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Passive Solar Design III- Multi-Splits

12/26/2013

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I hope everyone had a wonderful Christmas! 

Since I have the day off, it's time for another technical post.  Today I'll be going over the mini-split HVAC system we're planning to use.  I'll be using systems from LG as an example since LG provides a lot of technical info online and their cost is competitive.

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A mini-split heat pump system has a compressor outdoors similar to a central air conditioner.  It also has an interior air handler unit that provides heating and cooling.

A multi-split system operates the same way but it allows multiple interior units to be connected to a single outdoor compressor.  This allows each room to be a seperate zone with it's own thermostat.

The typical multi-split will supply 2-4 interior zones, though some units go up to 8.


The posts on Baseline Design and Air Infiltration worked through the overall heat loss, which is good for estimating total energy use and $$.  To determine capacity, however, we have to look at the the coldest temperatures rather than the overall energy used.

Heat pumps operate by extracting heat from the outdoors and bringing it into the house. I know it seems illogical, but you can extract a lot of heat from cold air.  Typical heat pumps will operate down to 10 degrees and Mitsubishi has a system that will operate down to -5 F.

LG publishes capacity and efficiency data down to 14 F so I'm going to work through an example for 14F.  It's a reasonable shortcut since we only get three or four days per year where the temperature falls below 14.

The figures below show the capacity data for two LG models. The LG LMU369HV, which is rated at 36,000 BTU and the LMU247HV rated at 24,000 BTU.

Note that both the capacity (BTU/hr) and the efficiency (COP) decline as the temperature gets colder.

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Although the LMU369HV is rated at 36,000 BTU/hr, it's capacity will drop to 22,300 BTU/hr at 14F, so a single unit will not be sufficient our projected heating load.  At least two units will be required.

The following chart shows three potential solutions plotted against the projected heating requirements in BTU/hr.
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Temperature Sensors - Science Project

12/20/2013

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Since neoTerra is a passive solar project we are incorporating thermal mass in the form of concrete slabs in the basement and on the first floor (I'll talk about the construction of the suspended first floor slab in a future post).

There's a lot of theoretical info on how much heat can be stored in the slab but there isn't very much practical info on how long it takes for the slab to warm up in the sun, how warm does it actually get and how fast does it cool off at night so I wanted a way to measure the temperature of the slab throughout the day.

My brother Phil is an experienced software engineer so he came up with a really cool system that allows us to embed temperature sensors right in the slab.  It uses a very small computer called an Arduino and very precise temperature sensors from Dallas Semiconductor.   I'll post details on the project in the How-To section.

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The sensors are encased in stainless steel sleeves so we can bury them diretly in the concrete.  We are putting seven sensors in the basement slab. 

Three will get direct sun, three will be in areas that get partial sun and one will be in an area that doesn't get any sun.

We are tying them to the remesh before we pour the slab.

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Fun with Fans - Science Project

10/6/2013

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You know how the basement always stays nice and cool in the summer.   In our present home there is at least a five degree difference at each level.   The basement will be comfortable at 75 while the second floor is at 85. 

Our open floor plan makes the problem worse.  When we run the air conditioning the second floor cools off.  But as soon as the AC stops, the cold air all settles back down in the basement.

neoTerra will also have an open stairwell. We thought it would be reallly nice if we could circulate that cool basement air up to the main floor, so I did a little science project to test the theory.

For this test, I selected a 340 CFM Panasonic WhisperLine fan.  These are among the quietest and most energy efficient on the market.

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I put the fan on the basement floor and used flexible duct to push the air up the stairwell to the second floor. 

It worked better than expected, though the cleaning lady was a little baffled by the shiny metal tubing running up the hallway!

The 340 CFM fan actually moves too much air for a single room so I'm planning to buy two smaller units for neoTerra.  We will locate them at opposite ends of the house to make the air travel as far as possible to get back down the central stairwell.


Here's a link to the Panasonic web site:
http://www.panasonic.com/business/building-products/ventilation-systems/products/whisper-line.asp




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Passive Solar Design II

8/4/2013

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In this section we will look at a few different types of glazing to see how the Solar heat Gain Coefficient (SHGC) will affect how much passive solar heat we can collect
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This table shows the R factor and SHGC for a few different windows.



Old-fashioned dual-pane glass without Low-E coatings doesn't have a very good R factor.  I won't consider it for the project but it is listed in the table for reference

The next table shows the solar gain for one square foot of an Integrity window with Cardinal LoE-180 glass.  This is a product that has a very good R factor and a fairly high SHGC.  I know the table is a bit hard to read. 

The upper section shows the total solar gain for each orientation (i.e. north, east, south, west).  It is interesting to note that even the north windows gain some solar energy in the winter.

Unfortunately, windows have a fairly low R factor so they also lose energy. The lower section of the table shows how much energy is gained (or lost) for each orientation.  It is not surprising that north windows lose more energy than they collect throughout the heating season.   East and west facing windows will also lose energy during the coldest months but will have a net gain in February, March, October and November.

The south facing windows have a net energy gain all through the heating season (sorry for the tiny print)

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We can do similar calculations for each of the different window types.   I was surprised to discover that the Anderson SmartSun glazing (very low SHGC) actually loses energy even through the south facing windows all winter.  It would be a very poor choice for a passive solar home.

The bottom line in any cost-effective design is to determine how much energy and $$ will actually be saved by the alternative designs.  The following table shows the potential savings for Integrity high-SHGC windows versus the Anderson mid-SHGC windows.  Note: to make the tables more readable I'm only showing January through April.
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Conclusion

Although the high-SHGC windows perform much better during the coldest months, there are four months of the year (March, April, October and November) where the heat load is so low that both products perform, about the same.

The bottom line is that we can predict a savings of about $797 / year with the high-SHGC windows and a savings of about $614 / year with the mid-SHGC glazing so there is only a $180 per year difference.   If the products cost the same then the high-SHGC glazing would be a clear winner.

Unfortunately, the Integrity windows cost about $8,000 more than the Anderson 100 series so it would take roughly 44 years to pay for the difference.

We will use the Anderson 100 windows and look for other ways to save $180 per year on energy.

Photo-voltaics are starting to look more interesting even though Georgia is not a very solar-friendly state.
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Congratulations Tracey!!

8/3/2013

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Last week, Tracey won the Chief Architect Green House Design contest with her entry of neoTerra.

The design will be featured in a live webinar on August 7th.  Check the contest results out here:
https://www.facebook.com/ChiefArchitect#!/photo.php?fbid=691554027540519&set=a.136631409699453.21564.130385066990754&&theater

Congratulations, Tracey!!
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Passive Solar Design I

6/30/2013

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Our first step is to estimate how much heat we could theoretically collect through the windows.  In a future post I'll compare a couple of different types of glass to see how the Solar Heat Gain Coefficient (SHGC) affects the amount of heat gained and the potential $$ savings on heating.

To estimate the potential heat gain, I started with the 1993 ASHRAE solar tables for 32 degrees north latitude, which is fairly close to our actual location.  I got the tables from "The Passive Solar House", appendix 2 -table 14 and entered it all into an Excel spreadsheet.   The following table shows the BTU's collected by 1 square foot of glass each day by month.

Note that this is calculated data based on an optimal perfectly clear sunny day.
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I know the table is a little hard to read.  The important info is that each square foot of south facing windows can potentially receive about 1700 BTUs per day during November, December and January.

However, most days are not perfectly sunny.   Most days have some clouds, haze or overcast that diminishs the solar energy. To get a better estimate of actual solar gain, we can use data from NREL that estimates the average % of sunlight that is actually received each month in different cities.   I used the % sunlight data for Atlanta to make the next table.  This shows the  BTUs per day adjusted for actual sunlight received.   In general, the average amount of sunlight is only about half of the theoreticl optimal clear day.

One interesting note is that the Fall tends to get more clear days than the Winter, so November gets more actual sunlight than December or January.
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This is summarized in a nice graph from NREL.   The following chart shows the solar energy received for Athens, Ga.
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The baseline design has about 475 square feet of south-facing glass.  If we used perfectly clear glass then we might expect to collect about 400,000 BTUs on an average day from October through February.   Unfortunately, a lot of that heat is blocked or lost by the glazing.   In the next post we will look at the effect of the glazing Solar Heat Gain Coefficient (SHGC). 
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Air Infiltration - How Tight is Good Enough?

3/9/2013

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Air Infiltration Standards
There are several energy standards to choose from.  Each one has a different intent.  A standard that works for Germany or Minnesota might not be the most appropriate for Miami or the Bahamas.

Consequently, the standards for air infiltration vary dramatically and don't provide good guidance on their own. The three main standards are IECC, Energy Star and Passive House.  Passive House is the most strict while the IECC is absurdly lax.   Here are the maximum ACH50 rates for the various standards.

    - IECC (Georgia code)                          7
    - Energy Star for climate zones 3,4       5
    - Energy Star for climate zones 5,6,7    4
    - Passive House                                  0.6

So, Passive House is more than ten times better than current code and about 8 times better than Energy Star requirements.

But how much is good enough?  We can do some calculations based on the previous Baseline design to estimate the energy savings.  As a spoiler alert,  fans of Passive House might want to skip this section.
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The table on the right shows the annual BTUs and cost of air infiltration for different levels or air tightness.  This is assuming an Air Heat Pump with a cost of $1.76 per therm.

There is, of course, a law of diminishing returns. Cutting from 7 -> 2 (roughly a factor of 3) saves about $350 per year.   Further improvement from 2 -> 0.6 (roughly a factor of 3) only saves an additional $98/year.

It would seem that meeting the Passive House air infiltration standard is not a cost effective goal.  We can probably find less expensive ways to save another $98 / year on energy.

Going forward, I'll assume a goal of hitting an air tightness of ACH50 = 2.   Recalculating the heat load and cost shows that this reduces the annual heating load by about 20M BTU and that air infiltration now accounts for only 12.82% of the heating load.

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By far, the windows are the greatest heat load (almost 45%).  In future posts I'll look at the solar gain and show that the south facing windows can collect more heat than they lose.
Primary Sources of Air Infiltration
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Baseline Design Part III - Estimating Energy Costs (without solar)

3/3/2013

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In this post I'll calculate the approximate cost for heating the Baseline design.  At this stage, I'm still not accounting for any solar gain.... this is the worst case analysis if the sun never shines or if we choose windows with poor SHGC characteristics.

The table below, lists the energy content and approximate cost for different solutions.  Natural gas isn't available where we're building, but I included it in the table for comparison.   The efficiencies listed for Air and Geothermal heat pumps are just approximations.
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In a moderate climate, the Air Heat Pump is a cost effective alternative to burning natural gas or propane.

The next table shows the approximate BTU heat load for various aspects of the structure and the cost of providing the heat using different solutions.   As mentioned in previous posts, the biggest opportunity for savings is by making the structure more airtight and improving the air infiltration.
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Clearly, LP and resistance electric heating are poor choices so I'll eliminate them from further consideration.   LP, in particular, has been through volatile price swings over the past several years so we've decided that neoTerra will be all electric with a fireplace or wood stove for backup heat.
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To summarize, heating with an Air Heat Pump will cost approximately $1300 / year, based on the estimated baseline heating load and current cost of electricity.  We will keep improving the design from this point.

In addition, going all electric will avoid the costs of installing an underground propane tank and plumbing the house for LP. 

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Baseline Design

2/24/2013

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There are many, many tradeoffs to consider when you're designing an unconventional house.   To help us evaluate different strategies for building a sustainable home we decided to start with a baseline model that would meet minimum IECC and Georgia requirements.  This allows us to make changes to the model and see the effect.  My primary focus has been on heating and passive solar design.

Aside from the size and orientation of the house itself,  the major factors that go into the baseline model are Climate, Insulation, Air Infiltration and Windows.
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Climate

Climate, of course, plays a major factor in the design of an energy efficient home.  An approach that's appropriate for a cold climate may not work for a hot climate.

North Georgia is in the southern edge of IECC 2009 Climate Zone 4.


We are lucky to have a small weather station on a mountain just a few miles from our site and at the same elevation (about 2800 feet).  http://www.canoodlenest.com/weather/

The station has established an average of 4147  Heating Degree Days (HDD) and an average of 857 Cooling Degree Days (CDD) over the past ten years.

For a passive solar design it's also important to have an estimate of the solar energy available at your site. The National Renewable Energy Lab (NREL) publishes detailed solar data for all 50 states here  http://rredc.nrel.gov/solar/pubs/redbook/  and has a dynamic map visualization site here http://www.nrel.gov/gis/
Insulation and Air Infiltration

The DOE recommended insulation levels for Zone 4 are:
    - Attic or Ceiling                     R38
    - Walls                                  R21
    - Floor over unheated space    R25
    - Slab                                    R5

Georgia code requires a maximum ACH50 score of 7 air changes per hour  (not very good).
Windows

There is some confusion on the allowable characteristics for windows.  The prescriptive window requirements in the 2009 IECC specify U of 0.35 for Climate Zone 4 and place no restriction on SHGC for Climate Zone 4. 

See the Efficient Windows Collaborative link here http://www.efficientwindows.org/code_overview.cfm
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Energy Star, however, has somewhat different zones and sets different standards than the IECC.  The Energy Star North-Central Zone roughly corresponds to IECC Climate Zone 4.

The table lists the Energy Star maximum U-Factor and maximum SHGC allowed for each region.

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In future posts I'lll compare the overall heating loads for a base design with no solar consideration and then compare the effects of high SHGC windows versus the low SHGC windows madated by Energy Star.
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