ᐅ Gas with solar thermal or heat pump? And possibly photovoltaic panels as well?

Hello everyone,

We are a family of five building a solid-constructed house in southern Germany (160 sqm (1722 sq ft) living space + basement). Now we need to decide on the heating/hot water system. Since the question mainly concerns solar/photovoltaics, I hope this is the right place to ask.

We have chosen underfloor heating with gas. To meet the energy-saving regulations, an additional renewable component is required, such as a combination with solar thermal.

After consulting with the house provider, we have three options. Since I am unsure, I would be very grateful for some feedback on our situation.

Option 1:
Gas + solar thermal
Total cost: approx. €10,000
Payback period for solar thermal: savings of about €300 heating costs per year. According to the house provider, the solar system pays off in 15-20 years.
This seemed to me like a worry-free solution that works quite reliably.

Option 2:
Gas (only for underfloor heating) + separate domestic hot water heat pump Brötje BTW 300 (to meet energy-saving regulations instead of solar thermal)
The domestic hot water heat pump is installed in the basement equipment room and is intended to warm the domestic water using the ambient air in the basement.
Total cost: approx. €6,000
I like the low total cost. It is clear that we will have a higher electricity bill due to the heat pump. I wonder if the system produces enough domestic hot water for five people, especially if several shower one after another.

Option 3:
Gas + domestic hot water heat pump (like option 2)
+ photovoltaic system to partially power the heat pump with generated electricity
Total cost: approx. €15,000
Payback period for photovoltaic system: savings/feed-in tariff about €1,000 per year. According to the house provider, it pays off in 8-10 years.
The downside is of course the higher total cost. However, if you really "earn" or save €1,000 per year, that would be acceptable for me.

Question 1: Domestic hot water heat pump Brötje BTW 300
Does anyone have experience with this heat pump or similar systems? Does it produce enough hot water for five people? Are these systems mature and reliable?

Question 2: Photovoltaic system
So far, I haven’t really dealt with this topic because I thought it no longer made sense with the low feed-in tariffs. I do not want a battery. If any, just a photovoltaic system on the roof where part of the electricity is consumed directly and the rest is fed into the grid.
Would you currently still plan to install a photovoltaic system?

Question 3: Which option would you prefer?
We would like a system that requires as little maintenance as possible, is reliable, uncomplicated, and affordable.

I am very grateful for any feedback.

Best regards

Hausbau129 schrieb:

Savings of about €300 in heating costs per year

That is probably not accurate. The total annual gas consumption usually amounts to around €600-700, of which 20-25% is used for domestic hot water heating. Solar thermal systems can save you about 50% of that. Calculate it yourself.

Hausbau129 schrieb:

Option 2:
Gas (only for underfloor heating) + separate domestic hot water heat pump Brötje BTW 300 (to comply with energy saving regulations instead of solar thermal)

I’m surprised that this would be permitted. The renewable energy share must contribute to heating the building. This is not the case with a separate domestic hot water heating system. The low initial costs of this solution also seem questionable to me. Are you sure you understood that correctly?

Hausbau129 schrieb:

Savings/feed-in remuneration of about €1000 per year. According to the house provider, the photovoltaic system pays off in 8-10 years.

It is difficult to assess without knowing the system size. However, 8 years is rather optimistic; 10 years is more realistic, provided it is purchased at a good price.

Either choose gas with solar thermal and possibly a photovoltaic system if your budget allows, or simply an air-to-water heat pump with photovoltaic, if you have the budget.

Hausbau129 schrieb:

Question 3: Which option would you prefer?
We would like a system that is as low-maintenance, reliable, straightforward, and affordable as possible.

Either a heat pump or a gas system combined with a ventilation system featuring heat recovery. The latter is not something every energy saving regulation planner can calculate and is rather the exception. However, it is possible. Ventilation is a comfort improvement; other measures tend to be similarly uneconomical and offer no real benefit to you.

A heat pump is somewhat cheaper to maintain, more ecological/future-proof, and would be my preference. You do need to take a bit more responsibility with a heat pump, as the crucial planning involved is often neglected in practice. Whether heating with a heat pump is ultimately cheaper depends, besides the planning, on how much it is sold to you for.

Solar thermal systems are never economically viable on their own. A sufficiently large photovoltaic system always makes sense, regardless of the heating system, and pays off through feed-in tariffs over about 10-14 years. Batteries are uneconomical.

We completed a KfW70 standard house in 2018 with a gas boiler and a controlled residential ventilation system including heat recovery. I had conduit installed for photovoltaic panels, so I can add them later if needed. Otherwise, nothing else.

PS: For 5 people using 30 liters (8 gallons) of hot water each (heated from 10 to 55°C (50 to 131°F)), that’s about 8 kWh per day. At 6 cents per kWh for gas, that costs around 50 cents per day. With solar thermal, you can save about half of that. So in total, it’s roughly 100 euros per year… unless you take very long showers.

As far as I have understood so far, solar thermal systems are not worthwhile, but photovoltaic systems always are.

Have you compared the heating costs of gas with those of a heat pump? Don’t just compare the initial purchase costs. Especially considering CO2 taxes and similar. Just the chimney you can save, plus the regular chimney sweep fees.

guckuck2 schrieb:

I am surprised that this would be approved. The renewable energy share must contribute to heating the building. This is not the case with a separate domestic hot water system.

Actually, it is possible. If you calculate very conservatively and, for example, have decentralized ventilation with heat recovery, you can rely solely on the heat recovery without solar panels or the domestic hot water system and still comply with the energy saving regulations.