about 80% efficient so we might expect to get about 200 Therms of heat from one cord.

The cost of firewood varies a lot from on area to another, but here in the woods of North Georgia a full cord of split firewood costs about $250 - $400. Assuming that you're a good negotiator (and buy off-season) this equates to roughly $1.25 per Therm.

Based on our computer model and two years of actual heating, the cost with the air heat pumps is $1.46 per therm.

Surprisingly, heating with wood is only slightly less expensive than running the heat pumps. Considering the inconvenience of stacking the wood, tending the fires and cleaning the stove, I probably would not use the stove if I had to pay for the wood.

However, we live on six wooded acres and get a fair amount of free wood from dead-fall and natural attrition. It will be interesting to see how much electricity can be saved using this free resource.

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Winter 2016 Energy Report

6/4/2017

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As usual, I am way behind on updates to the blog. The heating season ended in April and it's June already. We have been busy working on the basement and I've started on design of a solar panel array that will go on the roof... but that's a topic for a future post!

Wood Stove

We installed an Osburn Matrix high efficiency wood stove in December but, with the basement still under construction, we hardly used it this past winter. We consumed roughly 1/2 face cord of mixed hardwood from January through March. That's so little I'm not going to try to calculate the effect on our energy use until next winter.

On a side note, our heating cost has been so minimal that the stove will never pay for itself in reduced heating bills. However it has 4 big plus factors:

It provides a backup heat source if the power goes down.
We live in the forest and the wood is free.
It heats very quickly
Great ambiance

We used left-over / scrap galvalume siding to make reflective panels behind and to the side of the stove. I really like the way it turned out and I'll post more details in a future blog.

2016 Heating Season

This year I've decided to include October and April in the heating season report. We don't use a lot of heat in either month but they average about 250 HDD each and together account for 10-15% of the annual heat load.

For comparison the following graph shows HDD by month for Winter 2015 and 2016.

Both of the past two winters have been warmer than average with winter 2016 being close to record warmth. For comparison, the 2015 heating season had approximately 3413 HDD while this past heating season was only 3022.

The second graph shows the energy used for heating by month for both years.

The total energy used for heating was 3330 kWh in 2016 compared to 4358 kWh in 2015. Put another way, our total cost for the 2016 heating season was less than $350.00.

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2016 Energy Report

3/8/2017

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Wow, it's already March and I'm finally getting around to the 2016 Energy Report. So... here it is.

Background

First, for those of you just tuning in, the neoTerra project is our home and it is also a living experiment in sustainable design and energy efficiency. We strove to use durable, low-maintenance materials and we utilized recycled or recyclable materials wherever practical.

The house is all electric based on a philosophy that it will eventually be cost effective to produce our own electricity using solar panels but it will never be practical to make home-brew propane.

The house is a passive-solar design with an expectation that we will be able to receive 30% or more of our winter heating from the Sun. We use Diakin mini-split heat pumps for the remainder and we recently installed a high-efficiency wood stove to supplement the heat pumps and provide backup in the event of a power outage.

The house has roughly 4000 square feet of conditioned space (2000 on the main level shown below and 2000 in the basement level).

The house is located in the North Georgia mountains at an altitude of 2800 feet. The climate here averages about 8 - 10 degrees cooler than nearby Atlanta so heating is a larger factor than air conditioning.

This is an engineering experiment of sorts and we installed an array of temperature sensors in the ceilings and floors to capture detailed data on the actual performance of the passive solar design.

As an example, the picture below shows the current, real-time temperatures on the main level. It's 4:00 on a bright, sunny day and the concrete floors receiving sun (i.e. to the south) are in the mid to upper 70's while the shaded areas are in the mid to upper 60's. The floor in the guest bedroom is at an amazing 87 degrees! This is free heat.

I'll discuss the passive solar design in greater detail further down in this report.

2016 Energy Report

2016 Climate Data
Since heating is our single largest use of energy it's important to consider the actual climate data for the year. Our historical average (14 years of data) is 4069 Heating Degree Days (HDD) and 825 Cooling Degree Days (CDD).

2016 started out close to the historical average with January slightly colder than normal and March slightly warmer. After that, we had the warmest summer on record and a mild winter. Overall, we accumulated 3717 HDD (9% warmer than average) and 1054 CDD (28% warmer than average).

The following chart shows the 2016 HDD, 2016 CDD and the respective historical averages. The dashed lines show the historical averages from our climate model and the solid lines show the actual data for 2016.

2016 Energy Use
The next chart shows the total energy used as well as the amount used for heating and air conditioning. The total energy used was approximately 13000 kWh (about $1300.00) while the total for HVAC was 4539 kWh (approximately $450.00).

It's no surprise that we use a LOT more energy in the coldest months when we're heating and less in the warmer months.

The big surprise this year was how little we used for air conditioning. This was the warmest summer on record in Atlanta which is about 50 miles to our south. Good shade, cool mountain nights and superior insulation kept the house comfortable without AC. We ran the AC only four or five days and spent less than $20.00 on AC for the entire summer.

Heating a roughly 4000 square foot structure for $450 here in the mountains is very good and is actually better than what was predicted by the computer model used in designing the house.

As detailed in earlier posts, we use a TED energy monitor to collect daily data on actual energy use. Overall, HVAC accounts for about 1/3 of our energy. The water heater and the refrigerator each consume about $11 / month while the well and electric dryer are almost insignificant.

About 1/3 of the energy consumed is not monitored. This includes cooking, lighting, TVs, computers and so forth.

Passive Solar Performance
The computer model we used while designing the house predicts the passive solar performance based on the following factors:
- Historical average HDD per month
- Historical average percentage of sunshine for each month
- The estimated heat pump efficiency based on the historical average temperature for the month
- The overall heat load of the entire house

The result is an estimate of the the total energy used (in kWh) per HDD. Lower numbers indicate better performance (i.e. less electricity consumed per HDD).

The following chart illustrates the results.

The upper gray line shows the amount of electricity that would be used if there was no solar input. In other words, this line is showing how a conventional house would perform. Note the results are a bit different for each month since the heat pumps are more efficient in the warmer months (March, April, November) and are less efficient in the colder months (Jan, Feb and Dec). The performance ranges from approximately 2.5 in the coldest months to about 2.1 in the more moderate months. Based on an average of 2.25 we would expect to spend approximately $900.00 on heat in an average winter.

The passive solar design model is shown by the yellow line. Heat from the sun pushes the performance to roughly 1.5 in the coldest (least efficient) months and achieves a factor of 1.0 or better in the more moderate months. Thus, the passive solar design reduces the predicted expense to $508.00, which is a reduction of better than 30%.

Finally, the orange line shows the actual performance for 2016. This must be analyzed in the context of the actual climate for 2016 shown earlier.

January 2016 was colder than average and the house performed slightly worse than the model (i.e. the orange line is above the yellow line).

February was almost exactly average and here the actual performance is almost perfectly aligned with the model.

March was somewhat warmer than average and the house performed better than the model.

In October and November the house performed much better than the model due to warmer temperatures and a prolonged drought that provided increased sunshine.

As mentioned earlier, the total heating expense for 2016 was only $450.00 which is 1/2 the cost for a similar sized non-passive design.

Conclusions
The results from 2016 show that the computer model is accurately predicting the performance of the passive solar design.

The model could be improved to model the actual year as it progresses rather than comparing against a historical model. This will be a project for 2017.

In 2016, the Sun delivered over 30% of our winter heat and the passive solar design saved approximately 3000 kWh of electricity.

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Winter 2015 Energy Report

4/2/2016

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I'm so far behind on getting the February report written that I'm going to consolidate February and March into this report which will cover the entire 2015 heating season (November 2015 - March 2016).

February Report
The February temperatures came in at an almost perfect average of 747 Heating Degree Days (HDD) while March was pretty mild at only 388 HDD (about 75% of the 13 year average).

As I mentioned in the last report, the house performed very close to what was predicted by the computer model and averaged 1.656 kWh/HDD in January.   While this was very close to the model, I was disappointed because November and December were so much better. I speculated that part of the difference was due to heating the basement during January.

In February we turned the basement heat off to see how that would effect the overall efficiency.   I was surprised that the efficiency in February was 1.528 which is almost exactly as predicted by the model. Clearly, heating the basement has much less effect than I speculated in January.

At this point I must conclude that the exceptional performance in November and December was primarily due to greater efficiency of the Daikin heat pumps at warmer temperatures. I improve the computer model to use a better estimate of efficiency based on the average monthly temperatures.

March Report
March was only 388 HDD and also benefited from a lot of sun. We used only 341 kWh for heating (about $30.00) for an amazing efficiency of 0.879 kWh/HDD. This is almost twice as good as January.

2015 Heating Season
Though we will certainly use some heat through April, it will be minimal and I'm going to wrap up the heating season reporting now.   The following chart shows the performance of the house from November 2015 through the end of March 2016.

The upper graph shows efficiency (red line) versus HDD (blue bars) while the lower graph shows the temperature of the first floor slab (red), the basement slab (blue) and the outside (green). The first floor slab temperature provides an indication of sunny days (spikes) versus cloudy days (flat).

The upper gray bar is the computer model prediction for how the house should perform on cloudy days with no solar gain. There are seven instances where the house under-performed. I only have an explanation for two of these occurrences which is described in the next section.

Issues with Mini-Splits
Overall, the Diakin mini-splits perform better than expected and average efficiency is better than 200%. However, we do experience climate conditions here that can make the heat pumps perform poorly.

This area is a borderline temperate rain-forest. There are many days where we are literally in the clouds and the humidity is at or near 100%.

Heat pumps extract heat from the environment so the exhaust temperature is several degrees lower than the outdoor ambient temperature. If the outdoor temperature is near freezing or below freezing then ice will build up on the coils of the heat pump. The units are designed to handle this and will automatically perform a defrost cycle when the outside condenser becomes clogged with frost/ice.

The following plot from the Energy Monitor illustrates what happens when there is a lot of moisture in the air and the outside temperature approaches freezing.

Prior to 6:00 PM the Mini-split was running continuously and was heating the space with no difficulty. As the exterior temperature dropped below 40 degrees the unit began to go though periodic defrost cycles. These can be seen as spikes in energy consumption followed by a short shut-down. During these periods, no heat is being delivered into the house. Fortunately (actually by plan.... not luck) we have enough reserve capacity that the house never got cold.

This is not a flaw in the mini-split system but there are a couple of lessons to be learned.

First, we fortunately went with three smaller 3-head units rather than one giant 8-head unit. This was slightly more expensive but has proven to be a wise choice. If we had one giant unit then the entire house would be shut down during the defrost cycle. With three smaller units, there was always at least one unit running when the others were going through defrost,

The other observation is that mini-splits will probably perform even better in arid environments where there is less moisture in the air to cause frost build up. I would not hesitate to recommend mini-splits in New Mexico, Arizona or southern Colorado, for example.

Conclusions
Overall, for the entire 5 months we consumed 3910 kWh for heating and had 2886 HDD during the period. This comes to an average of 1.355 kWh per HDD which is in line with the computer model. Of course, this was a mild winter so future heating seasons will probably be higher and more expensive.

Over this 5 month heating season we spent approximately $380.00 on heating. If I add October, the total comes to about $400.00. This is a dramatic improvement over Chicago where we spent an average of $1200.00 per year to heat a house of similar size. Yes, the climate in Chicago is 50% colder, but the cost of electricity is about 400% greater than natural gas.

This demonstrates that passive solar design, tight construction and high efficiency heat pumps can make an all-electric house practical and economical.

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December Energy Report

1/24/2016

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I'm almost a month behind... but here's the December energy report.

At only 431 Heating Degree Days (HDD) December 2015 was one of the mildest on record. The historical average is about 720 HDD so this was only 60% of the average. We used 494 kWh for heating which comes to about $46.44 at the winter rate of 9.4 cents per kWh.

During the first half of the month only two of the seven interior units were turned on. On December 17th I turned two basement units on in anticipation of a cold front. Those units remained on until the 24th. The graph below shows temperatures versus energy consumed for the month.

The upper chart shows energy used per HDD. Lower values indicate better performance.

The lower chart shows temperature (Red = First floor slab, Blue = basement slab, Green=ceiling, Yellow=outdoor). Note that the ceiling sensors are located inside the insulation above the drywall so they don't indicate the actual room temperature. The interior temperature runs 3-4 degrees warmer, so a ceiling reading of 65 corresponds to an interior temperature of about 68.

The charts clearly illustrate the effect of passive solar heating. The 3rd through the 8th were clear or mostly sunny. The temperature of the slab reached mid to upper 70's each day (except the 6th which was partly sunny). Energy use was less than 1 kWh per HDD, which is quite good.

The 9th through the 12th were mostly cloudy or rain and there is very little heating of of the slab. Energy use was 50% greater during this time than the previous sunny days.

As mentioned earlier, I ran two basement units between the 17th and the 23rd. This was a mistake... there are no walls in the basement yet and we have an open stairway. All the heat rises right up to the main level and the basement units run constantly. On the 23rd the energy use was twice as high as the 11th even though the outside temps were a bit warmer.

I shut the basement units off on the 24th and performance improved dramatically. The lesson here is that I should only turn the basement units on when it's so cold that the 4 main level units can't keep up.

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November Energy Report

12/16/2015

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Intro
Now that heating season has arrived, I'm planning to post a monthly energy report to summarize how the design is working.

November and the first half of December have been particularly mild this year so we can't tell how the house will perform when it gets really cold. However, with the temperature sensor array and the energy monitor it's possible to start making estimates.

Review
As a brief review, neoTerra is a passive solar design using Daikin multi-splits for HVAC. There are three independent mini-split systems with a total of seven interior units. These are:

HVAC WEST 2 interior units in the kitchen/great room
HVAC EAST 1 interior unit in each of the two bedrooms
HVAC BASE 3 interior units in the basement

Each of the three systems is independently monitored for energy use and data is collected each minute.

In addition there is an array of digital temperature sensors embedded in the floors and ceilings. Right now, it looks like THIS >>>>

Wow, it is 60 degrees out on December 16th. Nice!

It's fairly sunny out and it's easy to see the effect of the sun on the slab. Areas in direct sunlight are at 73 - 78 degrees while areas that don't get sun are at 68.

Before construction, we created a computer model of the house to estimate energy efficiency over a wide range of scenarios such as increasing or decreasing the amount of glazing, different levels of insulation, and different types of HVAC systems.

Of course a computer model is based on an endless number of assumptions. If any of the assumptions are incorrect then the model will make poor predictions of actual performance. One of the goals for this heating season is to refine the model and replace assumptions with actual data.

As an example, the computer model estimates that the heating load is approximately 15,816 BTU per Heating Degree Day. This is based on the amount of insulation and the estimated air infiltration. The actual heat load may be quite different based on how well the insulation was installed, and how wind speed might affect the rate of air infiltration (totally unknown).

In addition, the model assumed an average efficiency of 200% for the Daikin mini-splits but the actual efficiency varies with temperature. One of the goals for this heating season is to determine the actual efficiency at different temperatures and build that data into the next generation model.

November Report
November 2015 was a very moderate month. At 411 Heating Degree Days (HDD) it was only about 80% of the historical average for November. We used a total of 429 kWh of electricity for heating (about $43).

During most of this period only two of the seven internal units have been turned on. One unit in the kitchen and one unit in the master bedroom have supplied more than adequate heat for the entire house.

Since every month is different, the best way to view the HVAC energy use is to calculate the average energy (in kWh) versus the HDD. For November we averaged 1.04 kWh/HDD. This was a pleasant surprise since the computer model predicted 1.58 kWh/HDD.... in other words, the house performed about 50% better than the computer model predicted.

We had 15 straight days of rain between October 26th and November 9th. This is an absolute worst case scenario for a solar house. During this period the house used 2.03 kWh/HDD versus 2.32 predicted by the model for days without sun.

This is within about 10% of the model but it doesn't prove that the model is accurate. For example, an experiment a few weeks ago indicates that the Daikin units are actually operating at an efficiency higher than 200%. This implies that the heat load is greater than the model predicted. This doesn't surprise me since we keep the house at 68 degrees and the HDD calculations are based on an average of 65 degrees. The next generation model will utilize actual temperatures from the sensor array to calculate the heating load in real-time.

We had 5 sunny days between November 13th and November 17th. During this period the house averaged .61 kWh/HDD which is much better than predicted. Put another way, we averaged about 6 cents per HDD on sunny days versus 20 cents per HDD on days without sun.

Since the average for November was 1.04 kWh/HDD and the average for days without sun was 2.03, the sun provided almost 50% of our heat load in November in spite of 13 rain days. I'm pretty happy with that!

Well
We strongly considered rain water harvesting early in the design phase. Budget overruns made me defer the rainwater project.

We have been collecting energy data on the well over the past three months. During this time we have averaged about $21 per month to operate the well. September was more than October and November combined since we were landscaping and watering plants.

This really (pleasantly) surprised me since the well is 800 feet deep and I expected it to cost a lot more.

At this point I have decided to abandon plans for rainwater harvesting. It would cost almost $4000 to save about $10/month on energy. At that rate the project would take about 33 years to break even.

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Happy Equinox

2018 Energy Report

January 2018 Energy Report

2017 Energy Report

Fall 2017 Energy Report

Winter 2016 Energy Report

2016 Energy Report

Winter 2015 Energy Report

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