April 6 2008

Earth Tubes

This ultra-low tech energy saving system first caught Greenline's attention when a mechanical engineer proposed one for a Ziger/Snead building.

With contemporary buildings so well sealed from the outside, indoor air quality suffers from a lack of fresh air. As a result, national building codes began to require that hvac systems bring in a minimum amount of outside air to maintain air quality in a building. Newer codes have increased this ventilation requirement and some in the building industry promote even higher ventilation standards, some as high as 100%.

The trick then becomes how to bring in outside air without expending enormous amounts of energy bringing it up (or down) to indoor air temperature. One primary methodology is to transfer heat from outgoing exhaust air to incoming ventilation air in the winter or vice versa in the summer. (See Jonas Risen's recent post on Energy Recovery Ventilation for an excellent primer on the topic. )

An earth tube is system is generally meant as a supplement to energy recovery ventilation in high performance buildings. The basic idea is to run incoming air through a tube in the ground, taking advantage of the earth's relatively constant temperature to pre-cool or pre-heat the air before it enters the building's ventilation system.

In practice, things are a bit more complicated. Ductwork or pipes rated for underground use are employed, the ducts must slope to drain to avoid water and mildew build-up and additional filtration is sometimes required to eliminate mold and bacteria.

The Aldo Leopold Legacy Center, a LEED Platinum building in Baraboo, Wisconsin employed a fairly large earth tube system are part of a comprehensive energy efficiency strategy. Their system includes over 600 linear feet of 24" diameter concrete pipe. Here it is under construction:

aldo leopold center earth tubes

We like the simplicity of the earth tube, but as with any hvac component, the proof is in the successful execution of an idea. We'll post a more detailed update if our project moves forward with it's own earth tube system so stay tuned!

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November 19th, 2008 at 3:13 PM

Mark Clareza

A question in mind came up. Why not use water as cooling medium to run through the pipes? Would it perform better than air? Thank you for your consideration.

January 26th, 2009 at 3:48 AM


Mark- The earth tubes essentially precondition the incoming ventilation air to take the edge off what the air conditioner has to do... Using water is a great idea but then it is really more of a geoheatexchanging system using tempered water to cool the incoming air... the incoming air is then still at a greater delta to the inside air and more energy is needed to cool/heat it... In a recent project my firm decided to forgo the earth tubes and use an ERV instead since the results were predictable and the system less costly for our application.

July 13th, 2009 at 2:43 PM

alex varsany

Seems to me, from the photo, that air is entering in the middle of the five-tube assembly and exiting at left: consequently more air is going to flow thru the left side.(Right way: middle in -- left and right out; corner in -- opposite corner out.)