Interpretation of Earth Drilling Experiences

Is my assumption even correct that a borehole with 120m would be cheaper than 2 boreholes with 60m each?
(I am aware that you probably can't simply calculate this by looking at the performance of the ground probe).

Are 2 boreholes advantageous in any way?

With 2x60m you will not need 1x120m, but less, because the extraction performance per meter increases with depth.

But yes, a single borehole with the same total depth is cheaper to install. The drilling rig does not need to be repositioned, the entire work process is carried out only once and not twice. You save space on the property, have a direct and shorter pipe run, and do not need an inspection shaft where the two probes merge.

Two boreholes have an advantage over one borehole in terms of reliability. In terms of efficiency, one deeper borehole is more advantageous than two shallower boreholes. I cannot confirm the blanket statement that a deeper borehole is always cheaper to produce based on my experience. It depends on the individual ground conditions underground. For example, if there is rocky ground in deeper layers.

Regarding the questions in the initial post: heat load and the annual heating energy demand including hot water preparation are different things. However, it is basically true that a buffer is already included in these calculations. Therefore, the heat generator (heat pump) does not need to be additionally oversized in terms of good efficiency; this is detrimental to efficiency.

An oversizing of the heat source (= deeper borehole), on the other hand, increases efficiency by reducing the operating costs of the heat pump. Whether this gain justifies the higher drilling costs is not so easy to calculate and must be decided individually. But basically, a few more meters of drilling is a good thing.

Yes, exactly. Just because you can drill 60m deep, it doesn’t necessarily mean that you can also reach 100m or 200m deep. With every meter, the risk of hitting rock increases.

Are there any experiences from your area?
Why does the company want to drill 2x60 and not 1x100m? Have they perhaps had bad experiences with deeper drillings in your vicinity or know that it simply isn’t possible?

Just because it worked at X and Y doesn’t automatically mean it will work at Z...