Earth Tube Cooling Length Calculator
Model the length of an underground pipe required to temper ventilation air by exchanging heat with the soil.
Introduction to earth tubes and ground-coupled air cooling
Earth tubes (ground-coupled air heat exchangers) temper incoming ventilation air by running it through a buried pipe. Because soil temperature a few meters below grade changes slowly compared with outdoor air, the ground can act as a seasonal heat sink (summer cooling) or heat source (winter preheating). This calculator estimates the tube length needed to cool air from an inlet temperature to a desired outlet temperature using a simplified exponential heat-transfer model.
The idea is old — Persian wind towers and Roman hypocaust channels exploited ground coupling centuries before mechanical refrigeration — but it fits modern low-energy buildings well. A tube buried 2 m deep in a temperate climate sees soil near the annual average air temperature, often 10–14 °C, while a summer design day may bring 30–35 °C outdoor air. Passing ventilation air through that soil before it reaches the heat-recovery unit or supply grille can shave several kilowatt-hours per day off cooling loads, protect heat-recovery cores from frost in winter, and do it all with no compressor, only fan power.
What this calculator is doing (conceptually)
As air flows through a long pipe, it exchanges heat with the surroundings. The temperature difference between the air and the ground is largest near the inlet and shrinks along the pipe. That produces diminishing returns: every extra meter helps less than the previous meter as the outlet temperature approaches the ground temperature. In an idealized steady-state model, the air temperature approaches (but never goes below) the ground temperature.
Governing relationship and the length formula
With a constant overall heat transfer coefficient and constant properties, the outlet temperature can be modeled as an exponential approach to the ground temperature:
Solving for required length L (when the target outlet temperature is above the ground temperature) gives:
Plain-text formula: L = (ṁ · cp) / (h · π · D) · ln( (Tin − Tg) / (Tout − Tg) ); with ṁ = ρ · Q, Q = (flow in m³/h) ÷ 3600, ρ = 1.2 kg/m³, cp = 1005 J/(kg·K); air velocity v = Q ÷ (π · D² ÷ 4); sensible cooling power = ṁ · cp · (Tin − Tout).
Source/version metadata: exponential-approach (NTU-style) duct model per standard heat-exchanger theory as applied to earth-to-air heat exchangers in the engineering literature; default overall coefficient h = 10 W/(m²·K) is a screening value that lumps in-tube convection with pipe-wall and near-soil conduction; air properties at ~20 °C. Last reviewed July 2026. For design-stage work validate against site soil data and a transient tool.
Variable definitions
- Tin: inlet air temperature (°C)
- Tg: ground (soil) temperature at burial depth (°C)
- Tout: target outlet air temperature (°C)
- D: pipe inside diameter (m)
- L: pipe length (m)
- h: overall heat transfer coefficient (W/m²·K), treated as a constant here
- ṁ: mass flow rate of air (kg/s)
- cp: specific heat of air (J/kg·K)
How mass flow is obtained from your input
The calculator converts volumetric flow rate to mass flow rate using an assumed air density:
ṁ = ρ · Q, where Q = (flow in m³/h) / 3600 and ρ ≈ 1.2 kg/m³.
Interpreting the result
- Longer length pushes the outlet temperature closer to the ground temperature, but the improvement per meter decreases.
- Higher airflow increases required length because more heat must be removed per second (higher ṁ).
- Larger diameter increases surface area per meter (
π·D) and tends to reduce required length in this model (all else equal). Real systems may differ because diameter also affects velocity and convection. - Targets near Tg require disproportionately long tubes. If you want air within a degree or two of the ground temperature, expect the length to grow rapidly.
How to use this earth tube length calculator
Enter the inlet air temperature you are designing against (a hot-afternoon design value, not the annual average), the ground temperature at your intended burial depth, and the target outlet temperature you want delivered to the building or to the downstream ventilation unit. Then describe the pipe: inside diameter in meters, the airflow the fan will move in m³/h, and, if you want to explore sensitivity, the overall heat transfer coefficient h (leave the default of 10 W/m²·K for a first screening pass).
The result panel reports the required length along with three sanity checks: the air velocity in the pipe (aim for roughly 2–4 m/s in residential work), the mass flow the model used, and the sensible cooling power the tube would deliver at the target outlet. If the velocity lands outside the comfortable band, resize the diameter before trusting the length: a 5 m/s tube may need less length on paper but will pay for it in fan energy and noise every hour it runs. Then stress-test the design by rerunning with the ground 2 °C warmer (late-summer soil) and the flow 25% higher; if the length explodes, your target outlet is too close to the ground temperature.
Worked example: cooling 32 °C air to 22 °C with 15 °C soil
Given: Tin = 32°C, Tg = 15°C, Tout = 22°C, D = 0.15 m, flow = 150 m³/h, h = 10 W/m²·K.
- Convert flow to m³/s:
Q = 150 / 3600 = 0.0417 m³/s - Mass flow:
ṁ = ρ·Q = 1.2·0.0417 ≈ 0.050 kg/s - Air velocity:
v = Q / (π·D²/4) = 0.0417 / 0.01767 ≈ 2.4 m/s— inside the 2–4 m/s comfort band. - Compute the log term:
(Tout − Tg) / (Tin − Tg) = (22−15)/(32−15) = 7/17 ≈ 0.4118-ln(0.4118) ≈ 0.887
- Using
h = 10 W/m²·Kandcp ≈ 1005 J/kg·K:(ṁ·cp)/(h·π·D) = (0.050·1005)/(10·π·0.15) ≈ 10.7L = 10.7 · 0.887 ≈ 9.5 m
- Sensible cooling delivered at the target:
ṁ·cp·(Tin − Tout) = 0.050·1005·10 ≈ 503 W.
This indicates that, under the model assumptions, roughly 10 meters of 150 mm tube could cool 32°C air down to about 22°C when the ground is 15°C at the tube depth, delivering about half a kilowatt of sensible cooling. Enter those defaults above and the calculator reproduces each figure. Notice the leverage in the last degree: asking for 17 °C instead of 22 °C (2 °C above the soil) raises the log term from 0.887 to 2.14 and the length from 9.5 m to about 23 m — two and a half times the trench for five more degrees.
Typical ground temperatures (rough guide)
Ground temperature depends strongly on depth, moisture, shading, and seasonal history. The values below are only broad, order-of-magnitude guides for “a few meters depth” in different climates.
| Climate zone | Typical average ground temperature (°C) | Design note |
|---|---|---|
| Cold continental | ~5 | Large seasonal swing; verify depth/insulation and frost effects |
| Temperate | ~10 | Often favorable for summer cooling with modest lengths |
| Subtropical | ~18 | Cooling potential depends on peak outdoor temperatures and humidity |
| Tropical | ~24 | Limited cooling if the ground is warm; moisture/condensation matters |
Assumptions & limitations (important)
- Constant overall heat transfer coefficient: The model lumps internal convection, pipe conduction, and soil conduction into one adjustable
h(default10 W/m²·K) that stays constant along the tube. In reality, h varies with air velocity, pipe roughness, pipe material/thickness, soil type, and soil moisture; values quoted for earth tubes commonly span roughly 5–20 W/m²·K. - Constant ground temperature: The soil is treated as an infinite reservoir at a constant
Tg. Real soil temperature changes with depth, season, rainfall, and prolonged operation (thermal saturation around the pipe). - Steady-state, 1-D model: Startup transients, daily cycling, bends/manifolds, and entrance effects are ignored.
- Air properties fixed: Air density and
cpare assumed constant (reasonable for quick screening, but not exact across wide temperature/humidity ranges). - Condensation and latent heat not modeled: If humid air is cooled below its dew point inside the tube, condensation can occur, affecting heat transfer, pressure drop, drainage requirements, and hygiene/maintenance considerations.
- No pressure-drop / fan power check: Required fan energy and acceptable pressure losses are not evaluated here, yet they often constrain practical tube diameter and length.
- Target constraints: Cooling below the ground temperature is not physically achievable with this passive model. If
Tout ≤ Tg, the logarithm term becomes invalid (or implies infinite length).
Use this calculator for early-stage sizing and comparisons (e.g., “How does required length change if airflow doubles?”). For design-ready decisions, use site-specific soil data and a model that accounts for moisture, seasonal variation, condensation, and pressure drop—or consult a qualified HVAC professional.
Earth tube design questions, answered
How long should an earth tube be for effective cooling?
For residential airflows of 100 to 300 m³/h, useful designs typically land between 15 and 50 meters of buried pipe. The exponential model shows diminishing returns: the first meters do most of the cooling, and chasing the last degree toward ground temperature can double the length. Size for a target 2 to 4 °C above the ground temperature rather than for the ground temperature itself.
How deep should earth tubes be buried?
Most designs bury tubes 1.5 to 3 meters deep. Below about 2 meters the soil temperature swing is a fraction of the surface swing and lags the seasons by one to two months, which is exactly what makes summer cooling work. Greater depth improves stability but raises excavation cost and complicates the drainage slope.
What pipe diameter and air velocity work best?
Common residential earth tubes use 150 to 250 mm pipe with air velocities of roughly 2 to 4 m/s. Lower velocities improve heat exchange per meter but need larger pipe for the same airflow; higher velocities raise pressure drop, fan energy, and noise. This calculator reports the velocity implied by your flow and diameter so you can stay in that band.
Can an earth tube cool air below the ground temperature?
No. The ground temperature is the physical floor for a passive earth tube: the exponential model approaches it asymptotically and never crosses it. If you need supply air cooler than the soil, add mechanical cooling downstream; the earth tube then acts as a pre-cooler that shrinks the mechanical load.
What about condensation inside earth tubes?
When humid summer air is cooled below its dew point inside the tube, water condenses on the pipe wall. Designs need a continuous slope of about 2 percent to a drainage point, smooth pipe with sealed joints, and access for inspection, because standing condensate can create hygiene problems. This simple model does not include the latent heat released by condensation, which reduces the sensible cooling actually delivered.
Ground Drift Mini-Game
Guide air through winding earth tubes for 80 seconds. Keep outlet temperature in the green cooling band while pressure pulses and hot gusts strike.
Controls: drag/tap to tune flow split. Keyboard fallback: A/D or ←/→.