Geothermal energy: pros and cons?!

100 m drilling is already quite deep, at least if you do it 3 times. For us, the heat pump was designed for 9.0 kW and due to predominantly clayey soil with rather low extraction capacity, this recommendation was made. The "cheapest" offer was, as far as I remember correctly, around 18 k something.

In the meantime, we have a gas condensing boiler + [Solarthermie] and are (at least at the moment) very satisfied with our decision.

was not a criticism of your post. At this price, we would have also passed.

Since the price for geothermal energy is often a main argument against it, I wanted to post our numbers here so that users can see that it can vary.
At first, we were considering geothermal energy, but it was already ruled out before the planning stage and we considered the gas condensing boiler + solar instead. Precisely with the argument of costs.
After our general contractor quoted that geothermal energy all in (with the Vaillant pump) would cost between 13,000 and 15,000 EUR, it was clear to us that we would go with geothermal energy after all.

By the way, my sister has also been satisfied with her gas heating + solar for 14 years.

I did not take that as criticism either. I actually just wanted to express that good planning is important.

The problem is that with many GCs/developers... (certainly not all) there is a standardized procedure. For example, there is the heat pump with an output of 8 kW and a drilling depth of over 50 m (just as an example). No attention is paid to who and how many people live in the house, whether it is 100 or 200 m² of living space, or whether one person likes to sit in the living room at 18°C with a sweater and another at 24°C in a T-shirt. And it is the same with the drilling. Whether clay, sand, or rock is present as the substrate does not interest most people, but that different extraction capacities result from this and therefore planning has to be different is irrelevant. Therefore, everyone is well advised to get an independent specialist to clarify exactly these things and to find the heating option that makes the most sense for them personally.

We ourselves also know two negative examples. There, the heat pump and the drilling were incorrectly sized, and this is now reflected in electricity consumption.

I know what you mean.
For us, the forecast from the GU (drilling depth and associated price) was increased again, so the drilling became about 1,500 euros more expensive - the kW was increased accordingly - the next larger model of the pump was ordered.
Here, the calculation showed that more power is needed for the square meters of living space.
Still very annoying, since we had only planned the (lower) costs in our financing. But we then considered this as an unavoidable cost increase and covered it from our reserve (as well as the additional costs for plumbing, electrical, doors).
Regardless, the drilling was carried out by an external company specialized in this (they seem to be primarily used in our area for geothermal drilling - they also drilled for my colleague - another GU).

And by the way, the sun does not cool down, but explodes in about 5 billion years. Before that, it expands so much that it swallows all the inner planets, including the Earth.

I think that should not go unmentioned in this thread.

Hello.... We are looking for geothermal drilling companies for our Vaillant heat pump vwf 88/4.
What were your offers?