Earth Air Heat Exchanger Calculator

Advanced Earth Tube Calculator

Outdoor inlet air temperature, °C

Undisturbed soil temperature, °C

Inlet relative humidity, %

Total airflow, m³/h

Pipe length, m

Inner pipe diameter, m

Pipe wall thickness, mm

Pipe thermal conductivity, W/mK

Soil thermal conductivity, W/mK

Burial depth to pipe center, m

Number of parallel pipes

Total fitting loss coefficient

Fan efficiency, %

Operation hours per day

Air density, kg/m³

Specific heat of air, J/kgK

Formula Used

The calculator uses an exponential heat exchanger model with pipe, air, and soil resistance.

Outlet temperature:

T_out = T_soil + (T_in - T_soil) × e^-NTU

Number of transfer units:

NTU = UA / ṁC_p

Heat transfer:

Q = ṁC_p(T_in - T_out)

Total thermal resistance per pipe:

R = 1 / (hπDL) + ln(D_o/D_i) / (2πk_pipeL) + ln(2z/D_o) / (2πk_soilL)

Pressure drop:

ΔP = [f(L/D) + K] × ρv² / 2

The tool estimates Nusselt number, Reynolds number, friction factor, effectiveness, fan energy, and dew point risk from the entered values.

How to Use This Calculator

Enter the outdoor air temperature and the soil temperature at pipe depth.
Add relative humidity to estimate condensation risk.
Enter airflow, pipe diameter, pipe length, and number of parallel pipes.
Set pipe wall, pipe conductivity, soil conductivity, and burial depth.
Add fitting losses for bends, inlet screens, filters, exits, and transitions.
Press calculate and review outlet temperature, heat transfer, and pressure loss.
Use CSV or PDF download buttons to save the result.

Example Data Table

Case	Inlet °C	Soil °C	Airflow m³/h	Pipe Length m	Diameter m	Expected Use
Summer ventilation	38	23	500	40	0.25	Pre-cooling fresh air
Winter tempering	5	16	350	30	0.20	Pre-warming inlet air
Greenhouse air loop	34	21	700	60	0.30	Daytime heat reduction

Earth Air Heat Exchanger Planning Guide

An earth air heat exchanger uses buried pipes to temper fresh air before it reaches a room, greenhouse, or ventilation unit. The soil stays more stable than outdoor air. In summer, warm air can lose heat to the ground. In winter, cold air can gain heat from it. The useful result depends on pipe length, air speed, soil temperature, depth, and heat transfer resistance.

Why pipe geometry matters

A longer pipe gives air more contact time. A wider pipe lowers pressure loss, but it can reduce internal convection when velocity becomes very low. Parallel pipes can carry more airflow while keeping each pipe slower. Burial depth also matters. Deeper soil changes temperature more slowly, so it often gives steadier performance.

Interpreting the result

The calculator estimates outlet temperature with an exponential heat exchanger model. It also reports heat transfer, pressure drop, fan power, residence time, Reynolds number, and condensation risk. These values help compare early design options. They should not replace local engineering design, drainage planning, filtration, moisture control, or code review.

Good design practice

Use smooth pipe where possible. Keep slopes that allow condensate drainage. Add cleanouts for inspection. Avoid materials that release odors. Provide insect screens and filtration at the inlet. Check radon, flooding, and soil contamination risk before installation. The system should be accessible for cleaning, because damp pipes can support microbial growth.

Improving accuracy

Measure real soil temperature at the planned depth when possible. Use realistic airflow from the fan curve, not only rated airflow. Enter the total equivalent loss coefficient for bends, entrance fittings, filters, and exits. Compare several pipe lengths and diameters. A design with slightly lower heat transfer may be better when it saves fan energy and maintenance effort.

Limits to remember

The ground around a pipe can warm or cool during long operation. Performance may drop if the pipe runs nonstop. Moist climates may create condensate during cooling. Dry climates may show less moisture risk. Insulation near the building entry can reduce unwanted heat gain. The calculator is best used for screening choices, estimating trends, and preparing questions for a qualified designer. Always confirm drainage, access, and local ground conditions before final construction decisions are made.

FAQs

What is an earth air heat exchanger?

It is a buried pipe system that uses stable ground temperature to pre-cool or pre-warm ventilation air before it enters a building or equipment unit.

Can this calculator design a final installation?

No. It supports early design comparison. Final work should include local climate checks, drainage design, soil review, filtration, hygiene planning, and professional approval.

Why does pipe length affect outlet temperature?

Longer pipe gives air more contact time with the pipe wall and surrounding soil. This usually improves temperature approach, but pressure drop also rises.

Why is airflow important?

Higher airflow carries more heat, but it moves faster through the pipe. That can reduce residence time and increase pressure loss and fan energy.

What does condensation risk mean?

Condensation risk appears when estimated outlet temperature is below the inlet dew point. Drainage, slope, cleanouts, and safe materials become very important.

Should I use one pipe or several pipes?

Parallel pipes can reduce velocity and pressure drop while keeping total airflow high. They may need more space, better balancing, and more installation work.

What soil temperature should I enter?

Use measured soil temperature at the planned burial depth. If unavailable, use a conservative seasonal estimate for the expected operating period.

Why is fan power included?

Fan power shows the energy needed to push air through the buried pipe system. A design with strong heat transfer may still perform poorly if pressure loss is excessive.