Understanding the earth-air heat exchanger: principle, benefits and limits in renovation
How the air/ground exchange works in shallow geothermal systems
The earth-air heat exchanger circulates fresh air through a buried duct. At 1.5 to 2 m deep, the ground stays more stable than outdoor air. In winter, the air warms up as it passes underground. In summer, it cools down. The gain depends on the length, the airflow, the type of soil and the quality of the installation.
What the earth-air heat exchanger really changes for ventilation and comfort
Its main benefit is pre-treating incoming air. It can smooth out cold and heat peaks, and limit the draughts linked to fresh air intake. It doesn't replace mechanical ventilation. It's more of an add-on to a ventilation system (single-flow or heat-recovery) and is all the more relevant if the envelope is well insulated.
Cases to avoid: moisture, unfavourable soil, unsuitable uses
Three contexts rule the project out before any calculation.
To go further on this principle, see preheating air using the ground.
Sizing an earth-air heat exchanger: the parameters that drive performance
Airflow and needs: starting from the ventilation system (single-flow or heat-recovery)
An earth-air heat exchanger is sized first on the actual airflow of your installation. With single-flow ventilation, you aim to preheat the incoming fresh air. With heat-recovery ventilation, you're rather looking to relieve the heat exchanger and limit cold peaks. Base your sizing on the regulatory flow rates and the ventilation system's operating flow rates, then check that pressure losses stay compatible with the fan.
Length, diameter, depth: practical benchmarks and common mistakes
Performance comes from a good trade-off, parameter by parameter.
| Parameter | The trade-off to hold |
|---|---|
| Length | too much length increases pressure losses and complicates maintenance |
| Diameter | too small a diameter speeds up the air, causes noise and encourages condensate |
| Depth | bury the ducts where soil temperature is more stable |
| Bends | avoid tight bends |
| Condensate | plan for a slope, drainage and an access hatch for cleaning |
Sizing
Size your earth-air heat exchanger
Set the airflow, the length and the depth: the supply temperature, the pressure losses and the condensate follow.
250 m³/h
40 m
200 mm
2.0 m
1
Supply air
23.8 °C
8.2 °C cooler than outside
Exchange effectiveness
51 %
Exchanged power
690 W
Air velocity
2.2 m/s
Pressure losses
22 Pa
Condensate
6.9 L/day
Annual preheating
784 kWh/yr
2.2 m/s
The velocity stays in the comfortable band, between 1 and 3 m/s.
6.9 L a day
Water to drain away in muggy weather. A continuous slope, a low point and a collection chamber are not optional.
€2,900 to €4,800
Teaching model: steady state, homogeneous soil, no bypass and no recovery from the ventilation unit. The ground cools down around the duct over the season, so the annual figure shown here is optimistic. A contractual quote needs a soil survey and a full airflow calculation.
An Argile tool
Soil type, temperature, moisture: what you need to check on site
The soil does the work. Wet, dense ground exchanges better than dry, sandy soil. Record the soil type, the presence of water, flood risk and the possibility of draining moisture away. Also check the network's air-tightness and the local radon risk to avoid any stray suction.
Design and installation: key points for a clean job site
Layout, slope and drainage: managing condensate without bad surprises
For an earth-air heat exchanger, the layout must stay simple and accessible. Aim for a continuous slope towards a low point. This avoids water pockets and odours. Plan a collection chamber, a trap and perimeter drainage if the ground is wet. Keep bend radii wide to limit pressure losses.
Choice of ducts, chambers and filters: durability, maintenance, hygiene
Choose ducts with a smooth inner wall, suited to contact with air and buried environments. Limit the number of joints, and place an inspection chamber at each change of direction. On the air intake side, install an easily replaceable prefilter, then a finer filter upstream of the ventilation system. Also plan for a cleaning system.
Air-tightness and connections: interface with the building's ventilation
Performance often comes down to careful air-tightness. Take care with seals, sleeves and wall penetrations. Plan a bypass and dampers to manage the seasons and avoid air backflow. The connection to the mechanical or heat-recovery ventilation system must remain removable, with access to the filters, without disrupting flow balancing.
Field feedback: what works in 2026 and what disappoints
Measured gains in summer and winter: realistic expectations by region
On site, the best feedback comes when you mainly target summer comfort. In an oceanic or Mediterranean climate, a few degrees of cooling at the supply point are often observed if the flow rate is well set and the bypass works. In a continental climate, the effect is good in mid-season, then drops if the soil "recharges" poorly. In winter, expect modest preheating. It's useful, not miraculous. The earth-air heat exchanger performs best alongside a well-balanced ventilation system.
The most frequent failures and callbacks: odours, condensation, fouling
Callbacks come back to three issues, each with its own origin.
| Symptom | Most frequent origin |
|---|---|
| Persistent odours | a poorly placed air intake, or a trap that loses its water seal |
| Condensation | the slope, drainage or insulation of cold sections overlooked |
| Fouling | light filtration |
What works is simple access to inspection points and planned maintenance.
Job-site feedback: single-family homes, deep renovation, small commercial buildings
For single-family homes, it works well if the earthworks are planned for and if air-tightness isn't full of holes. In deep renovation, it disappoints when the network is "squeezed in" too hastily, or on wet ground. In small commercial buildings, it's relevant for stabilising ventilation, provided there's flow-rate monitoring and contracted maintenance.
Pricing, regulations and grants: fitting the earth-air heat exchanger into your offer
Cost items and installation time: earthworks, networks, associated ventilation
An earth-air heat exchanger is priced mainly on three lines: earthworks and removal of spoil. Supply and installation of the buried ducts, chambers, condensate trap, air intakes. And the associated ventilation, often a mechanical ventilation unit or an insufflation box with filters. For a house, generally allow 1 to 2 days with an excavator, then 1 day for connections and adjustments.
Useful rules and documents: soil survey, ventilation, hygiene good practice
Before digging, request a soil survey or at least a serious site assessment. The rest fits into a list you run through from earthworks to handover.
- Frost-free depth, water table, radon, pollution, easements, it all matters.
- An accessible, filtered air intake.
- Cleaning, access to chambers and condensate management planned for.
- At handover, a sizing note, an as-built plan and a commissioning report.
2026 grants: how to position the earth-air heat exchanger within a global renovation (MaPrimeRénov', CEE)
In 2026, position the earth-air heat exchanger as an added comfort and efficiency feature within a global renovation pathway. Grants are secured via the eligible work items (insulation, high-performance ventilation, heat pump). On the CEE side, there's generally no dedicated standardised operation. So sell it as a priced option, and anchor the file on the subsidised works and the audit.
