getting hot and bothered about geothermal

The Architect has been hot about geothermal and, having a degree in geophysics and hearing about the potential energy savings (up to 50 to 60 percent!), I’m warming up to the idea. However, that geophysics degree cuts both ways, so I’ve not been entirely convinced, especially since we heard the city’s HVAC guy allude to heat buildup issues.

Ground source heat pumps have been around for quite awhile. The first ground source heat pump (coils in trenches) was installed at a home in Indianapolis in 1945. Since that time, more than a million systems have been installed across the United States. Despite that, it’s still considered an innovative technology, primarily because there are not many system installers and, despite the one million installations, not all that common.

First, a word about terminology: Merriam-Webster defines geothermal as “of, relating to, or utilizing the heat of the earth’s interior.” Traditionally, geothermal refers to tapping into the earth’s heat for energy. For example, Iceland is a world leader in green energy due, in large part, to its geothermal resources. Geothermal as used in the context of HVAC systems is actually a ground source heat pump. Yes, heat from the earth is harvested to heat the home (albeit with a heat pump), but the earth is also used to dump heat accrued during the cooling season. Other names for this technology are geothermal heat pumps, earth-coupled heat pumps, earth exchange systems, and GeoExchange systems, the latter a protected trade name. Despite the usage of geothermal in the title of this post, I will use the term “ground source heat pump” because it’s a more technically accurate term than geothermal.

Ground source heat pumps use the earth to increase the efficiency of heat transfer for air conditioning and as a source of heat for heating. Below a certain depth, typically 10 to 20 feet, the earth is at a constant temperature, reflecting an average of the year’s overall temperatures (unless you live near a magma body, in which case you probably shouldn’t be living there!**). For example, the ground temperature in northern Minnesota is about 37 degrees F while the ground temp in Austin, Texas, is about 71 degrees F. Given that the ground temperature is a function of the average air temperature and air temperature is governed in large part by the sun, ground temperature is a form of stored solar energy.

Heat naturally flows from higher temperatures to lower temperatures via conduction. A heat pump induces heat to flow from a lower temperature to a higher temperature, hence the word “pump”. A standard air conditioner is an air source heat pump. It pulls heat from your inside air and transfers it to the outside air. A refrigerator is also an air source heat pump, pumping heat from the fridge interior and transferring it to the exterior (which is why it’s important to keep those coils on the back dust free and breathable). The reason ground source heat pumps are more efficient than air sourced heat pumps is because the temperature difference is greater at the heat sink. Dumping heat into the ground at 71 degrees F consumes a whole lot less energy than dumping heat into the great outdoors at 104 degree F.

There are several types of ground source heat pump systems, including closed loop and open loop. I’m only going to dwell on closed loop systems because (1) that’s what’s most common in Texas and (2) the water conservationist in me is appalled with open loop systems which commonly pump groundwater and then dispose of it. There are also horizontal loop and vertical loop systems. Again, because they are most common in Texas and in urban settings, I will focus on vertical loop systems.

A typical HVAC system, such as the one we have in our current house, has an air handling unit inside the house and a condensing unit located outside. The air-handling unit moves air from the house into the unit and over cool evaporator coils coursing with chilled refrigerant. The heated refrigerant is then moved to the condensing unit where the refrigerant is cooled and condensed (which is why there’s always a fan blowing out there) before it’s moved back into the house to start the whole cycle over again. An HVAC system using a ground source heat pump works in a similar way except that instead of transferring heat to the air via a condenser the heat is transferred to the ground by first transferring the heat to a water/antifreeze solution via a heat exchanger and then pumping that solution through tubing buried in the ground. The tubing forms a loop through which the water is circulated such that the solution is cooled by the time it makes the full loop. The cool thing about this (besides the resulting conditioned air) is that there is no need for an outdoor condensing unit: more room outside and no fan blowing and going all summer!

Heating in a typical HVAC system is accomplished by pulling air into the air handler and across an electric heating element or a gas-heated element. A ground source heat pump works in the opposite direction during the heating season where the coils on the inside are now used as a condenser instead of an evaporator and the earth tubing is used as a source of heat instead of a sink.

The length of the tubing exposed to the earth is an important consideration and is a function of the thermal properties of the ground at your particular location, how big of a system you need (tonnage), ground temperature, groundwater flow, the diameter of your borehole, what you fill the borehole with, and how close your boreholes are to each other (or any other ones in the area).

Distance between boreholes is important to prevent thermal interference between the boreholes. The rule of thumb is 20 to 25 feet between each borehole with about 150 to 300 square feet of land per ton of system capacity. Ideally, borehole spacing should be based on the more specific thermal attributes of your location. The National Ground Water Association says that rules of thumb should not be used when sizing these systems (I include them here as a screening test. For example, if your contractor wants to place your boreholes 5 feet apart, you may have an issue and can start asking questions). There are manual and numerical methods for calculating the length of borehole needed to achieve the desired cooling and heating effect. Hopefully your contractor knows how to run these numbers. Given that a lot of assumptions are made on thermal properties, you'll want to go along with any safety factors built in, even though it may increase the cost. While it's possible to test and quantify the thermal properties at your location, this is typically not done unless your system is expected to require more than 100 tons of cooling.

There needs to be a good thermal connection between the borehole and the earth. Early ground source heat pumps struggled because installers used non-thermally enhanced grout (grout is the stuff that backfills the borehole around the tubing). Thermally enhanced grouts can reduce the depth and number of boreholes. When grouting, it’s critical that the grout doesn’t bridge in the hole creating large air gaps. Air is a terrible heat conductor and will greatly decrease the efficiency of your system. Being a hydrogeologist, I have some experience with drillers. Drillers are infamous for cutting corners on the way to getting the job done, especially if they’ve fallen behind (it’s difficult to check their work when it’s hidden…). Therefore, it’s critical to have a technician babysitting the rig and the roughnecks. Preferably the technician can cuss like a sailor: It was sitting on drill rigs that I learned how to use the f-bomb as a noun, verb, adjective, and adverb, often all in the same sentence.

Drilling is a messy business; therefore, it’s critical to keep the inside of the tubing clear of mud and dirt. According to some experts, lack of cleanliness has caused many a system to fail or serve at a less-than-ideal efficiency.

A shallow water table helps to enhance the thermal connection to the earth because the fill materials will likely become saturated with water. Even enhanced grout typically has lower thermal conductance than the ground itself; therefore, smaller diameter boreholes tend to have better performance (but they increase the chance of bridging…).

High-density polyethylene pipe is typically used for the tubing. High-density polyethylene pipe typically has a 50-year warranty with independent tests suggesting a 200-year lifespan. Pipe joints should be thermally fused (that is, melted together). This material and fusing is what is used for natural gas lines (with a reported 1,000 year expected lifespan). Other methods of joining pipes have been shown to fail over time.

Most installers use geothermal transfer fluid, something the cool kids called GTF, which consists of water and methyl alcohol (to prevent freezing), in the tubes. If the tubes leak for some reason, you won’t have a hazardous waste site on your hands.

Using a ground source heat pump will impact temperature in the earth, albeit locally. Ideally, you live in a place where the amount of heat you put into the ground during the cooling season equals the amount of heat you pull out of the ground during the warming season. Because of unretrievable heat loss around the fringes of your boreholes, this ideal climate would require slightly more cooling than warming. Unfortunately, very few of us live in this ideal climate; therefore, the ground will have a net heat loss or heat gain resulting in a decrease or increase in ground temperature over time. This overall change in ground temperature will decrease system efficiency over time. That’s the bad news. The good news, at least for homeowners, is that this typically only happens with larger commercial systems (many more system tons; therefore, many more boreholes) and not home systems (although it could if the system is not designed properly). Nonetheless, boreholes need to be spaced farther apart in Texas than elsewhere in the country because of our cooling dominated climate.

Design software typically only considers the conduction of heat in the ground. The movement of heat via groundwater flow (advection) is another potential source of heat dissipation; however, water has to be able to flow quite freely for there to be a benefit. Given that our lot is over the Austin Chalk and that groundwater doesn’t move very fast through the chalk (it still has a fraction of its original seawater in it, fer cry eye!), only considering conduction makes conservative sense. The density of the Austin Chalk as well as its lower porosities and saturated condition maximizes its thermal properties.

An addition to a ground source heat pump system that can bleed off some of that heat and also save energy is a device called a “desuperheater” (I picture the little guy on Fantasy Island opening the water heater closet and yipping “De Superheater! De Superheater!”). A desuperheater harvests heat from your central air conditioner’s compressor to heat your water (according to my engineer bride, desuperheaters are also used at power plants). For a ground source heat pump, instead of dumping all that heat into the ground, it gets used to warm your water. Brilliant! A supplemental heat rejector to deal with our cooling-dominated climate! In fact, desuperheaters are highly recommended for cooling-dominated climates (they don’t help you out at all when you’re not using your air conditioning).

A desuperheater can provide about 5 to 8 gallons of hot water per hour per ton of cooling capacity. In an average climate, it can provide 20 to 40 percent of a home’s hot water )and you gotta believe that's higher here in Texas). It provides a greater cost benefit in homes with electric water heating than gas water heating because heating water via electricity tends to be more expensive. Given Austin’s warmer-than-average climate, a desuperheater can reduce water heating energy costs by 85 percent (!!!) if you heat your water with electricity and by 60 percent if you use gas. Given that heating water by natural gas is already 60 percent less expensive than using electricity, the cost saving are considerably less (48 bucks a year for gas versus 152 bucks a year for electricity). However, despite the lower savings, there should be a hidden efficiency benefit for the entire system by using the desuperheater as a pre-earth circulation heat rejector.

If you’re using electricity to supplementally heat your water, you can get a single tank, although some recommend a separate buffer tank for the desuperheater regardless of the energy source. If you are using gas, you’ll need a separate desuperheater tank to preheat the water before it goes into your water tank (haven’t quite figured out why yet…). Kind of a bummer, but there are desuperheater tanktoppers to reduce the footprint of the extra tank. An added plus of a tanktopper is that it pre-preps you for solar water heating (something to consider for the non-cooling months). And yes, you have to have a tank for the desuperheater (the harvested heat has to be stored somewhere); however, none other than the U.S. Department of energy says you can use a desuperheater with a tankless water heater.

Here’s a comparison of annual cost between different types of heating and cooling systems in Austin according to Action Mechanical Systems:

Heating AC Hot water Total

All-electric: $864 $689 $179 $1,732

Natural gas: $418 $689 $75 $1,182

Geothermal: $182 $415 $27 $624

A ground source heat pump with a desuperheater can result in energy bills 64 percent lower than an all-electric system and 47 percent lower than a natural gas system. That’s rather amazing! And according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (I hear their conferences are off the hook…), maintenance costs for ground source heat pumps are 11 percent lower than a natural gas system and 17 percent lower than all electric air source system.

Designing and installing a ground source heat pump requires some special skills and experience. It’s recommended that the contractor be certified by the International Ground Source Heat Pump Association and have considerable installation experience. Engineers and system designers can become Certified GeoExchange Designers via the Association of Energy Designers. Drillers should be certified by the International Ground Source Heat Pump Association. In addition, the State of Texas licenses water well drillers for the drilling of closed-loop geothermal wells.

But what about cost? Unfortunately, up-front costs for these systems can be quite a bit more than a standard system (one source suggested twice as much). In defense, practitioners point customers to the much lower cost to heat and cool after the system is put into service. One source noted that a geothermal system can be a money maker right away if the additional costs are wrapped into a home loan. For example, if financing the additional cost of the geothermal system adds $40 a month to your house payment but you’re saving $70 a month, you’ve just put an extra $40 a month into your pocket. A pretty convincing financial argument assuming you have some headroom on your financing. There also may be financial incentives to take the edge off. Last I checked, Austin (may) offer a $500 to $1,250 rebate depending on the efficiency of the system and whether or not you include a desuperheater. The feds may offer a rebate as well.

Some folks around the corner from us (we met them via the blog) recently finished a Craftsman-style green house (literally and greenily!) with a ground source heat pump, and they were kind enough to give us a tour (they even showed us their desuperheater, which I now have a far greater appreciation for…). They seemed pretty happy with their system.

So, after almost writing a thesis on this topic (sorry...), where does that leave us? I’m just about convinced that we should put one in. Next step is to talk with a ground source heat pump contractor and see what the cost difference is. As you know, it’s all about the green: the green green and the Benjamins…

Sources used for this post:

Geothermal Heating & Cooling Systems

Geothermal Heat Pump Design Manual

Heat Pump Water Heaters

Builder Guide: Improve Energy Efficiency with Desuperheaters

Residential Heat Pump Water Heaters

Advances in Modeling of Ground-Source Heat Pump Systems

Guidelines for the Construction of Vertical Boreholes for Closed Loop Heat Pump Systems

An Information Survival Kit for the Prospective Geothermal Heat Pump Owner

Retrofitting the Workforce: Geothermal Heat Pumps (focused on Texas)

GeoExchange

** As you go deeper the temps gradually increase because of the geothermal gradient caused by a molten hot core bleeding heat to the surface.

Photo from Wikipedia Commons.

the appliance from hell: the lowly clothes dryer

One thing that’s neat about passivhaus is the unholy fixation practitioners have on heat sources and sinks. Stuff you (or at least I…) don’t think about. Like the thermal impact of flushing a toilet. While that impact is (presumably?) minor, some thermal impacts are not. The one impact that has kept me up at night sweating and shivering in the corner is the appliance from hell, the lowly clothes dryer.

Before considering the lowly clothes dryer in the context of passivhaus, I thought of that white jiggly box in our house as something to (ahem) dry our clothes. You put clothes in, you turn it on, you come back later: dry clothes! Instead, I now see it for what it is: A well camouflaged thief—an embezzler, really—who skims off the top and steals your money.

The crime the dryer commits is multi-faceted. First, it pilfers your cool air. Just plumb takes it. I was enjoying that cool air, fer cry eye! And I paid to cool it! That's right, the dryer pulls air from its surroundings to do its deed. Second, it then uses valuable electricity to warm that cool air to drying temperature. Begin sobbing here. And third, it ejects that air to the outdoors, creating a negative pressure in the house through which hot air seeps back in as make-up air from Gawd knows where, which I then have to cool so the dryer can warm it up again. It’s a miracle I’ve gotten any sleep at all lately!

Amazingly, there isn’t a dryer on the market (that I am aware of) that uses outdoor air to dry your clothes unless your dryer is already outdoors such as in the garage or on an unconditioned back porch. The dryer in our current house is on our back porch. I figured we were low-rent: turns out we are passivhaus pioneers! Tellingly, the local passivhaus dude plans to put his dryer on a back porch.

There are options. Such as the helpful suggestion by the passivhaus creator, Dr. Wolfgang Feist, to dry your clothes on a clothesline. And while I find drying clothes on a clothesline vaguely romantic (we indeed do it from time to time), it’s not a good all-the-time option. There is also something called a “drying closet” which is essentially an enhanced indoor clothesline in a rather large box. There are also condensing dryers, built for cases where there is no place for an exhaust. However, reviews of these critters are mixed with none other than the Canadian government advising against them.

Our current dryer had to get serviced recently, and I took the opportunity to gaze at its innards to see if I could connect hosing to supply it outside air. The repairman thought I was nuts (“Dude: It doesn’t use that much air!” “But, dude, have you seen that sucker blow! It‘s 200 cfm!!!”). I’m thinking we’ll chose our next dryer based on whether or not we can make this modification.

Given that it seems to be getting hotter around here (two days of record-setting temps at 105 degrees…), I’m hoping that manufacturers will start making appliances for different climates. For example, having a fridge with a condenser that could be placed outside (we don’t put our AC condensers in the house, now do we?). Until that time, it appears you’ll see me stumbling about town with dark bags under my eyes (and higher electric bills).

can you overinsulate?

I was wondering about this in a recent post: can you overinsulate? Apparently I'm not the only one. Reduced to first principles, heat flows as heat flows, so it seems that what's good for keeping heat in is also good for keeping heat out. My bride (the engineer) and I have chatted about this from time to time (it's that kind of exciting relationship...), and it's mentioned in this article and debate over at Green Advisor: perhaps it's due to temperature difference. The temperature difference in Minnesota is larger than the temperature difference in Texas. Up north, you may be fighting to keep the T at 75 inside while it's 10 below zero outside (a difference of 85 degrees!). Down south, you may be fighting to keep the T at 75 inside while it's 105 outside (like today...), a difference of "only" 30 degrees. Perhaps this is why energy recovery ventilators work better (greater efficiency) in heating climates than in cooling climates?

Furthermore, there are a lot of heat sources in a house: the stove, the computer, the TV, the cat, you. In a heating climate, these heat sources help you. In a cooling climate, they hurt. And there's the humidity issue. I like the comment in the article about someone sometime designing appliances for a hot climate. I'm still losing sleep over how a clothes dryer takes indoor (cool!) air, warms it up, and then disposes it the outdoors. Not cool! (so to speak...). Certainly not efficient.

light speed

Just read a blog over yonder about lighting design which inspired me to work up the above schematic (click on it for a larger version). I have no idea if what I've done is actionable, but it sure felt good! I love me some pendant lamps (read the haiku below), so I may have gone a little crazy there (my bride is not so crazy about pendants; O the things you learn about someone after you say "I do!"). I may have gone a little crazy elsewheres too since I followed the (developed-during-the-process) cardinal rule of "If in doubt, put a can there."

The collage shows lights we've been looking at and, in a few cases, acquired. Don't worry: we're not putting all of these in the house! Some are included just because they're too darn cool (that cubist ceiling massing toward the bottom). My bride is intent on hanging the beautiful linear chandelier above the dining room table. Saturn's ring light (with part of an orange wall behind it [our neighbor's house!]) is the same as two vintage pendants we recently purchased for the kitchen island (they're from a midcentury modern schoolhouse). We also have the pendant with the little bird on it to hang over the master tub (something I believe we'll have to install after the home gets inspected...): snagged it for 66 percent off! Love love love the concrete up-light with the pink cord. All the "flying saucers" are potential can light covers.

We recently toured a house assembled by a potential builder for a homeowner with an impressive collection of art. The owner (who greeted us with wine and cheese!) had directional can lights just to illuminate his paintings, so I've sprinkled in a pinch of those as well since we loved the effect. As far as outdoor lighting goes, I added some spots for the sandcrawler. It moves slow, but people need to be able to see the sandcrawler coming. I reckon at some point The Architect will weigh in on lighting design. Perhaps this is a start...

As promised, a haiku (in the American Standard form) for pendant lamps:

reaching from the sky

illuminated sculpture

whispers from the sun...

urban inspiration

One plus about building a modern house is that inspiration can be seen driving home from work or eating scrambled eggs in a spiffy restaurant. Speaking of which, there’s a neat new restaurant around the corner from our current place that must have had a fabulous interior designer because the place is tres cool.

a place for the bubbles

We're not gotta-park-the-darn-car-in-the-darn-garage kinda folks, but we do need garage space for our bubbles, a couple of late fifties microcars often referred to by aficionados as bubble cars. The green one is a 1958 BMW (you read that right: B. M. W.) Isetta and the other one is a 1957(?) VELAM Isetta. If you've seen a Smart car, well, everything old is new again. Originally designed in Italy, BMW and VELAM licensed the design from a company called Iso (Isetta means "little Iso" in Italian). We bought the VELAM from, no joke, the heir to the Dubble Bubble bubblegum fortune who turns out to be an avid collector of bubblecars (he has quite a museum in Madison, Georgia). And, no joke, he delivered the car to our place in the back of a tractor trailer filled with bubble gum. Anyway, I digress...

Since we're talking to builders to get preliminary estimates, The Architect quickly put together a mockup of the garage/carport, and it's pretty darn cool. Plus, that shed roof will make it easy to collect rainwater. I'm thinking it needs a couple windows and maybe is a tad too big (knowing how I'm going to be using this space, I'm going to have to park my daily driver outside [the bride gets the carport]). Initially, I wasn't real keen on having part of the space a carport (a money-saving strategy...), but seeing the elevations and wanting to save and covet a large fig tree in that corner of the lot, I think this will work just fine.

the windows are the windows to your soul

Been thinking about and struggling to understand energy efficiency and windows, which ties into broader subjects related to insulation, energy efficiency, and climate (more on that later…). In short, as far as I can tell, insulation, window characteristics, and wall (and roof) design depends on where you are at climatically. At first blush, this seems obvious, but it becomes less so when you wade into the details. For example, one would think that whatever is good for keeping heat inside in a cold climate would work just as well for keeping heat out. But that ain’t so. As near as I can tell, it all boils down to the (apparent) fact that it is easier to control heat than it is to control cool. But I’m still thinking about that…

My handwringing over this comes from our starting-point decision to use storefront aluminum windows for the house. Storefront aluminum windows are sleek and durable, and they sure look mighty fine in a modern house. However, passivhausers would rather poke their eye out with leaky ductwork than put aluminum windows in their houses. And for good reason: Aluminum is a great conductor of heat, even better than steel. Thermally broken aluminum windows are available, but even these windows don’t approach the overall energy-efficiency of wood, fiberglass, and vinyl windows.

One thing that has made it difficult to achieve an apples-to-apples comparison between different window designs is the lack of comprehensive data. The passivhaus subculture and the window manufacturers that cater to that subculture know how to do it. To do it right, you need to know the properties of the glass as well as the properties of the frame. Pretty basic stuff. However, most window manufacturers don’t report this data. Instead, they report numbers for a complete window of a seemingly random size. I say seemingly random because some manufacturers, if you read the fine print, test a ridiculously oversized window, presumably to minimize the less-than-desirable thermal properties of the frame. One manufacturer appeared to size its test windows to achieve a certain U-value. No apples to apples there even among windows of the same manufacturer (although it creates the mathematical possibility of teasing out an estimate of frame properties…). Some manufacturers only report the numbers for the glass, which is unfortunate and horribly misleading because the frame is often the thermal weak point.

So what’s a tossing-through-the-night-because-I’m-worried-about-them-darn-windows dude or dudette to do? Ultimately, I climbed the mountain to visit the Efficient Windows Collaborative. These fine folks show comparisons of energy costs among windows of different designs in different climatic settings across North America. What I get from poking around this site is that in Austin’s warm climate, having a low solar heat gain coefficient (SHGC; a measure of how much solar radiation comes through the window to warm your house) is the most important factor. In a cool climate, you want a higher SHGC because you want the sun to warm your house. In a warm climate, you want a low SHGC because the solar radiation is your thermal enemy. The U-value is a measure of non-solar radiation heat flow through your window. Having a low U-value appears to be less important in a warm climate than in a cool climate (see my graphic modified from the Collaborative above). A thermally broken metal frame is less efficient, but “only” results in heating and cooling costs some 5 percent higher than non-metal windows for San Antonio (some green-tinged folks would cringe at me using the word “only” here; see my modified graphic above).

The Efficient Windows Collaborative has a window selection tool to help folks evaluate different windows and identify manufacturers that meet your goals. They also recommend checking out some free software from Lawrence Berkeley Labs if you want to do your own thermal modeling. I don’t know about you, but that sounds hot!

what lies beneath...

Back when we were courting lots, one of the factors we considered before making an offer was the surface geology. What lies beneath your lot has huge implications for your foundation. Ultimately a geotechnical firm will come out to drill a few boreholes before you build to see what lies beneath, but you can get a hint of what’s under those weeds before you buy the lot. And what’s under those weeds could save you, or cost you, a lot of money.

The key words I used above were “state geologic survey”. These are the folks that have pulled on their hiking boots and figured out the types of rocks and where those rocks are. As a result of this geologic mapping, they’ve also put together geologic maps, colorful renderings of what lies beneath. In some cases, particularly urban areas, the geologic survey may have even put together special maps of environmental geology geared toward, among other things, building and what the geology means for foundations. Most potential homebuilders aren’t aware of these resources. As it turns out, I have a degree in geophysics and worked, in my deep dark past, at the state geologic survey for a number of years. That gave us a leg up in these geologic matters.

Ideally, you want to stay away from clay and shale. Clay typically shrinks and swells depending on its water content. Shrinking and swelling means the ground beneath your foundation actually moves up and down. This would be fine if the ground moved the same amount at the same rate, but this is rarely, if ever, the case. All that shrinking and swelling winds up cracking your foundation if you are on a slab. Depending on how the ground moves, walls may crack and pipes may break. And messing with broken pipes in a slab is an expensive and miserably messy experience. Note that not all clays and shales are the same. Some move much less than others, but most move. Solid rock is best for building.

Austin sits squarely on a geologic transition point, and this has important implications for foundations. Millions and millions of years ago, there was a mountain range here, an ancestral extension of the Appalachians, that ran through Texas via Dallas through Waco through Austin through San Antonio and then out yonder to big Bend (where you can still visit remnants of these old mountains). The mountains are long gone, eroded and buried by other sediments, but they strongly influence the present lay of the land.

When the Rocky Mountains pushed out of the Earth some 65 million years ago and again 23 million years ago, they also lifted up much of Texas up to and including the old Appalachians. This caused sediments to the east and south of the old mountain range to sluff off toward the Gulf of Mexico. This created the Balcones Fault Zone, which gives the central spine of Austin its geologic complexity (don’t worry: this fault zone has been dead for millions of years). Because of all this faulting, you can have solid rock here and, ten feet later, mushy clay there.

Fortunately, Austin has a report on environmental geology (“Environmental Geology of the Austin Area: An Aid to Urban Planning” published by the Bureau of Economic Geology. Yeah, it was published in 1976 but, trust me, the rocks haven’t changed much since then…). Included in this report is a simplified geologic map that shows the location of clay, limestone, sand and gravel, and basalt (Austin had a volcano pop off back in the good ole days!). The report also includes the more detailed geologic map; however, this requires more interpretation for a non-rock person. If your area doesn’t have such a report, I reckon you could talk to someone at your geologic survey about the rocks in the area or even try talking to a geotechnical firm to get a heads up on what to look for.

One thing we avoided when looking for lots was the Del Rio Clay. Living in a house on this clay is like riding a slow motion roller coaster, including the screaming… Del Rio Clay was a deal breaker for us. If you’re looking to build close to a creek, then you’ll probably have to deal with alluvium, sand and gravel in the creek bed, which can extend for quite a distance from the present location of the creek. Friends building a house nearby and close to the creek had to install piers 30 feet deep on one side of their house to hit competent rock for their foundation. Another think we looked for was faulting. Building over a fault can be a problem, especially in California! Here in Austin it’s a potential problem if there’s different geology on the other side of the fault. For example, limestone on one side and shale on the other can be a challenge if your foundation has to straddle the fault. Same rocks on both sides? No problem.

So what did we find under our lot, at least according to the geologic maps? Austin Chalk. Good solid fine-grained limestone that is great stuff to build on. Nice neighbors around the corner from our lot that just built a house with geothermal (more on that later…) were kind enough to show us their geotechnical report. Thin topsoil, 10 feet or so of weathered (tan) but competent chalk, and then many feet of unweathered blue chalk. It’s a beautiful sight (and not just because I did my dissertation on the Austin Chalk south of Dallas).

One thing to note: geologic maps are often interpretive. Geologists get clues of the geology from where the rock is exposed, from boreholes, and from the vegetation growing on the surface (certain plants prefer certain geology). In other words, they have to fill in the blanks by connecting the geologic dots. That means that the maps may not be accurate, especially down to the resolution of a city lot. For example, if we had seen that a fault had Del Rio Clay on our neighbor’s lot but Austin Chalk on our lot, we would have been a little nervous. That fault probably wasn’t mapped at a lot-level of accuracy, in which case we might have Del Rio Clay under our lot or, even worse, Del Rio Clay and Austin Chalk. The other thing to note is that these maps show surface geology. For example, that limestone you see may only be a few feet thick or it maybe a hundred feet thick. If that the limestone is underlain by shale, a far less competent rock (as is the case with the Austin Chalk underlain as it is by the Eagle Ford Shale), thickness is important. However, if your property is in the middle of a lot of stuff of a certain preferred geologic flavor (as we are), you are probably OK. Ultimately, your geotechnical contractor will confirm your situation.

mug shots

It occurred to me the other day that elevations are like mug shots: You see what the suspect looks like from the front and in profile and how tall said suspect is. hmmm... Glad to have this one in custody...

Here comes the rain(water harvesting) again

"Rainwater harvesting..." The words faded and the potential builder smirked a wee bit. He may have even winced from biting his tongue. "How big of a tank are you planning?"

"Five thousand gallons."

"Five thousand gallons! Wow! You know what you're talking about!"

Indeed, we plan to include a five thousand gallon rainwater tank in our build. The builder smirked a wee bit because he knows what we know: water goes shockingly fast. We have an 800-gallon tank at our current house, and it's amazing how quickly that water goes, especially in this wacky flood-now, drought-later Central Texas climate. A typical family in Austin will use about 2,000 to 4,000 gallons a month for the yard during the summer months. More if you have a pool. Even more if you have a garden. And even more than that if you’re washing your pigs every Thursday night. The more rain we can harvest, the better. I'd love to do 10,000 gallons (my original plan), but I'm not sure where we'd put the other tank...

Fortunately we live in a town where the city gives a rebate for rainwater harvesting systems. Austin provides 50 pennies for each gallon of your unpressurized system with the total rebate not to exceed 50 percent of the cost of your system or $5,000 over your lifetime. That's a pretty healthy rebate for rainwater. Tanks tend to run about a buck a gallon, generally less the larger the tank gets. In addition, you don't have to pay state sales tax on rainwater equipment.

Note that I wrote “unpressurized” above. If your system is pressurized, that is, you have a pump hooked up to it, something you'll need to do to run your sprinkler or even a drip system, the rebate is even better: 100 pennies per gallon. However, there's a catch: You have to install and maintain a reduced pressure zone backflow preventor, something known in the bidness as an RPZ. An RPZ has to be installed at the street near your city meter by a certified plumber (I hear estimates of $600 to 1,000) and it has to be tested annually ($100 a test) by a certified RPZ tester. The cost of that annual testing pretty much kills any money savings you might hope to achieve with a pressurized system. And RPZs are kinda ugly, sticking up out of the ground all bony and whatnot (although I imagine the good folks at the American Backflow Prevention Association [and their Texas chapters] find them rather heart-racing...).

The RPZ requirement drives urban rainwater types bug-eyed batty. The RPZ is required to protect city water supplies. If there's a pressure drop in the city system and you have a hose in your rainwater tank, your rainwater (termed “water of an unknown quality” in the RPZ subculture) may get sucked into the city’s distribution system. In that case, your neighbor may be drinking your rainwater (be sure to send him a bill!). What? You don’t plan on ever putting a hose in your tank or, if you do, you plan on having a substantial air gap (a lo-tech but effective way of protecting the city’s water supply)? Tough luck: you still need that RPZ. And the annual testing.

Until this last legislative session, it was against state law to use rainwater for potable uses in your house if you were hooked up to a public water supply system. You could use rainwater for non-potable uses such as flushing toilets and washing clothes. However, drinking rainwater is a whole nuther deal that requires filtration and a little UV to kill the wriggling nasties you don’t want in your gut. Furthermore, if you wanted to be as reliant as possible on rainwater for all of your home's needs, you'd need tanks in the 20,000 to 30,000 gallon range. There’s generally not enough back yard in town for tanks that size.

Nonetheless, I feel deeply committed to using rainwater for outdoor use at our new property. Rustling the rain conserves water, the plants like it, and, quite simply, collecting rainwater makes us happy. The tank at our current house is made by Texas Metal Cisterns, and it’s a fine tank. But my new love is made by an Australian company called BlueScope. BlueScope produces portable, galvalume, install-in-place systems with a rubber-lined storage system. And the tanks are beautiful and modern with an optional roof fascia that gives the tanks some rings-of-Saturn mid-century cache. With a diameter of 11 feet and a height of 7 feet 3 inches for the 5,000 gallon tank, she's got big feet, but she can sure hold her liquor. Because Central Texas is rainwater central USA, we can get the tank locally.

Our plan is to put the tank in the back rear of the lot and feed her from the garage roof and part of the house. We love the idea of an aqueduct running 15 to 20 feet from the garage to the tank. The local BlueScope dealer says there's no need for a first flush system if the gutters are adequately protected, but it appears the city requires one. And any system greater than 500 gallons applying for the rebate needs to have plans approved and the final system inspected by the city. So there.

How much water can you collect? The Texas Water Development Board has an easy-as-pie and coo-as-poo spreadsheet to help size your rainwater system whether it’s for indoor and/or outdoor use (much rainwater literature is directed toward rainwater drinkers…). The City of Austin uses roof area multiplied by 5 or landscaped area multiplied by 4 as a rule-of-thumb that seems somewhat reasonable.

Postnote: It turns out that the tank I love (the one pictured above) is 3,000 gallons. A smaller footprint of 8 feet 10 inches. Decisions, decisions...

It’s raining links!:

Previous post on rainwater harvesting.

City of Austin FAQ on rainwater harvesting (including quite a few on the RPZ requirement).

City of Austin rainwater harvesting factsheet.

HarvestH2O.com: An online rainwater harvesting community

Texas Rainwater Catchment Association

American Rainwater Catchment Systems Association