Air-water and geothermal water-water heat pumps both support low temperature heating, but they are not the same project. Air-water is easier to deploy. Geothermal is more stable and longer term, but only when land, access and administrative context allow it. In Belgium, feasibility often decides before preference.
COP and winter behavior
An air-water heat pump uses outside air. In mild weather, performance is strong. During cold spells, the COP drops and defrost cycles may appear. A good design accounts for that instead of pretending the rated COP applies every day.
A geothermal water-water heat pump uses a more stable source. Vertical probes access ground temperatures that move less than air. That can deliver higher seasonal performance and better winter stability.
The emitters still matter. Underfloor heating or low temperature radiators help both systems. Old high temperature radiators can reduce performance, especially for air-water.
Drilling and land
Air-water needs an outdoor unit, hydraulic integration, noise management and sizing. It is technical, but it avoids boreholes.
Geothermal needs drilling access, vertical probes and checks around soil and groundwater. In Wallonia, environmental rules and local constraints must be reviewed. This can stop a project even when the owner likes the idea.
The land question is therefore practical. A dense urban plot, narrow access or protected underground context can make geothermal unrealistic. A large property with long term ownership can make it attractive.
Grants and budget
Wallonia supports eligible heating heat pumps through a base amount multiplied by income category. The base grant alone should not be used to sell geothermal as automatically better. The investment and drilling work must be justified by long term performance.
Air-water usually wins on initial accessibility. Geothermal can win over a longer horizon, especially for larger homes and stable heat demand. VAT rules may also affect the project, but they do not remove the need for a feasibility study.
EcoChaleur approach
EcoChaleur starts with the building: insulation, heat emitters, domestic hot water, electrical capacity, outside unit constraints and land. Only then does the air-water versus geothermal choice become clear.
For many homeowners, air-water is the balanced route. For the right property, geothermal can be excellent. The honest answer is not a universal winner, but a match between technology and site conditions.