Air heat pump vs geothermal energy, new build 400m2

Hello dear forum members,

I have already read quite a bit in the discussion about ground and air heat pumps here, but it mostly involved construction projects in the size range of 150-200 m2. Since we are planning to build somewhat larger, I wonder if that makes a difference in the consideration and have therefore created this thread.

Briefly the most important key data for the construction project:

- Construction project on the outskirts of Berlin
- New single-family house, not KFW 55 (because according to the energy consultant, an additional ventilation system costing over €20,000 would be required, which we do not want)
- Exterior walls aerated concrete 36cm, no extra insulation
- No chimney planned/wished for
- approx. 300m2 heated living area from ground floor to attic (GF, FF, attic)
- approx. 100m2 heated basement area (BS)
- total heated area thus 400m2
- Heating exclusively planned via underfloor heating (BS-attic completely)

We have decided against a gas heating system because although it is currently possibly more economical than a heat pump (of any kind), the idea of sustainability plays a role for us and therefore we are also accepting additional costs (especially investment costs).

For this reason, we have developed the following two variants with our energy consultant:

Always included is a photovoltaic system (nominal output approx. 4.5 kWp) with buffer storage (probably in the range of 7.5 kWh) – the "idea": We produce the electricity for the heat pump ourselves. This will of course not cover the entire demand of the heat pump (see below), but even if we reach 50% on an annual average, that is already something. (If too much electricity is produced in summer, it will of course be sold.)

Variant a) Ground source heat pump: According to the heat load calculation, we need a system with approx. 18 kW output and must carry out boreholes totaling 440m depth in the area. This is divided into 5 boreholes of 88m each.

We have an offer here, but unfortunately individual items are not visible. Without going into too much detail (because the forum is not supposed to judge my offer), the costs for the boreholes, a Vaillant heat pump with 19.7 kW / Cop 4.7 (unfortunately it is not clear under which standard), 300l buffer storage, 500l fresh water preparation + all additional costs amount to €42,500 gross.

Variant b) Air source heat pump: Here, of course, the borehole is omitted. The heat pump is a heat pump from Heliotherm with 18.5 kW / Cop 4.14 (A2/W35) in fully modulating design, connection is hydraulically decoupled, buffer and water storage analogous to a), total costs: €25,000 gross.

(Additionally, about €18,000 for just over 400 m2 underfloor heating costs plus €15,000 for photovoltaic, but these are the same in both variants and so I leave them out.)

So what remains at the end of the day for the heating system:

- Costs for ground source heat pump: €42,500
- Costs for air source heat pump: €25,000
- Difference: ground source is €17,500 more expensive in investment costs
- However, the air heat pump is less efficient than the ground source heat pump, especially in winter when it is needed, so ground source is cheaper in ongoing consumption

Now I have the following questions for the experts:

- Do these considerations make sense overall?
- Is the idea of covering part of the power demand of the heat pump with one’s own photovoltaic system wise?
- From your point of view, is an additional cost effort of €17,500 for the ground source heat pump variant worthwhile (also considering the heat load of approx. 18 kW)?
- What other factors should I pay attention to?

The overall goal is: to realize a sensible but also as sustainable as possible energy system for the future, which of course does not cause unnecessary costs (since unfortunately I have not won the lottery and the money is therefore finite).

Sorry that the post has become somewhat longer, but I wanted to be as precise as possible. If you need more information, please let me know.

Thanks!

*Edit* I just see that you have a heating load calculation... 18 kW?! Wow. We heat 190 sqm with 5.3 kW.

*Edit* I just see that you have a heating load calculation... 18 kW?! Wow. We heat 190 sqm with 5.3 kW.

Thanks for the info. I will check again with the energy consultant on how exactly that comes about. We have planned several floor-to-ceiling window areas - that surely plays a big role.

I suspect the lack of ventilation... at least for 3-4 kW. Otherwise, it fits.
I would still consider the buffer with both concepts.
Otherwise, calculate it... annual performance factor 4.5 for brine-water heat pump and 3.5 for air-water heat pump... then you can draw lines with your house parameters... assuming 20 kWh, you would break even after 57 years... questionable. At 25 kWh in 45 years. If you include the BAFA subsidy, it would be 35 years at 25 kWh accordingly. The drilling lasts longer... nevertheless, I find it difficult with the price difference of the brine-water heat pump.
All assumptions, without considering photovoltaic – only good for a trend.

I also find the heating load quite extreme. We have just under 300m² and are at 7kW. So I would question whether that is correct. Either the wall construction and insulation are rubbish, or someone just roughly estimated without any real basis.

That you rule out gas is not quite understandable to me given the statement "Your money is finite." With that heating load, you will probably end up with about €300-400 in electricity costs for a heat pump during the winter months. Whether it’s worth it...?!
And regarding sustainability: especially in winter, the heat pump turns into a disguised coal heater because nothing comes from the roof... so the sustainability suggested by the green lobby is very questionable.

If a heat pump for your house, then geothermal. However, the extra cost for the drilling is so extreme that you could also take an air-water heat pump... with the money saved you could heat for a very long time.

I would consider a fireplace with which you can add heat in winter, a ventilation system as well, and then rely on an air-water heat pump + photovoltaic. Otherwise, I would choose gas, save money, wait 10-15 years, then get a heat pump of the latest generation. For now, we are foregoing heat pump + photovoltaic, but taking the ventilation system and fireplace in combination with gas. After long (1 year) back and forth consideration and calculations, that was the most economical solution. In 10-15 years we will see further.

Even according to the Energy Saving Ordinance standard, there is already a high heating load, but there could also be a considerable hot water demand, depending on how many people. Otherwise, the rule applies: the higher the demand, the more worthwhile a more expensive heating system that provides heat at a lower cost. By the way, the difference in the annual performance factor between air and geothermal is significantly greater than the assumed 3.5 to 4.5. The posting above me is unfortunately unprofessional, as numbers are thrown around without any comparison or indication of where they come from.

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