- Earth Tube Design
- The Equations!
- Pressure Drop
- Reynolds Number and Turbulence
- Earth Tube Material:
- Metal ducts
- Clay or cement duct work
- PVC (Polyvinyl Chloride)
- HDPE (High Density PolyEthylene)
- Corrugated Drain Pipe
- Earth Tube Length:
- Earth Tube Diameter:
- Earth Tube Layout:
- Earth Tube Depth
- Camels Nose Heat Exchanger?
- Air Intake:
- Ventilation Requirements:
- 1 Earth Tube Design
- 2 The Equations!
- 3 Earth Tube Material:
- 4 Earth Tube Length:
- 5 Earth Tube Diameter:
- 6 Earth Tube Layout:
- 7 Earth Tube Depth
- 8 Camels Nose Heat Exchanger?
- 9 Air Intake:
- 10 Ventilation Requirements:
Earth Tube Design
Earth tubes (also known as Earth-Air Heat Exchangers, EAHX in professional circles, or as “Cool Tubes” in Australia, or by a variety of other names) can be designed many ways. Most home owners who install them realize that, while they are dealing with very ancient and intuitive technology, Earth Tubes are still poorly understood and somewhat “experimental”. While specific performance data is somewhat lacking, there are many experts with their own rules of thumb… Here I will try to sort it all out.
Since I plan to invest thousands of dollars in my own Earth-Air Heat Exchangers (a more sophisticated term than Earth Tubes), I have done a lot of research in order to try and optimize my designs. I have found that there are a wide variety of approaches out there. Some earth tube systems are long (hundreds or thousands of feet), others are short (less than 50 feet). Some propose 4 inch diameter PVC, others are using 24 inch diameter cement tubes. Various layouts, depths and other design features (weep holes, insulation, flexible connectors, etc) are talked about with great pride or warning. Many of these designers talk about how their method is the only good method, one particularly rambling blogger warned that any changes from his specific design could only result in disaster. However, almost all of the designers talk about how successful they were (few have really good data to back it up, but some do and that fraction is growing). I have concluded that because of dynamic effects (such as fluctuating operating temperatures and moisture levels), the wide range of parameters and the lack of information regarding material properties or other boundary conditions, it may be difficult or impossible to be sure of an optimal earth tube design (at the current state of the art). However, the wide variety of success indicates that the tolerances for a “pretty good” earth tube design are quite flexible and there is good understanding for how the parameters affect performance.
There has also been sufficient experience to make certain “danger zones” clear. As far as I can tell, the main dangers include…
- Moisture leading to mold and other problems => Make sure to slope your earth tubes and provide good drainage so condensation can exit. Stay above the water table and keep rain water out. Be careful with corrugated flex pipe as it could potentially provide lots of places for water to collect.
- Blockage rendering the tubes useless => Keep bugs and critters out, design the tubes so they won’t cave in, provide multiple paths as a more robust system, etc.
- Flow resistance making it too difficult to move air thru the tubes => system losses (both friction and dynamic losses) add up. Try to keep the pipe short enough (or large enough diameter) to avoid this problem. Smoother pipes can be narrower or longer. Avoid constrictions in the flow, such as two pipes coming into one of the same cross sectional area. Avoid fittings that try to turn the flow too quickly (such as 90 degree elbows) as these can double the back-pressure…
Other factors, such as how much heat is transferred, etc. are just performance details.
In the end, the key to good duct design (earth tubes are buried ducts) is compromise for economy. The ideal design uses the minimum length of very standard pipes with as few dynamic losses as possible driven by an appropriately sized and affordable fan.
Professional engineers and HVAC designers calculate the pressure drop (head loss) for a given duct system. You can use similar equations for earth tube design. If the pressure losses in the system exceed the pressure driving the flow (passive or active), the flow will stop and the duct will become useless. When air flows thru a duct, there is pressure drop due to friction losses as well as dynamic losses which are caused by change in direction or velocity (usually at the fittings). For a commercial HVAC duct system, 1 Pa/m loss is typical.
This page is long enough, so I will move the actual calculation to this page (link coming). However, even without entering actual numbers, the equations tell about the relationship between the parameters, which leads to design insight.
The friction losses are due to viscous interactions between the air and the pipe walls and can be expressed with the D’Arcy-Weisbach Equation. In this equation;
Some things are immediately apparent from the equation; for instance, the pressure drop is proportional to the friction factor. In other words, the rougher the pipe wall, the higher the frictional pressure loss (which seems pretty obvious). This should affect your material choice; PVC is 200 times smoother than concrete, HDPE is even smoother. Increased length is also a factor; while we want length to provide more contact for heat exchange, too much and the flow could stop. Velocity is very important because this is squared. In other words, if you double the velocity, the pressure drop is affected by a power of 4. If we reduce our velocity from 700 to 175 ft/min, we reduce the velocity by a factor of 4, and our pressure losses by a factor of 16. Passive systems tend to move the air relatively slowly, but adding a fan to increase the velocity may actually be counter productive (choose a fan with high pressure rather than high velocity ratings). On the bottom of the expression, we see the hydraulic diameter (proportional to pipe radius), which means increasing the pipe radius reduces the frictional pressure drop.
The dynamic pressure losses can usually be found in tables. For instance, for a given velocity, the pressure drop around a 90 degree mitered turn may be 50%. The same flow around a smooth bend may only lose 15%. The problem is that most of the tables are for higher velocities used in home or industrial HVAC systems. However, looking at these charts, you quickly get the idea that fewer fittings is a good idea. You can also see that lower velocities have lower losses for a given fitting.
Diverging or converging sections should be as gradual as possible. Most HVAC texts suggest that divergence should not exceed 12° and convergence should not exceed 30°.
Reynolds Number and Turbulence
A second equation, important for understanding earth tubes, is “Reynolds number”. When scientists were studying flow, they knew that it was sometimes laminar and then as the velocity would increase, it would transition to turbulent. They could graph it but it took until 1883 before Osborn Reynolds showed that the change depended on ρVd/μ, which was named “Reynolds Number” in his honor (and usually shown as “Re”).
This is important because the turbulence of the flow affects both the pressure drop and heat transfer rate of the system. By calculating the Reynolds number for a given design, you can predict if the flow will be turbulent or laminar. In a viscous flow, friction is able to stop the air molecules adjacent to the wall. If the flow is laminar, layers form with each layer away from the wall moving a little faster than the layer below it. This forms a “boundary layer profile”. The flow is called “laminar” because these layers are stable. Streamlines stay nice and straight and never cross. While the flow at the walls is stopped, the majority of the flow moves easily and smoothly thru the duct/pipe. The problem is that heat transfer between the walls and the majority of the flow is greatly reduced. In turbulent flow, the fluid is always mixing and the system is better able to transfer heat from the walls to the majority of the flow or vice versa.
Many Earth tube designers incorrectly assume that their flow will be laminar. They tell you that you can “add turbulence” and increase heat transfer by choosing tubes with rough or corrugated surfaces, laying the tubes in serpentine patterns, etc. These “enhancements” increase pressure drop dramatically. A few quick Re calculations show that they are not necessary for most earth tubes.
Reynolds number is proportional to the density, velocity and diameter and inversely proportional to the dynamic viscosity. The density and viscosity are properties of the fluid (such as air) and are both inversely proportional to the temperature. Thicker fluids (like syrop) have higher viscosity and tend to form laminar flows (Low Re), air is not very “viscous” and goes turbulent easily. The velocity and diameter are aspects of the duct design, increasing either parameter will increase your Reynolds number and turbulence.
ρ = fluid density
- V = mean flow velocity
- d = hydrolic diameter (inside tube diameter) (keep in mind that this may be different from the “nominal diameter”)
- μ = dynamic viscosity of the fluid
Lower Reynolds number flows are laminar. Higher Reynolds number flows are turbulent. For a round duct/pipe, this transition happens around Re~2300. We can easily calculate a table of Reynolds numbers for various nominal duct sizes (actual diameters would vary based on duct material). In this chart, I have colored Reynolds numbers >2300 red. These are turbulent flows.
Using the velocity and the nominal diameters (again, the actual internal diameters would vary based on duct material), we would get this table showing cubic feet per minute. Again, the “turbulent” flows are colored red. If you can stay below these flow rates, you may have laminar flow (flow could still be made turbulent by seams/joints, dirt, upstream turbulence from the fan, etc.) which would flow with less resistance, but much less heat transfer (much less slope for pressure drop over velocity).
A related question is how quickly the turbulence will form. Assuming the flow enters the duct as laminar flow (unlikely), how far will it go before it becomes fully turbulent?
Friction between the flow and the walls (friction exists even in a relatively smooth pipe) will bring the molecules immediately adjacent to the wall to a stop. This slows the flow next to it, and the flow next to that, etc. The result is a growing boundary layer profile that shows the gradient between the stopped flow at the wall and the free stream velocity. This boundary layer grows as the flow moves down the pipe until it meets in the middle and a stable flow profile develops. If the viscosity is high enough that the Re<2300, the flow can remain laminar. However, if the flow is not viscous enough, the friction at the wall can actually cause some flow reversal (wall roughness can cause this to happen even sooner). This flow reversal starts transition to a fully turbulent flow. A boundary profile can still develop in a turbulent flow, but it is really the “mean” turbulent velocity profile; the average of many small fluctuations in velocity and direction. Since this average is relatively constant, the resulting wall shear is constant and the pressure drop becomes linear with X.
The distance before this stable profile develops is a function of the Reynolds number and Diameter and can also be calculated. Often this distance is expressed over the diameter. For all the Reynolds numbers on the above chart, this works out to between 18 and 20 times the diameter, which for these pipes is between 6 and 20 ft. Any ridges, fans, screens or other upstream obstacles will only cause this to happen sooner.
Just in-case my point got lost in the engineer speak… here it is plainly. Turbulence is good for heat conduction, but bad for pressure loss. In designing your system, you should assume the flow in your earth tubes will be turbulent no matter how smooth the walls are. There is no need to add features to increase turbulence, they will only increase your back pressure and reduce your flow.
Earth Tube Material:
This is a very important decision. There are a variety of materials to choose from, from baked clay tiles, to steel duct work, to common PVC or the most modern HDPE plastics with anti-microbial coatings… Perhaps I will eventually come back and put this in a table, but for now, I will just list some of the pros and cons to each.
Note that the thermal conduction properties of the material do affect the rate that heat conducts thru them, but it doesn’t seem to affect the overall performance of the earth tubes. Partially, this may be because the total resistance to thermal conduction includes both the R value and the thickness. Although concrete conducts heat better than plastic, concrete pipe is typically much thicker and 2 inches of concrete ends up with a thermal resistance similar to 1/4 inch of HDPE. It is also somewhat because a somewhat stable temperature gradient is setup that eventually lets the heat thru. But the real reason the material conductivity doesn’t matter very much is because it is the conductivity of the earth that is the bottleneck. Aluminum conducts heat very quickly, but can’t draw it from the earth any faster than a plastic pipe can.
More important aspects to consider include durability, cost, ease of installation, environmental concerns and the interior wall friction factor that has a direct effect on the frictional pressure losses of the system.
are commonly used in homes as part of HVAC systems, so there are a wide variety of connections/fittings available and it is not hard to put the system together yourself or to find someone to do it for you. The prices are also reasonable and it is somewhat intuitive to believe that the metal will conduct heat better (I don’t think it actually matters since the earth limits the conduction speed anyway).
However, buried metal ducts will corrode over time, particularly in moist or acidic soil, even galvanized ducts are not recommended for burial. Rectangular sheet metal ducts, commonly used for indoor HVAC systems, are particularly poor in an outdoor/underground environment where their shape does not help them resist earth loading. Their joints open up and bugs, water, earth and roots get into the pipes. While I could not find experts who recommend using regular HVAC ducting, I did find corrugated steel duct earth tubes being used in a variety of projects. Mostly these were “earth ships” in dry areas of the southern states where corrosion is less of a problem, but the attached image is of an installation used to ventilate a large First Nations (Na-Cho Nyak Dun) tribal center in the Yukon (Canada). No comments were made on the expected life of these ducts.
Clay or cement duct work
has also been used. The idea is that if it is good enough for drainage tile or sewer systems, it is good enough for air. Their durability is not in question, however they are brittle and could be cracked with impact, most often during assembly when these heavy sections are lowered into the ground (typically with expensive equipment). The rough walls of these pipes provide a lot of resistance to airflow. The Friction factor for cement pipe is 200 times that of PVC. This friction has a direct effect on the frictional pressure losses. I suspect that the larger standard diameters can more than make up for the higher friction. The surface roughness can also make cleaning them impossible. The many joints are appreciated by bugs and mold.
It sounds like a bad idea to me, but proponents say that you can seal the joints against radon and insects while the permeability of the pipe allows moisture to escape (thwarting mold). Because many of these materials can absorb and release moisture, they can actually solve some of the humidity problems often associated with earth tubes.
PVC (Polyvinyl Chloride)
is a frequent choice. It is popular because you can go to any hardware store and buy as much or as little of it as you want. There are also a wide variety of fittings available. You can easily buy the tools and glue needed to assemble it or find someone to do that work for you. The downside is that PVC infamous for being one of the most hazardous consumer materials ever invented. Not only is it toxic in is fabrication, but many of those production chemicals are not actually bonded in the plastic and can leak out over time. No one wants dioxin or other carcinogens in their air supply. Structurally, PVC is brittle and gets more brittle over time. It is easily broken during installation (as testified to in the blogs of many who installed them). Flexible rubber joints have been used to repair breaks and some recommend them as a way to prevent breaks (flex instead of crack). Even after a successful installation, cycling temperatures cause thermal stress and micro-fractures. The joints can catch and hold water and make the pipes difficult to clean thoroughly.
I also found it can be quite expensive (~8$/ft for 6″ Dia) compared to other options such as HDPE (~$3/ft for 6″ SDR17). Of course, there are various grades of PVC; for instance, PVC SDR 35 (thinner) Sewer pipe can be purchased for less than 3$ per foot, but it breaks relatively easily. The equivalent HDPE pipe (6″ DR 32.5 pipe) is much tougher and can also be purchased for less than 3$ per foot, but will require a couple more dollars per foot to fusion weld it together (if you hire someone else to install).
HDPE (High Density PolyEthylene)
is probably my favorite choice right now. It is an inert plastic with none of the health concerns of PVC. It is also more flexible, smoother, stronger, and tougher than PVC or any other tube material I could find. This toughness is important during installation, burial and for the life of the tubes. HDPE handles the thermal cycling with ease. You can bury it and it will last as long as you need it, probably forever, but it is also recyclable. Sections of HDPE are fusion welded together in a way that results in joints that are as strong as the rest of the pipe and provide almost nowhere for water to collect. This type of pipe has the lowest friction factor available, which has a very direct impact on reducing frictional pressure losses.
One downside to HDPE is that you may need to hire a professional with the right tools to make those fusion welds. You can’t just pick up the pipe or the fusion tool at home depot and do it yourself (which was the main advantage of PVC). It comes in long pipe lengths that you will need to order in bulk and then unload when it is delivered. It also has a fairly high coefficient of thermal expansion, so if you plan to solar heat the air (as I do) flanges are recommended to prevent the HDPE from pulling itself thru the wall when it cools down.
I looked it up and noticed that the fusion welding temperature on the professional rigs was not very high (450°F), so I experimented with a piece of scrap HDPE pipe that I was given. I tried it three ways. First, I used my wife’s electric frying pan, which has a nice temperature dial. Second, I used my benzomatic torch directly. Third, to get a more even application of heat, I used the benzomatic to heat a thin piece of metal on one side and then touched the plastic to the other side…. In all three cases, I was able to soften the HDPE plastic and fusion weld it with ease. When I used the benzomatic directly, I was worried the HDPE would burn, but it didn’t. It just softened nicely. When I used the metal plate to transfer the heat, the plastic stuck a little (I over heated it past softening), but adding “parchment paper” solved that problem. The electric grill worked perfectly, but is probably overkill considering the other methods worked so well. I cut the samples up later and looked at the fusion cross sections… They looked good, although I could have gone with less softening. However, aligning the pipes was a little bit tricky. It would be good to make a simple jig for that purpose. I am pretty confident that I can do my own fusion welding for this low pressure application without hiring a pro.
Some people may prefer to have an expert fusion weld the HDPE pipes together… If you do that and want to keep your HDPE installation costs down, you will need to plan ahead more. Ordering all your HDPE for one delivery is a good idea (be ready with a fork lift to unload it), but you should also plan to have the fusion welder out for just one day. This will require organizing to make sure your trenches are dug at the right stage (after the house is cited and perhaps after foundations are poured). If you are planning for a geothermal ground loop, it would be at this same time also… You would then have all the HDPE pipe laid and fused at once. It will be important that both ends of the tubes are protected from critters from the start. These trenches will then need to be filled in (protected) before the next construction phases can begin.
Some builders create a temporary connection box to terminate the earth tubes in while other construction details are taken care of. The remaining distance to create the final connection to the house would then need to be done later and would require additional expense to mobilize the fusion equipment and operator.
Most of my earth tubes, specifically the in ground loops and the fresh air loops that connect to the basement, can be laid at once, but some other pipes will need to be installed along with the rebar (before the shotcrete). For those, I may want to use a 500 ft coil of 4inch HDPE pipe that I can simply cut to length (won’t need fusion) and wire into place.
An HDPE expert I was talking to told me that wrangling a 500 ft coil is very difficult and requires special straightening equipment that heats up the pipe as it is unwound, so maybe this isn’t really an option.
Another downside of HDPE is the availability of the pipe. As I noted, you can’t just walk into Home Depot and pick up a few pieces. You will need to find a proper supplier, a supplier that is used to dealing with much bigger customers (think cities or oil companies). The supplier may keep some HDPE pipe in stock, but there is a good chance the stuff you want won’t be… Larger-diameter thinner-wall pipe for low-pressure flows isn’t something a lot of people are ordering. Basically, the factory has a large extrusion pump that pushes the plastic thru a die to make the pipe. It pushes pipe out continuously and they slice off the lengths they need. When you order, you are asking the factory to stop the machine and switch dies for your order. If the factory is moderately busy, they are going to need a minimum size order to even consider doing that. It may be something like 500 or 1000 ft of pipe. You also need to wait your turn. Other customers are ahead of you and priority customers with larger orders may cut in line, so order early.
My local HDPE pipe distributor was very friendly and helpful, even though my job was small potatoes. He tried to push me towards the thicker pipe they had in stock (for higher pressure water or oil applications). He explained the factory processes and warned me that a customer order may take some time to fill. However, the thinner pipe also takes a lot less plastic and the price is about half as much. It will be easier to move around and easier to fusion weld, so maybe the hassle is worth it. I will come back and let you know how it actually works out for me.
You can buy very expensive HDPE with an anti-microbial inner coating designed specifically for earth-tubes and marketed towards people concerned about microbial growth. However, i suspect that the other properties of HDPE, particularly its very smooth walls and joints and inert chemical makeup, combined with proper installation, already prevents most of the problems and the expensive coating is not needed.
Corrugated Drain Pipe
is another polyethylene product, so, like the HDPE pipe, it is tough, long lasting, inert, etc. However, It is much thinner than HDPE, so, in order to keep it from collapsing, it is corrugated for extra strength. This pipe is definitely the most flexible and lowest cost of all the piping options, which is why it has been so enormously popular for earth tube applications. It is also very commonly used in perimeter drain systems used by both conventional and earth sheltered homes. As with the other types of pipe, the 6 inch corrugated drain pipe costs more than two 4 inch pipes (probably more due to lower production than increased cost of manufacture). You can get solid corrugated drain pipe, or you can get perforated or slotted pipe. There is also “leech” pipe which has even larger holes and is commonly used in septic fields. On average, 4 inch drain pipe costs less than 40 cents a foot (2012 pricing), while 6 inch can easily get up to $1.20 per foot. This sort of pipe is easy to install yourself, for additional savings.
Of course, there are drawbacks… In fact, I suspect that much of the bad press surrounding earth tubes comes from the use of this sort of pipe. Because the pipe is corrugated, regardless of how well it is laid, water will never run out of it completely. Water can sit in the corrugations. This can be worsened if it is not laid straight, which is not always easy with coiled pipe. Using perforated or slotted pipe can help by letting that water out, but those holes are notorious for letting bugs and radon (and possibly more moisture) in. Also, the factory slotted pipe has the slots on inside ridges, so there is no draining the outside ridges (I assume this is to prevent the slotted pipe from snagging while it is uncoiled), it will never drain completely. This pipe can come with a fabric sock that will help keep plant roots and many of the larger bugs out. It may also be better to slot your own solid pipe so that you can restrict the slot to a single side (don’t cut thru the pipe or it will collapse, just notch each of the outward corrugations so they will drain), but then you need to lay the pipe very carefully to make sure the notch is on the bottom.
My wife is particularly concerned about this sort of pipe and absolutely will not let me even consider it as fresh air inlets for our home… Fortunately, I agree. However, we will be using it as drainage pipe around the perimeter of our foundation. My plans for “By-Passive Solar” include earth tubes that would not go into the house, but would instead circulate solar heated air under my umbrella. The perimeter drains are already in a good place to do that second duty, I would simply need to lay them out a little differently so that I had a complete circuit and attachments to the solar air heater…
The corrugations also add wall friction ( very high surface roughness which leads directly to high frictional pressure losses) to this sort of pipe. If you are taking it more than a hundred feet, I recommend paying extra for the 6 inch pipe (even larger sizes would be better, but they are prohibitive expensive). If you use a duct fan, make sure it is the high pressure centrifugal type and not the low pressure axial “booster fan” type. It may not be practical for other reasons, but some suggest pushing the air (pressurizing the pipe) rather than pulling the air (reducing the pressure in the pipe). This positive pressure should help keep some things out (including Radon) rather than drawing them in.
Warning: Corrugated drain pipe seems great! It is tough, flexible, cheap, easy to install, etc. but it can also hold water (potential mold problem) and severely restricts the flow of air (high frictional pressure losses).
Some experienced earth tube experts (such as Larry Larson) recommend these corrugated tubes (but at the larger 8 inch diameter) because they feel the corrugations help mix the air, which improves thermal transfer. He also says you must lay them in a serpentine pattern to help with the mixing. I assure you (see the sections on Pressure Drop and Reynolds Number calculations) that the flow will be turbulent in even the smoothest pipe. The corrugations and serpentine path will dramatically affect pressure loss (Larson mentions that you can’t even feel the air moving). Larson’s site goes into detail on other steps you need to take to keep mold an other potential hazards at bay.
is a fiberglass duct type that I recently learned about. I have not had time to research it thoroughly, but it is used mainly in under-slab HVAC for commercial and industrial buildings. It is available in all the diameters and with all the fittings that you would need. I heard it was expensive, and it looks like it needs very professional installation but not sure how that cost compares to the alternatives. I will research it more when I have time.
Earth Tube Length:
Of course, all the parameters (length, diameter, smoothness, path) work together to determine the final performance. But all other things being fixed, increasing the length of an earth tube provides more length for heat exchange, it also increases the friction and back pressure. If it is too short, you have less than optimum heat transfer, but it it is too long, it is much harder to get air to flow thru it at all. At a certain point, the back pressure increases to the point where natural convection can not move the air. At a more extreme length, the back pressure could reach a level that not even a fan could blow the air thru it. Cost is a more complex function. There are certain minimum costs for delivery and getting a crew out to help install the pipes (HDPE pipe requires a professional). There may also be a minimum order to get decent pricing. Once you have a truck making a delivery, there is not much difference in delivery cost if it is a full truck or a half truck. Also, if the pipe needs professional installation, getting someone out for 1 hour usually costs the same as 4.
There are many tools for calculating the length of pipe that you need. Problem is they usually assume a certain flow rate and don’t take back pressure into account. They also don’t take the ability of the soil into account. I will link to some calculators here eventually, but for now I found that rules of thumb based on working examples are just as good. Passivhaus builders will tell you the diameter tubes that worked for them. I often see 6 inch pipe used in 250 ft lengths with a fan to keep things moving.
Talking with Adam Bearup recently and he mentioned some experience with the Earth Shelter Project Michigan. They setup a number of 6 inch diameter PVC earth tubes, each had a fan, but each was only about 50 ft long. He says they didn’t affect the temperature of the incoming air for very long after the fans were turned on. The tubes were probably not long enough for the size of the fans. The air was moving too quickly thru the tubes and stripped the heat from the tube walls faster than the soil could conduct it (not a thermally stable system). Even so, the ventilation was insufficient (such a large earth sheltered home would have needed 26 passive tubes running at 175 fpm or 7 tubes at high fan speed of 700 fpm) and so there were problems with humidity, etc. Adam acknowledged that longer or deeper tubes may have worked better, but at a greater expense. If he could do it again, he would probably just use a Heat Recovery Ventilator (HRV) instead, the cost would have been a fraction of the cost of the earth tubes. The HRV is an active system that requires electricity, but at least it is predictable technology. The HRV exchanges (averages) heat in real time without any storage capacity.
Earth Tube Diameter:
Again, this is only one of several parameters, but all others held constant, increasing the diameter reduces the back pressure (resistance to flow) by increasing the cross sectional area (π*R2) faster than the perimeter (2π*R) (proportional to surface area). Hydraulic Diameter is cross sectional area over perimeter and is in the denominator of the D’Arcy equation, which means that increasing diameter reduces pressure losses. Having a larger diameter means you can get more flow volume at a lower velocity which allows for more heat transfer. There is also more surface area (for a given length) which also helps increase heat transfer.
Many discussions on this topic will point out that smaller diameter pipe has a larger surface area to flow volume and therefore more heat transfer, but that argument assumes the same flow velocity (reduced flow volume). If the length and flow volume are held fixed, the larger diameter pipe has greater heat transfer than a smaller diameter pipe. However if you change the configuration and switch from a single 8 inch to four 4 inch pipes, you would have the same flow volume but double your surface area and therefore increase your heat transfer for the same flow rate. Changing to the smaller pipes would also increase your back pressure. Looking at the cost of this configuration, at least for HDPE pipe, the four 4 inch pipe configuration costs 8% less than the single 8 inch pipe configuration.
Increasing diameter appears to increase cost at a higher rate than increasing length. I noticed that prices also increased at a higher rate than the radius and even slightly higher than the cross sectional area. So while I would probably prefer to have 16 inch diameter pipe, I will probably install 6 inch diameter to get a lower price per inch of cross sectional area, even though this limits my total length.
Certain types of pipe, such as HDPE pipe, have a thickness that is proportional to the diameter. For instance, HDPE SDR 17 pipe has a wall thickness that is 1/17th of the diameter. The weight of the pipes is therefore proportional to the square of the diameter which means it increases at a faster rate. This means larger pipes are proportionally heavier (and also harder to bend), and therefore harder to work with, than smaller pipes. Six inch HDPE SDR 17 pipes are 3.34 lbs/ft, but 8 inch equivalents are 5.66 lbs/ft. That weight can add up quickly when moving around long sections of pipe. The insulative properties of the wall would also increase with thickness and reduce heat transfer.
There is a thinner HDPE SDR 32.5 pipe… Half the thickness of SDR 17 pipe, but it is very non standard and must be ordered from the factory in a large enough batch to make it worth their while (min 1000 ft). There is also a 6 inch HDPE pipe that comes on a 500ft coil, but I couldn’t find anyone with that in stock either. I was told that it takes 2 men to wrestle a 4 inch pipe off the coil, but no one I spoke to had actually tried the 6 inch pipe. One distributor told me that 6 inch coiled pipe has a tendency to kink rather than bend. The benefit of a coil would have been not needing to hire a professional to fusion weld the pipe. You could just lay it in one long piece.
Earth Tube Layout:
The layout of the earth tubes is an important decision. Some prefer a system with a manifold that separates the flow into parallel lines using T-connections or Y connections. These connections are usually more expensive than 10 ft of pipe and severely degrade the flow properties. It is very difficult to balance the flow thru a system of branching connections, and even more so when using a compressible fluid like “air”. If the system is not balanced, the majority of the flow may go thru only a few of the parallel runs, rendering the rest useless. I have seen a number of layouts where the pipe used throughout the system was all the same diameter, which constraints the flow rate of the whole system. Of course there are some benefits. This type of system is often easier to layout using straight sections of rigid pipe and if one of the pipes is crushed or clogged, at least the other pipes can continue to work. Often, this system is chosen by people using straight PVC pipe or who have a very limited area to run the earth tubes.
Radiant flooring used to be done this way also, but with smaller diameter PVC pipe. While water is famous for taking the path of least resistance, It is possible (but still difficult) to “balance” the flow of a relatively ”in-compressible” fluid like water, compressible air flow is much more difficult. Modern radiant flooring tube layouts are typically done with a single PEX or HDPE tube “snaked” across the floor. In addition to increased performance, this is easier and cheaper than the old branched PVC approach.
Earth tubes can also be done this way using U connections or flexible pipe if that is possible. Of course, if more land is available, fewer bends and larger radius curves can be used. Larger radius curves cause less resistance to the flow, but a straight pipe is actually ideal (if you have the land for it). However you lay it out, There are many advantages to this serial configuration, including flow uniformity and the ability to clean the pipe if necessary. (I plan to use a blast of air pressure to fire a bleach soaked Nerf ball thru mine if I ever need to clean it out.)
Another good option is running a number of pipes that are separate for their full length and only come together at the entrance to the home. Particular configurations can be used that make it easy to connect with a rectangular duct inside the home. This spreads the tubes out in the earth for maximum heat transfer and storage, but brings it all together for maximum flow volume thru a single opening into the home.
Since I have several acres around my home site, I plan to keep the earth tubes as straight as I can. I plan to use HDPE pipe that is flexible enough to allow for some bending without any special/expensive connections. The basic rule of thumb is that you can bend straight HDPE SDR 11 pipe around a radius no less than 50 times the diameter. Thinner pipe is more flexible, but there is always a chance of kinking if you try to bend it too tightly. PVC pipe is much less flexible.
Earth Tube Depth
Earth tubes work better when they are deeper. However, after a certain depth, the cost of burying them increases greater than the improvement in performance. Many “failed” earth tubes are buried less than 3 ft deep. At that depth, it is probably not worth doing. Many small excavators have trouble digging below the 8ft depth. 10 ft would be better, but I can understand someone limiting their earth tubes to the depth that their excavator can handle. Stepping up to the next size excavator could considerably increase the cost.
Working with deeper trenches can be more dangerous, particularly with certain types of soil that tend to cave in. In that case you would need to dig an even wider trench at further increased costs.
I have seen cases where earth tubes were laid at relatively shallow depths, such as 5ft, but then 1 inch of rigid insulation board was placed 2 ft above the tubes to further separate them from the air temperatures. This may be a good compromise if the costs of going deeper are prohibitive.
The choice of when to lay pipe is also important. I know of at least one project where the earth tubes were laid along with the earth cover over the home, which then settled (there is always settling) and damaged most of the PVC earth tubes. Laying the tubes earlier, along with the foundations, trenched into undisturbed soil, would get them deeper and with much less risk of settling.
Camels Nose Heat Exchanger?
Authors such as John Hait recommend putting these tubes close together so that the remaining heat from the exiting air can be transferred to the incoming tube. His book calls this earth tube design feature a “camels nose” after the well known biological advantage of camels. I found this excerpt The Biology of Human Survival.
The camel’s nose conserves water by lowering exhaled air temperature and removing water from it. During heat exchange inside the nose, the nasal passages are alternately cooled by inhaled air and warmed by exhaled air. The loss of heat from exhaled air to the nasal passages condenses water much as one’s breath condenses on a cold day, and this moisture is retained in the nose. In addition, the vascular anatomy of the camel’s nose and head provides a counter-current heat exchanger that protects the brain from overheating. A camel’s brain may remain more than 2°C cooler than its body. ~~~~Claude A. Piantadosi; The Biology of Human Survival, 2003
While this design improves the heat retention efficiency of the system, it may be less desirable from a flow efficiency perspective. The camel drives the airflow thru his system with a large diaphragm and lungs; but the passive earth tube equivalent is driven by low ΔT based buoyancy. The theory is that the cold air entering the inlet tube is warmed by the earth, expands and rises due to buoyancy where it enters the home at floor level. At some point the relatively warm, but used up, air exits home thru the return tubes. It is cooled by the surrounding earth and is drawn down the tubes by gravity acting on its increasing density. This is a very low force way to drive the flow and seems to always require fans (augmented passive solar?) to overcome the wall friction of the tubes and actually produce the required fresh-air flow volume thru the home.
The most famous earth sheltered home builders of the natural world are the prairie dogs. These rodents carefully engineer their tunnels (they live in earth tubes) to provide natural air-conditioning, fresh air, etc. They passively drive their earth tube system by separating the inlet and outlet by some distance that creates a pressure differential. Basically, they put the outlet at the top of the hill where the wind-speed is fastest and the air-pressure is lowest. This low pressure exit creates a suction that actually draws the air up into the earth tubes and thru the system.
This article touches on how prarie dogs use the Bernoulli principle to increase airflow thru their tunnels. I have read others about how they carefully balance the forks in their tunnels for even distribution and how even their beds of dry grass are kept mold free underground; I will connect back if I stumble upon articles like that again…
Termites and other creatures start with the Bernoulli principle, but take it a step further by actually constructing solar chimneys. These use solar energy to heat up the air in the exit pipe which creates expansion and much greater buoyancy than the small temperature difference applied in the Camel-tow method.
This article is about a mall in Zimbabwe modeled on the principles of a termite mound. This clever approach of copying (mimicking) natures master builders is sometimes referred to as bio-mimicry.
When applied to home construction, many PassivHaus experts suggest that your earth tubes should enter habitable space like living rooms and bedrooms to provide fresh air. This air can move thru the house and then exit from kitchens or bathrooms which require this sort of exhaust vent thru the roof by code anyway. The idea of venting moisture laden air from these rooms into cool underground pipes is probably not a great one anyway.
So, I do not recommend a Camels Nose arrangement for fresh air earth tubes, but I do think it would be a good idea for my by-passive solar heating earth tubes that must start and stop at the same point anyway. I plan to run the inlet and outlet tube as near each other as possible for as far as possible so that as much of the exit air heat is transferred to the intake air as possible. I may also insulate around these when they are outside of the umbrella so that more of the heat is exchanged and not wasted.
The air intake is an important component of the design.
Your earth tube needs to be secure to prevent insects and animals from using it as a subway into your home. In addition to unwanted pests, the tube could bring in odors or germs. These unwanted guests could also build nests that would inhibit or block the flow of air thru the tubes. I recommend two levels of screening. Thicker screen is needed to keep the larger (stronger) animals out. A second layer of thinner/finer mesh would be used to keep insects out of the tube. Of course, the ends of the pipe are only a small percentage of the surface and sealing them is not very effective against smaller insects if there are many other holes and joints along the way. Here again is where HDPE pipe can be a superior option.
Many earth tube designs feature a dust settling box at the inlet. This is a box that allows dust, pollen and other small particles to settle in a dry well before the air is drawn into the earth tube intake and into the house. Keeping the dust out also keeps the earth tubes cleaner and less habitable to insects. The key to this design concept is to let the air in thru a small inlet (about the same area as the earth tubes connected to the box, but then expand the volume rapidly so the air slows down and the particles can settle out of it. This is most simply done with a hole (covered with screen) in a box in front of the earth tube entrance. For maximum effect, don’t line up the box inlet and the earth tube inlet. For reduced pressure loss, create a gradual scoop to transition between the box and your inlet. If you have a centrifugal fan in your intake box, adding a splitter sheet can help balance the airflow and improve efficiency. Some keep their tubes even cleaner with the addition of standard furnace filters. Some have used “electronic air filters” that electrostaticly remove particles from the air without physically getting in the way.
Always keep an eye on system pressure losses. Intake screens, air filters, even the sudden change in area between the box and the inlet, can cause significant dynamic system losses.
The box can be further enhanced with a wind-scoop to catch air. This would increase the pressure in the box and help overcome more of the frictional resistance in the pipe. Of course, this scoop feature should be oriented to catch prevailing winds.
The earth tube intake “box” could be built any number of ways (I plan to build several and experiment with cord-wood, masonry, etc.), but should be sealed against insects and other critters. They should also have a lid or small door so you can access the earth tube inlet, filter or other features. I am also considering using some glass block elements to let UV light in for various health reasons.
This section about ventilation requirements was broken out into a separate page.