Ventilation Codes

As rising energy costs caused home owners to focus on increasing home heating/cooling efficiency, residential construction advances tackled the problem of infiltration, which had been the accidental fresh air supply for many centuries before anyone knew it was important.  This airtight building approach was great for reducing unwanted heat loss/gain, uncomfortable drafts, dust, pollen, bugs and more.  However, for better or worse, we need fresh air to live.  As above ground homes get tighter, building codes and organizations like ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) have developed standards to take some of the guesswork out of what is required (Specifically, ASHRAE-62).  These same ventilation standards are useful and required for Earth Sheltered home builders.  As ASHRAE says, “If you build tight, you must ventilate right!”     Another detailed document 

Codes allow most rooms to have natural ventilation provided by windows that can open to at least 4 to 8% o the floor area.  Where this is not available, a more intentional system for ventilation is required.  Since you may not always want to open a window (and let all your heat or cool out), a well designed ventilation system that doesn’t rely on windows is usually a good idea.  In fact, ASHRAE 62.2 recommends that windows are not actually sufficient to control the moisture and cooking by-products (grease that sticks to walls, etc.) generated in an average kitchen.

In earth sheltered homes, it is common for bathrooms and other rooms to be on the “back” of the house without the option of opening a window. These back rooms are also against cool earth, increasing the probability of problem causing condensation and making adequate ventilation even more important.

The following table gives a high level overview of ASHRAE-62.

ASHRAE 62, Ventilation requirements are helpful and required for Earth Sheltered homes also…  These same requirements can be found in the International Residential Code (IRC 303).

Living Areas

Living Areas include habitable space such as living rooms, play rooms, dining rooms, bedrooms, etc. (pretty much anywhere you sit or lay down).  It does not include storage rooms, mechanical rooms, large closets, laundry, etc.   Generally the 15 cfm per person dominates the requirements.  It is a judgement call as to how many people can be expected in a room, but as a rule of thumb, common rooms should be sized for the total number of expected residents.  Bedrooms should be sized for at least 2 people.

As an example calculation, lets assume a living room that is 15’x15’x10’=2250 ft^3.  If we must do 0.35 ACH, then that is 788 ft^3/hr or 13 CFM…  Even for a hermit, that is less than our minimum of 15 cfm per person.  Lets say we have 4 people and need 60 cfm.  Most forced air furnaces run at about 700 ft/min (8 mph).  These are driven by a fan and can only function for a relatively short distance, so lets assume that our more passive systems operate at a more leisurely rate of  175 ft/min (2 mph).    This would require nearly 50 square inches of inlet, which would require either four 4 inch pipes, or two six inch pipes or one 8 inch pipe…  Of course, we could make the room much larger (up to 32 x 32) and not need any larger duct work…  In many cases, the living room, dining room and kitchen essentially share the same space without walls to separate them, so we can consider them one living area for the purposes of the ventilation requirements.

Other smaller, but separate, rooms will each need to support 15 CFM per person expected to occupy them.  If we assume 30 CFM per bedroom, we could use a single 6 inch duct to carry fresh air to each room.  There are no exhaust requirements for these living areas, but air must be allowed to escape as fresh air enters or a pressure differential will occur.  It can escape thru return air vents or under doors.  In many pasivhaus designs, used air from the living spaces is exhausted thru the bathrooms or kitchen.  This promotes whole house flow.

Bathrooms

The exhaust requirements for bathrooms are often specified as 8 ACH (Air Changes per Hour).  This is not a continuous requirement, but is typically applied when sizing the exhaust fan.   Some local codes require the exhaust vent to be place at or near the ceiling.  This makes sense because the warm moisture laden air you are trying to remove is typically buoyant and rises to the ceiling.  The code also requires that the moisture laden air be exhausted to outside; it does not permit exhausting to internal duct systems that may lead back to an air to air heat exchanger or earth tubes.

I have heard of local codes that specifically require the exhaust to vent thru the roof.   This can be problematic for an earth sheltered home builder who would rather run this thru a wall than penetrate the waterproofing layer.   Where this rule is enforced, it is an attempt to prevent the builder from inadvertently dumping this humid air under the eves where it could be sucked back into the soffit vent and degrade the insulation above the ceiling.  If your earth sheltered design has no soffit vents, and it would be easier to run it thru the wall, you may be able to get permission to vary from the code (a variance).

A code variance or modification considered when there are “practical difficulties or undue economic hardship involved in meeting a construction code requirement.”

If you must (or would prefer to) run these exhaust vents up thru the roof, I recommend running them up thru the skylight curb (A skylight is a pretty standard feature in an earth sheltered bathroom) to reduce the number of roof membrane penetrations.  If the tube can be run up the sun-ward side of the skylight curb, they may be some potential for a solar chimney application.

As a quick calculation, a 5′ x 8′ bathroom (about the smallest layout that “works”) with 10′ ceilings has about 400 cubic feet of volume.  With 8 ACH, we are looking at exhausting 3200 cubic feet of air per hour or about 53 cfm.  Velocity is important.  Most bathroom fans are rated between 50 and 120 cfm, so it would be no problem to install one of those, or you could use more passive methods and a more leisurely pace, but you would need more or larger pipes.   Two 6 inch pipes at 175 ft/min would easily clear 68 cfm.  Fortunately, the “continuous” ventilation provided by a passive system would only require less than half of that and could be served by a single evacuation tube, even at low temperatures.  I plan to design for a passive system, but go ahead and install the standard bathroom fan with a switch anyway.

Bathroom fans are important for removing moisture, but many people refuse to turn them on due to the awful racket they produce.  It is possible to use a remote fan to suck the air out of the bathroom from further away to reduce the noise.  On the other hand, a friend of mine was a construction contractor and was often asked to install very high end bath room fans and then called back because the fans didn’t provide the comforting (and tinkle sound covering) noise that the home owner expected.  He would take it down, bang it up with a hammer and re-install.

Kitchen

In addition to the basic ventilation needs for the kitchen (listed as 100 cfm intermittent or 25 cfm continuous or an operable window), there is also the need for a range hood to deal with the moisture and odors of cooking.   Many homes exhaust this air directly to the outside and the rules for exhausting this air (along with its moisture and suspended oils) are very specific.   However, many home fires have resulted from oil coated ductwork catching fire.    It also seems very wasteful to eject heated air from your home at a rate of 150 cfm.  More modern range hooks have a charcoal filter to remove the cooking oils and other particles from the air and then return it to the room, with any wasted heat.   In fact, if the heat from cooking is simply filtered and returned to the room, it can be considered an internal heat gain when performing a home heat loss calculation.

IRC 2003 – RANGE HOODS
M1502.1 General.
Range hoods shall discharge to the outdoors through a single-wall duct. The duct serving the hood shall have a smooth interior surface, shall be air tight and shall be equipped with a back-draft damper. Ducts serving range hoods shall not terminate in an attic or crawl space or areas inside the building.
Exception: Where installed in accordance with the manufacturer’s installation instructions, and where mechanical or natural ventilation is otherwise provided, listed and labeled ductless range hoods shall not be required to discharge to the outdoors.

If I had a gas range (officially referred to as a “combustion appliance”), particularly if it was a large gas range, I would strongly consider venting to the outside.   But with my electric range, I am happy to have any heat generated be filtered and returned to the room.

Your refrigerator is really an air to air heat exchanger that needs proper ventilation for maximum efficiency.

Another interesting consideration is ventilation for your refrigerator.  A refrigerator is really a decent sized air to air heat pump that extracts heat from the inside and pushes it to the outside (behind).  Without proper ventilation, it can warm up the region behind the refrigerator, which makes it less efficient as it tries to dump more heat into the same space.  Most refrigerators have a built-in air-intake on the bottom front of the refrigerator, but assume you will leave space behind and above for the heated air to exit again.  Be careful not to surround your fridge with cupboards, and if you do, make sure to provide adequate ventilation.    I am considering adding a duct from the basement to the the space behind the fridge so that it can draw and heat basement air and add it to my living space as it cools my food.  Perhaps this would be a good entry point for an earth tube.

Laundry

Clothes dryer exhaust is the main concern here.  A clothes dryer works by heating up room air to lower its relative humidity, and blowing the warm air thru the tumbling wet clothes where it can pick up the moisture.  The dryer exhaust contains a lot of moisture and often lint along with the heat.

You can get a natural gas clothes dryer that uses electric motors for the fans and rotating the drum, but a natural gas flame to heat the air.  These can be cheaper to run over the long term (assuming the price of gas stays low relative to electricity), but they are more complicated so they cost more to purchase and need a professional installation.  There is always a potential for problems with any gas appliance, including leaks of the methane its self or combustion by-products such as carbon monoxide, nitrogen dioxide (linked to asthma), VOCs, fine soot, and many other trace chemicals, so I don’t recommend them for well sealed earth sheltered homes.  If you do go with gas, make sure you add what ever extra ventilation you need to stay safe, plus a bit more.

In the cold dry winter, the moisture and heat (and perhaps the smell of fabric softener 😉 ) may be welcome additions to the home.  It is a shame that most Americans pump this “enriched” air outside.  You can purchase a simple dryer heat diverter for less than $10 that will divert this air back into the room.  Unfortunately, if this exhaust air, already saturated with moisture, is sucked back into the dryer, it will not be able to pick up much more water and the dryer efficiency will be reduced.  Ideally, the exhaust air can be diverted into another room that needs it instead of being sucked back into the dryer.  Reusing the dryer exhaust in winter is like giving the dryer a second job as a heater/humidifier.

In hot humid summer, it is very important that any hot humid dryer exhaust exit the home as quickly as possible.   If the duct is long or constricted or contains many turns, the back-pressure increases and the (built in) dryer fan is not able to push as much air thru the system and the efficiency of the dryer is reduced.   Dryer booster fans can be purchased to compensate, but because they are designed to manage the heat, moisture and (worst of all) dryer lint, they are expensive.  A much cheaper solution is just to design for as short and direct an exhaust duct as possible.  This can be a bit more difficult in an earth sheltered home where the laundry is often against a back wall where the earth is thickest, but make sure to keep it under 25 feet and count any 90 degree turns as an extra 5 ft.

Another way to make it difficult for the dryer is to limit the amount of incoming air.  Air must be able to get into the laundry room before it can be blown thru the dryer.  If not, the pressure in the laundry room will drop and the shortage of air will reduce the efficiency of the dryer fan causing it to work harder and take longer.  Most homes are not built tight enough for this to be a serious problem, and ASHRAE guidelines actually exempt the laundry room from all ventilation requirements, but earth sheltered laundry rooms can get pretty tight.   It may not be worth ventilating directly, but consider adding a transfer grill or jump duct to let in air from the hall, or you could trim the bottom of the door to let air in that way.

Many home designs keep the laundry away from the bedrooms for a variety of reasons, including separation from unpleasant laundry room noise and vibration.  My heavy cement house shouldn’t let much sound or vibration between rooms, so I have placed my laundry close to the bedrooms where the dirty clothes are ;).   I need to let air in for the dryer, but I don’t want the sound getting out.  I have a false ceiling in my hall that I use to run duct work and electrical to the master bedroom.  I also plan to put a transfer duct above the hall ceiling into the laundry so air can get in, but the sounds of the laundry room will not have a direct route into my hall.

A piece of string makes a great passive solar clothes dryer and is probably the cheapest passive solar component you can add to a home…  Dryers use a lot of electricity, so it can make a big difference. 😉

Garage

My garage is essentially a buried Quonset hut, so I do not have the sort of heat gain problems associated with above ground garage structures.  ASHRAE garage ventilation requirements are 100 cfm per car.   Since it doesn’t make sense to run this sort of fan all the time, I will probably put mine on a switch or on a motion sensor.  I don’t recommend starting your car before opening the garage door anyway.

Do not use one of those rotating vents, it will run all the time and over ventilate your garage.

If you have a shop in your garage, you may need to consider that ventilation also.  If it is basically one room, meeting the ventilation requirements for the cars can count as your shop ventilation also.  Again, providing a switch so you can turn the ventilation on only when you need it is probably a good idea.

ASHRAE-62.2 is very specific about how duct work and air handling equipment that passes thru the garage should be well sealed to avoid carrying contaminants often found in garages into the rest of the home.

Flow Path

Many passivhaus designers recommend using earth tubes to bring fresh air into main living areas and then exhausting this air thru moisture laden (wet) areas such as the bathrooms or kitchen.  Exhausting this moisture directly to the atmosphere runs contrary to the “Camels nose” design that many earth sheltered home designers would rather implement, but I suspect that trying to draw moist bathroom air down into earth tubes is probably not a good idea.  I have seen some HRV web sites that do recommend running bathroom exhaust back thru the HRV (or HEV) to recover this heat (and moisture) in winter.

Passive stack (Bernoulli) and Solar chimneys are a great way to passively draw air flow thru a building.   While rare in the energy-rich USA, passive stack is very common in Europe, particularly in overcast England where it is found in 90% of single family homes.  Optimizing a stack design can require a study of the local weather patterns and home design, even the surrounding neighborhood structures and trees can affect the flow patterns.  However, as a general rule of thumb, taller stacks work better, especially if you add a rotating cap that works in any wind direction.  Larger diameter pipes work better than smaller diameter ones which have higher flow resistance.  Careful design may also be needed within the home to ensure that all rooms are equally well ventilated; this is more of a concern with passive homes because of the lower driving pressures.  Passive systems may need to be “augmented” to increase their effectiveness as necessary.  Modern “active” ventilation fans often come with an intelligent control system that can dial back the amps when the passive methods are working well.  As any kite flier knows, its not windy every day.   However, due to the way weather is generated by hot or cold fronts, calm days are usually moderate and good for opening windows.

If you are lucky enough to live in one of the sunnier areas of the world, you may want to add a solar chimney to add passive solar heating to the passive stack effect.

Fans can be purchased based on their specific cfm ratings needed for the various rooms, or whole house ventilation systems can be used (typically, 50 cfm is sufficient for a whole house ventilation fan system).  If these systems draw in fresh air directly from outside, you could end up in a situation where you are continuously heating/humidifying or cooling/dehumidifying fresh air and then dumping it outside along with its energy.  Earth tubes are one way to moderate this incoming air, but those above ground dwellers may have a cheaper/easier way…  Many well sealed above ground homes use an ecomonical “balanced ventilation” system such as an HRV or ERV.

HRV/ERV

A balanced ventilation system is one that controls the ventilating airflow in and out of the home, typically using a pair of fans.  These units usually pass the two airstreams next to each other so that some of the thermal energy can be transferred from one to the other.  They can do this thru a cube shaped heat exchanger or thru a turning wheel.   These are typically known as heat recovery ventilators (HRV) if they only transfer heat (Or as an ERV if the interface can also transfer moisture).   HRV/ERV systems usually have several fan speeds and range from 50 to 150 cfm, but at lower speeds, the heat exchange is more efficient (more time for transfer, less dynamic resistance), so running constantly at the lower speed is better than running 20 minutes of every hour at the highest speed.  Many also include timer switches that are installed in the bathrooms to boost incoming flow temporarily.   Underground passive solar homes can use the same mechanical systems, but often prefer to use more passive methods, such as earth tubes to temper the incoming air and a solar chimney to draw the air thru a home and out the top.

An HRV is a very simple device with 2 fans and a heat exchanging core, all nicely packaged in a metal box with duct work connections. They can be purchased in several sizes, based on square footage of the home to be ventilated, for between $500 and $1000.

I believe that all systems should start with a good passive design, but in many cases, adding some “active” assistance can dramatically improve performance for a very reasonable cost.  Cost wise, an HRV costs little more than the price of the two fans it contains, but provides a lot more value.  These units typically contain filters which also help improve air quality.  They almost certainly cost less than an earth tube system.

It may be that an ideal design would use a combination of earth tubes and an HRV or ERV.   Unlike an earth tube system, an ERV or HRV does not have the mass to store energy.   An earth tube system can store the warmth of summer and return it in winter.  Even a relatively short Earth tube can at least help moderate day to night fluctuations.  An HEV can only transfer energy from one stream to the other.  However, this can significantly reduce heat loss/gain while ventilating.  An ERV can also reduce moisture loss/gain, which can be helpful in an earth sheltered home.
The choice of an ERV vs HRV is more complicated than the “climate zone map” that the manufactures show.  They are interested in making the sale as quickly as possible and a complicated decision would just slow the sales down.  As an informed customer, you should think about what the equipment does and make your own decision about what would work best in your design.  Also, ERVs tend to be 10 to 20% more expensive than the equivalent HRV.

Using an earth tube upstream of an HRV should prevent frost buildup in the unit, which means you could purchase a lower cost HRV that does not include the “Re-circulation Defrost” option (should save about 10% of the unit cost).  This defrost option is typically the most complicated (and failure prone) part of the system and works by occasionally blocking the fresh air intake and allowing the warm air from the home to defrost the unit, in many designs, this causes cross contamination between the inflow and outflow air.  At best, this re-circulation option puts the ventilation out of balance and wastes heat when operating.

You may want to consider an ERV (Energy Return Ventilator that exchanges heat and moisture), you may see that many of the manufacturers (such as the Lennox web page) only recommend ERVs for warm climates, but never explain why.   Digging further, I found that these are not recommended for cold climates primarily because of the risk of freezing the moisture exchanging core.   This is not a problem when fed with an earth tube, or you could pay a little more to get the defrost option.   The benefit of going with an ERV is that it is able to transfer incoming humidity to the outgoing air stream.  The advertisements make it look like a complete transfer, but it reality the heat exchange is the same as for an HRV, except across a moisture permeable interface.    This happens best if the hot humid humid outdoor air is cooled while warming (transfers its heat to) the cooler dryer exhaust air stream.  Relative humidity changes force moisture out of the cooling air and it is picked up by the warming air on the other side of the membrane.   It is important to understand that this moisture transfer only works if the outgoing air is much cooler or dryer, which usually only occurs if you cooled and/or dehumidified it.  An ERV is NOT a dehumidifier or an air conditioner, but it can help you retain your conditioned air during ventilation. 

Earth sheltered homes don’t tend to need AC equipment which would automatically dehumidify the air as part of its process.  An ERV will not dehumidify the air, most earth sheltered homes will still want a “whole home dehumidifier”.  However, using an ERV will help reduce the summer humidity load.  Cold air can not hold much moisture.  During cold winter ventilation, cold dry air entering the home will pick up moisture from the inhabitants as it warms.  If it is expelled from the home with an HEV or exhaust fan, it takes that moisture with it.   Using an ERV in a cold climate gives that moisture a chance to jump onto the incoming air as it is warmed up in the core and could actually help prevent your home from drying out (process reversed from summer).

The exchange also works best if the system is in balance.  The outgoing flow should roughly match the incoming flow if you want to recover heat or moisture.  During the summer, I plan to use a solar chimney to draw the air out of my home…  Incoming air will still pass thru my HRV or ERV, but I wouldn’t expect any “recovery”, and I may even want to remove the core (pressure and dynamic losses) and turn off (or redirect) the outgoing fan.

I have read a number of articles (such as this one) on the proper installation of these HRV or ERV units and here are the top concerns.

• Use a dedicated duct system.  Some try to feed their forced air system and end up picking up the moisture off their condenser coil.  This increases the humidity and causes other problems.
• Exhaust high moisture sources (bathrooms and kitchen) directly outside.  It is not worth trying to recover this heat or moisture.  You will just be spreading too much moisture around.
• Separate the outdoor intake and exhaust ports to avoid drawing the exhaust hair back into the unit
• Balanced flow is key to good recovery.  Make sure to check the flow rates in both directions and make adjustments if necessary.  If you intentionally unbalance the flow, be smart about it.
• Use with a dehumidifier or AC unit in summer.  Don’t count on a ERV to remove moisture (unless it is a combo ERV/Dehumidifier), it can only help reduce the burden of fresh air on your other systems.
• Reduce the number of twists and turns to a minimum.  I recently saw an installation whose intake ran thru flex pipe along two walls and for more than 40 feet.  It included two tight 90 degree turns before a 180 degree turn into the unit.  I expect those dynamic and frictional losses will dramatically reduce the airflow rate.

Calculation Example

Using the above information, it is possible to size the earth tubes and fans necessary for healthy ventilation.

(obviously still in progress…  I’ll get back to this later)

Earth Tube Concerns

A variety of concerns are often stated as reasons why earth tubes should not be used or may even be dangerous.  These range from concerns about cost effectiveness to serious health concerns related to mold or radon.  In general, these concerns are based on anecdotes or experience improperly installed earth tubes.

Earth Tubes and Radon:

One earth sheltered home owner I spoke with told me that after many years of living in his earth sheltered home and loving his earth tubes (his wife told me how much she liked the natural humidity control), an air quality test showed that they were bringing in high levels of radon gas. Radon is an inert “noble gas”, 8 times more dense than air, that also happens to be the radioactive offspring of Uranium decay.   The inspector told them that the level found in their home was the cancer causing equivalent of smoking several packs of cigarettes a day.  They immediately plugged up their earth tubes.  But what had gone wrong?

While the danger of cigarette smoke in now almost universally accepted (well paid tobacco lawyers and tobacco executives excepted), there is actually still some controversy about how dangerous naturally occurring Radon actually is.  Controversy or not, it makes sense to assume that radioactive R86 is probably something you want to avoid if you have a choice.

Radon works its way out of the ground thru little gaps in the soil.  This is a natural process that happens all the time, but the radon is usually dissipated quickly.  The problem starts when earth tubes can provide a sort of Radon collection system, a path of least resistance that draws in all the radon from the soil around the home.  Once the radon is channeled inside an airtight home, the heavier-than-air gas can become concentrated.

Many earth tubes are made from flexible drainage pipe with weep holes along every inch of it.  This is great for humidity control as it lets moisture in an out as needed, but it also makes it easy for radon to enter the pipe.  Sloping the tubes toward the home also makes it a bit easier for the 8x-denser-than-air radon (and water) to enter the home.

Using sealed solid wall earth tubes (such as HDPE double wall pipe) and sloping down away from the house to daylight, prevent both of these problems.

Even if radon (which only has a radioactive half life of less than 4 days) does enter the home, it is really only a concern if it is trapped in the home and becomes concentrated.  Actually, this can even happen in “normal” homes with poorly sealed basements and poor ventilation.

Homes that allow for a proper number of air exchanges each day, should not have Radon problems. In fact, a good earth tube system that provides ample air exchanges can actually be the solution to a radon problem.

Earth Tubes and Mold

Every now and then an earth sheltered home or PassivHaus gets diagnosed with “Sick House Syndrome”.  Here is an example,  In this case, the problem was really in the walls, but the earth tubes were not sloped to drain and were actually collecting rain water directly...  Generally, these houses are unhealthy because they literally can’t breathe, and the levels of internal pollutants (chemical and biological) build up (become concentrated) to the point where they irritate the occupants and may even cause serious health problems.  In extreme cases, the home may even be condemned.

A properly designed and installed earth tube system could provide for healthy air exchanges, so it is ironic that the headlines often blame the earth tubes, specifically, mold growing in the earth tubes, for all the problems.

We all know that mold likes to feed on organic material in warm, moist dark places.  No one wants to breathe air that just passed thru a mold filled earth tube.   The idea that earth tubes provide this environment scares many people away from the whole idea…    Properly designed earth tubes should not provide the right environment for mold.  They should be warm and dark, but without the right level of moisture or anything organic to eat.

Keep in mind that even prairie dogs can engineer an earth tube system with enough flow to keep their dry grass bedding from molding.  The keys to a successful earth tube design are slope and airflow.  And choosing a nice smooth inorganic material (like HDPE plastic pipe) helps too.

Water can get into earth tubes by seepage thru joints or holes, but mostly it is by condensation on the inner surface as warm moist summer air flows thru a cool tube.  In some particularly poor designs, rain water can enter directly.  These problems can be avoided with careful design.

Condensation is unavoidable and happens when warm moisture laden air comes into contact with the cool pipe walls.  If this moisture sits there, it could make a suitable environment for mold spores that happen to pass thru.  So don’t let it sit there!

A stagnant earth-tube just sits there stewing… but good airflow can either bring in dry air that will absorb the moisture, or it can bring in more humid air that will condense further until the weight of the droplets exceeds the surface tension and friction holding it in place so that it will run down to the lowest point in the pipe…  A proper design anticipates this water and directs it out to daylight or into a drainage pit.

In some cases, builders have used HDPE pipe with an expensive antimicrobial coating on the inner surface.  I suspect the expensive coating is overkill.  However, the smooth walls and seams of HDPE pipe promote greater airflow and allow water to drain away more easily (provided the pipe is sloped), which probably does make a difference.

The real kicker is that scientific studies of microbial and fungal growth in earth tubes actually show a net benefit as these two quotes explain.

“The Earth-Air Tunnel is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of earth-air heat exchangers [earth tubes] not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.”                                           ~~~Rabindra Nath B, Shailendra Kumar M, and Pawan Basnyat; Use of Earth Air Tunnel HVAC system in minimizing indoor air pollution; Air Quality Monitoring and Management, proceedings of Better Air Quality 2004.

“This study [of 12 different EAHX] was performed because of concerns of potential microbial growth in the buried pipes of ‘ground-coupled’ air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spores and bacteria, with few exceptions, even decreases after passage through the pipe-system”       ~~~Flueckiger B, Monn C; Microbial investigations and allergen measurements in ground-coupled earth-to-air heat exchangers; AIVC 20th Conference and Indoor Air 99, the 8th International Conference on Indoor Air Quality and Climate, UK, Garston, BRE, 1999, proceedings, Edinburgh, Scotland, 8–13 August 1999, Volume 5.

Earth Tube System Saturation

Thermal saturation is a very real concern that is often underestimated.  Many books and articles on the subject repeat the false idea that the temperature of the earth at some magical depth (5, 10 or 20 feet depending on the writer) is stable at some fixed temperature all year long.  These articles overly simplify the situation and make the soil around the earth tubes seem like an inexhaustible thermal reservoir.  Of course, the laws of physics, including laws about transfers of energy, still apply 10 ft below the ground.  Please see the results of my own experiment to get an idea of an actual underground temperature profile…

The moment you bury an earth tube in that “stable” environment and start to run air thru it, you start to change the temperature profile of the environment.   The original temperature profile was somewhat stable because it was separated from the air by 10 or 20 ft of compact earth.  If you now bring the air down into the earth for the purpose of exchanging heat with it (Ground – Air heat exchange), you should expect the temperature of the earth to vary according to that heat exchange.   If the soil warms up air in the earth tube on a cold day, that thermal energy must come from somewhere.  Of course it comes from the soil, and in the exchange, the soil temperature will be reduced proportionally to its higher mass and heat capacity.   If the air is cold and is warmed by the earth, the earth will cool down during the exchange.  This is why it is called an “exchange”.  The rate that the soil is warmed or cooled is affected by things like the exchange surface area and conductivity, the volume flow rate of the air, the temperature difference (which drives the conduction and is constantly changing during the day), the mass of the earth in proximity to the tube, how quickly the heat from the surrounding earth is conducted to the earth next to the tube, etc.

Since this is a dynamic conjugate heat transfer problem, it is very difficult to predict the exact behavior.  However, if your tube is too short or your air flow is too high, or your ΔT is too great, you could quickly exhaust the thermal difference between the air and the soil near the tube.  As the temperature difference drops, the heat exchange will slow to the point where the earth tube becomes thermally saturated and ineffective.    In a cooling situation, it is possible that by 10 or 11 in the morning, the warm air could increase the temperature of the soil to above the desired room temperature.  For the rest of the day, hot air flowing thru the pipes would just keep increasing the temperature of the soil around the pipes with the theoretical maximum being the outdoor air temperature.  (I plan to use this to my advantage by solar air heating the soil during the summer, see the section on “by-passive” solar).  

In a truly successful earth tube design, the pipes are long enough and the flow speed is low enough that the exchange between the earth and the air happens at a rate the earth around the tubes can absorb.  Since the external air temperatures fluctuate on a daily cycle, it won’t be a truly stable system, but at least it shouldn’t run out of ΔT.  In an even better design, this air flow can be driven by completely passive processes.

In a closed loop system, air is cycled thru the system over and over again.  You don’t get fresh air, but the ΔT is lower, so the earth air exchange system can handle more airflow.

Earth Tube Cost

It is very possible for the install of an earth tube system to cost thousands of dollars.  Many have performed cost benefit analysis and decided against installing them (opting for an air to air heat exchanger instead). I myself have spent many hours agonizing over how to reduce the cost of an earth tube system (while also reducing other risks) in order to make it viable.   Costs can be broken up into 3 main areas; Materials, Installation and Operation/Maintenance.  Of course, the benefits of the system is mostly related to the effectiveness of the system, which is dependent on good design.  Take a look at the Earth Tube Design section of this site.

One way that I planed to reduce the cost of my earth tubes was by reusing the trenches already planned for drainage and sewerage.   Less additional trenching meant less additional cost.  I also needed to compromise on my depth to keep costs down, but I still ended up with depths of up to 15 ft (20′ would be better).

The cost effective depth depends on length of the arm on the excavator, so greater depth may just be a matter hiring an excavator with better equipment.  In my case, the contractor I hired had several large excavators to choose from and came with one that made short work of the trench for the septic line.  It went from the basement of the house (10 ft deep) and cut thru a bit of a hill (up to 15 ft deep) on its way to the septic field nearly 200 ft away (the location at the bottom of the hill was specified by the county health department).  I manually put back a foot or so of earth over the septic lines before placing my double wall earth tubes, that whole manual process took my wife and I most of a Saturday.   When the excavator back filled the hole, he probably took a couple extra hours because he had to be a bit more careful regarding my tubes during the process.

Earth Tubes in hot & humid environments:

If you have 80% humidity at 35°C entering your earth tube, and it cools to 25°C degrees, it will drop a lot of water as condensation on the earth tube walls, but this drop in temperature only increases the level of humidity.  In addition to manageable concerns about where all that condensate goes, the “cooled” air may enter your space at 100% humidity.

This is a real concern and many experts simply recommend against using earth tubes in these environments.

However, I have a two stage arguments for earth tubes in hot and humid environments.

1) The humidity level within the earth tube may increase to 100%, but then it enters the larger habitable space which probably has a higher temperature, so the humidity will automatically drop again. The total moisture in your habitable space will be less than without the earth tubes because some of it was left behind in the tubes.  The cooler your earth/tubes are, relative to your habitable space, the better the natural de-humidification.

2) Assuming the earth is cooler than your habitable space, the earth tubes will remove at least some of the moisture from the air. If you need to remove more with a dehumidification system, you are still better off than without the earth tube. Perhaps the earth tube will enable you to choose a smaller system or use a desiccant wheel or something like that.

After that, it becomes a cost benefit analysis to see if you can put in earth tubes economically enough to make it worth your while.  Often, these hot and humid areas have lower labor rates that make earth tubes more affordable than importing expensive dehumidification equipment.

I am in Michigan, which has cold and relatively dry winters.  However, the summer and fall seasons can be hot and humid.  Hot enough that I also expect my earth tubes to reach 100% humidity and condense out gallons of water per day.  This is why I used smooth wall HDPE earth tubes and sloped them carefully.  It is also why I plan to pass my earth tube air thru an efficient dehumidification system before being distributed thru my home.  My neighbors will simply draw in the hot humid air directly.  The difference will be how much harder their system has to work to dehumidify the air.

Here are some external “earth tube” links…

A Google search for “Earth Tubes” or “Cool Tubes” or “Air Earth Heat Exchangers” or “Ground to Air Heat Exchangers” or a variety of other terms can turn up many more

This is what Wikipedia has to say about it.  One of these days, I should improve their content…

Using earth tubes (which he calls GeoAir Energy), Russel Finch heats his home and green house (he also grows and sells citrus).

Oranges in Nebraska;  “Every time an expert tells me it can’t be done, I sit back, peel one of my oranges, and marvel at my ability to eat something that is impossible to grow in Nebraska.”  ~ Russel Finch  (A very similar article)  (Citrus in the Snow website)

See the link for more images and details of this Earth Tube installation.

This is an “Earth tube Exhibit”  done by the Clean Energy Exhibition.  They talk about a study done on the tubes, but i have not been able to get any data.   Still nice to see the pics of the install.  In this case, they “saved money” by stacking many pipes in a single trench.  Personally, I think this dramatically reduces the effect of the tubes, which is sad considering that digging the trench is probably the lowest cost portion of the project.

REHAU manufactures a system of pipes specifically designed as Earth Tubes (Ground Air Heat Exchangers). These include filters and other specialty sections.

Rehau is known as one of the few commercial Earth tube providers (they call them Ground Air Heat Exchangers and use the brand name “Ecoair”).  This is a link to a story about one of their homes.