ᐅ Roof renovation: Replace or keep solar thermal when adding PV panels and an oil heating system?
Created on: 1 Jan 2026 14:11
S
silverdok
Hello everyone,
Due to my age, I need to have my roof replaced. This inevitably leads me to consider installing a photovoltaic system. Since the scaffolding will already be up, it makes sense to do it at the same time.
My current setup includes two solar thermal panels combined with an oil condensing boiler (Viessmann Vitosolic 100, Vitocell 100-B/W 400 liters (100 gallons), and Vitoladens 300W). Apart from significant issues during the first three years after installation in 2009, this combination has worked quite well.
Originally, I planned to install 10 photovoltaic modules on the south-facing side and feed all the electricity into the grid, while keeping the two solar thermal panels. However, the solar thermal system has had no flow for a few days now, although the pump seems to be working fine. Now I am wondering what to do.
Should I remove the solar thermal system during the roof renovation in spring and install an additional 4 photovoltaic modules? Or should I have the solar thermal system flushed and the expansion vessel checked? But how much longer can the solar thermal system actually function? It would be really unfortunate to have to remove the solar thermal panels shortly after completing the roof and photovoltaic system.
Assuming I remove the solar thermal system, would I simply run the oil boiler year-round for domestic hot water and buy a bit more oil with the money saved? And what would this involve in detail? Would I have to remove all the piping? Would brass caps be installed on the connections at the Vitocell?
Or should I invest in a 10 kWh (kilowatt-hour) storage unit and an electric heating element for the Vitocell until I eventually switch to a heat pump? But I am wondering how the control system would work in detail? The battery charges during the day, and then hot water is produced from it at 5 a.m. The oil boiler starts at 6 a.m. and detects that the storage tank is still warm? Or is there a logic in the storage system or inverter that controls the heating element? I am not quite clear on this. How would this ideally be implemented?
That turned out to be quite a few questions. Perhaps someone has experience they can share with me.
Thanks in advance and best regards
Due to my age, I need to have my roof replaced. This inevitably leads me to consider installing a photovoltaic system. Since the scaffolding will already be up, it makes sense to do it at the same time.
My current setup includes two solar thermal panels combined with an oil condensing boiler (Viessmann Vitosolic 100, Vitocell 100-B/W 400 liters (100 gallons), and Vitoladens 300W). Apart from significant issues during the first three years after installation in 2009, this combination has worked quite well.
Originally, I planned to install 10 photovoltaic modules on the south-facing side and feed all the electricity into the grid, while keeping the two solar thermal panels. However, the solar thermal system has had no flow for a few days now, although the pump seems to be working fine. Now I am wondering what to do.
Should I remove the solar thermal system during the roof renovation in spring and install an additional 4 photovoltaic modules? Or should I have the solar thermal system flushed and the expansion vessel checked? But how much longer can the solar thermal system actually function? It would be really unfortunate to have to remove the solar thermal panels shortly after completing the roof and photovoltaic system.
Assuming I remove the solar thermal system, would I simply run the oil boiler year-round for domestic hot water and buy a bit more oil with the money saved? And what would this involve in detail? Would I have to remove all the piping? Would brass caps be installed on the connections at the Vitocell?
Or should I invest in a 10 kWh (kilowatt-hour) storage unit and an electric heating element for the Vitocell until I eventually switch to a heat pump? But I am wondering how the control system would work in detail? The battery charges during the day, and then hot water is produced from it at 5 a.m. The oil boiler starts at 6 a.m. and detects that the storage tank is still warm? Or is there a logic in the storage system or inverter that controls the heating element? I am not quite clear on this. How would this ideally be implemented?
That turned out to be quite a few questions. Perhaps someone has experience they can share with me.
Thanks in advance and best regards
N
nordanney5 Jan 2026 12:24silverdok schrieb:
1800 kWh per year consumption, feed-in tariff 12.4 cents/kWh, 9000 km annual mileage with long-paid-off car. And now? The numbers alone don’t tell you much. For example, what do the 9,000 km per year actually cost? For us, with the planned electric car, that’s about €300 in electricity—part of which can be self-generated, and that hasn’t even been taken into account yet (I already drive electric myself and in summer I often charge 50 kWh or more at once using the photovoltaic system).
As I said, everything needs to be considered and calculated. Then full feed-in can be worthwhile or not. I’m completely open to both options—but to assume you base storage and photovoltaics solely on the subsidies (i.e. feed-in tariff and VAT exemption on purchase) is careless. What use is a 12.5-cent tariff if in 10 years you’re driving electric, have a heat pump, and face an increased electricity demand with a 75-cent purchase price from the grid?
After all, you’re committing for 20 years... You need to think this through carefully.
S
silverdok5 Jan 2026 12:53To my knowledge, full feed-in is not limited to 20 years and can be switched to partial feed-in at any time.
Legal regulations: Systems commissioned after July 30, 2022, can choose annually between full feed-in and self-consumption (with surplus feed-in).
Legal regulations: Systems commissioned after July 30, 2022, can choose annually between full feed-in and self-consumption (with surplus feed-in).