Earth Tube Cooling Length Calculator

Cooling with the earth

Earth tubes, also known as ground‑coupled air exchangers, are passive ventilation components that route incoming air through a buried pipe before it enters a building. The surrounding soil, with its relatively stable temperature, acts as a heat sink or source depending on season. During hot weather, warm outdoor air loses heat to the cooler ground, arriving indoors at a reduced temperature and easing the load on mechanical cooling systems. The calculator above employs a simplified convective heat transfer model to estimate the pipe length required to achieve a target outlet temperature. By specifying inlet air temperature, average ground temperature at depth, desired outlet temperature, pipe diameter, and airflow rate, users can approximate how much buried length is necessary for effective tempering.

The underlying physics treats the pipe as a circular conduit with air flowing at a steady rate. Heat transfer occurs from the air to the pipe wall and onward into the soil. For design simplicity, the calculator assumes a constant overall heat transfer coefficient $h$ of 10 W/m²·K, representing combined convective and conductive effects. The temperature change along the tube follows an exponential decay relationship derived from Newton’s law of cooling: , where $\mathrm{T\_\{in\}}$ and $\mathrm{T\_\{out\}}$ are inlet and outlet air temperatures, $\mathrm{T\_g}$ is ground temperature, $D$ is pipe diameter, $L$ is length, $\mathrm{m\_dot}$ is mass flow rate, and $\mathrm{c\_p}$ is the specific heat of air. Solving for length gives $L=\frac{\mathrm{m\_dot}\times \mathrm{c\_p}}{h\times \pi \times D}\times -\ln \left(\frac{\mathrm{T\_\{out\}}-\mathrm{T\_g}}{\mathrm{T\_\{in\}}-\mathrm{T\_g}}\right)$.

Mass flow rate is determined from the volumetric flow and air density, assumed here to be 1.2 kg/m³. With diameter known, the surface area per unit length is simply the circumference, $\mathrm{A\_L}=\pi \times D$. The exponential term reflects diminishing returns: as the outlet temperature approaches the ground temperature, additional length yields progressively smaller cooling increments. This highlights the importance of realistic targets; no finite pipe can reduce the air temperature below the surrounding soil temperature. Likewise, if the desired outlet is only slightly cooler than the inlet, the required length drops sharply.

The table below presents typical average ground temperatures at 2 meter depth for various climates. These values, which lag seasonal air temperature swings, inform the $\mathrm{T\_g}$ input.

Suppose outdoor air at 32 °C is drawn through a 0.15 m diameter tube, with soil at 15 °C. The goal is to deliver air at 22 °C to the building at a flow rate of 150 m³/h. The mass flow rate is $150\times 1.2/3600=0.05$ kg/s. Substituting into the length formula yields $L=\frac{0.05\times 1005}{10\times \pi \times 0.15}\times -\ln \left(\frac{22-15}{32-15}\right)$, which evaluates to roughly 23 meters. This suggests that burying about 23 meters of 15 cm pipe at sufficient depth could cool the incoming air by 10 °C under the stated conditions.

Designers must also consider condensation within the pipe. When warm, humid air contacts the cool tube walls, water can condense, potentially leading to mold growth or drainage issues. Slope the tube slightly to a drain and use smooth interior surfaces, such as PVC, to facilitate cleaning. Intake openings should be screened to prevent debris or animals from entering. Periodic disinfection may be necessary in humid climates.

Pressure drop is another factor. Longer tubes and smaller diameters increase friction losses, requiring more fan power to maintain airflow. This calculator does not account for friction; users may need to balance cooling benefits with energy consumed by ventilation fans. Multiple parallel tubes can reduce velocity and pressure drop while increasing surface area.

Ground temperature stability depends on depth and soil composition. Sandy soils conduct heat less effectively than moist clay, potentially reducing performance. In regions with permafrost or high water tables, earth tubes may be impractical. Likewise, in tropical climates where ground temperatures remain warm year‑round, cooling potential is limited, though the same system could provide winter heating in cooler climates by warming incoming air.

Maintenance access should be included in designs. Cleanouts at bends or near the building allow inspection and removal of debris. Because the tubes are buried, retrofitting can be difficult, so careful planning of route and length is vital. Combining earth tubes with mechanical ventilation systems can provide hybrid solutions that leverage passive cooling while ensuring air quality through filtration and controlled flow.

In conclusion, the earth tube cooling length calculator offers a preliminary estimate for enthusiasts exploring passive geothermal ventilation. By understanding the interplay between airflow, pipe geometry, and ground temperature, users can gauge the feasibility of earth tubes for their climate and building size. Though simplified, the model underscores that meaningful cooling requires significant length and careful attention to moisture and maintenance. When implemented thoughtfully, earth tubes can enhance comfort and reduce reliance on energy‑intensive air conditioning, embodying a low‑tech approach to sustainable building design.