Air or geothermal energy? The decision has four levels

Everyone knows the short version: geothermal energy works more efficiently, air is cheaper to buy. That is true — but it does not help much with the decision, because both are already clear before you read the first quote.

More interesting are the four levels on which the two types actually differ. Two of them are regularly overlooked.

1. The heat source determines the effort

An air source heat pump takes heat from the outside air. It needs an installation location and electricity, nothing else. That is why it is installed faster and is cheaper — and why it is the standard case in Austria.

A brine/water heat pump (geothermal) extracts heat from the ground, via a deep borehole or surface collector. A water/water heat pump uses groundwater. Both require development — drilling equipment, excavations, space.

The advantage: the source provides a much more consistent temperature in winter than the outside air. Exactly when heating is needed, the gap to the desired flow temperature is smaller. This is the reason for the better efficiency — not a better machine, but a better starting position.

2. The permit — this is where it gets formal

This is the point that is missing in price comparisons and can cost weeks.

The Vorarlberg guideline explicitly separates the two cases:

• Brine/water and water/water: "The required water law permit must be obtained. The responsible authority is the respective Bezirkshauptmannschaft (district authority)."
• Air/water: "The required building law permit must be obtained. The responsible authority is the respective Gemeinde (municipality)."

Two different authorities, two different procedures. Water law is the more complex route — and drilling cannot begin before it is completed. If you are planning geothermal energy, you should put this step at the beginning of your schedule, not at the end.

Even with air source heat pumps, the permit is not a sure thing: the neighbourhood becomes an issue in the procedure, and then what matters is what arrives at the property line.

3. The conditions for public subsidies differ

This is also rarely considered: some federal states place stricter requirements on air source heat pumps.

Tyrol staggers the permitted design temperature by type — a maximum of 55 °C for water/water and brine/water heat pumps, but only 50 °C for air/water heat pumps. So if you have a house with a high flow temperature in Tyrol, you get five degrees further with geothermal energy before eligibility for a public grant ends.

That can change the calculation: if your house is just above the air limit, the more expensive geothermal energy might be the only eligible option for a public subsidy — or you lower the flow temperature, see the article about the flow temperature.

4. The installation location and the neighbours

An air source heat pump has a fan and is usually located outside. That is its biggest practical disadvantage — not because of the noise volume itself, but because the installation location determines the sound pressure at the property line. Details on this in the article about sound values.

A geothermal heat pump is located entirely inside the house. Nothing penetrates outside, there is no outdoor unit and no defrosting operation in winter. In densely built-up areas, this is sometimes the deciding argument — regardless of any efficiency calculation.

When which type makes sense

In favour of air: limited budget, no space for a borehole or collector, moderate temperature level in the house, sufficient distance to neighbours, quick appointment needed.

In favour of geothermal energy: high heat demand over many years, planned long service life, cramped neighbourhood, a house with a higher flow temperature in a federal state with staggered limits — and a property that allows development in the first place.

The honest interim conclusion: In most Austrian single-family homes, the air source heat pump wins because the surcharge for geothermal energy is rarely recovered over its lifespan. However, the exceptions are not edge cases — and they do not depend on the device, but on the property, the neighbourhood and the federal state.

What you should clarify before comparing

1. Is geothermal energy even possible on your property? Space, access for the drilling rig, protected areas.
2. Which permit does which variant need — and how long does it take here? A call to the Gemeinde (municipality) or Bezirkshauptmannschaft (district authority).
3. What design temperature does your house need? It helps determine whether both variants are eligible for a public grant.
4. Get quotes for both variants if the property allows it. Only then are you comparing real numbers instead of rules of thumb.