ᐅ Roof renovation: Replace or keep solar thermal when adding PV panels and an oil heating system?

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

Hello everyone, happy New Year to all.

Here are my considerations: The roof covering will be done with Erlus 58S Brazil brown or Wienerberger Alegra 15 old red.

I have also decided to dismantle the string inverter (ST), as I do not want to deal with the roof again anytime soon. I will then have 14 panels measuring 1762 x 1134 mm (69.4 x 44.6 inches) each (Aiko 475 Watt AIKO-A475-MCE54Db), which corresponds to 6.6 kWp. I will remove the pipes from the string inverter; for photovoltaics, I still have other empty conduits leading to the basement. In this context, I also plan to remove the old cable from the rooftop power supply to the basement. The house is now underground wired. Or could this thick cable still be used for something else?

I am considering the following three inverters:

1. SMA STP6.0-3SE-401
2. Growatt MOD 6000TL3-XH BP
3. Sungrow Hybrid SH8.0RT / Sungrow Hybrid SH6.0RT

All with HEMS and potential battery options.

Any opinions?

Thanks & regards

Hi,

interesting line of thought, and yes, I understand wanting to only deal with the roof once—most people feel that way after the second scaffolding. From a technical standpoint, removing the ST system is not a big deal; these units can last around 20 years, but only if the pump, expansion tank (MAG), and heat exchanger don’t start clogging, which statistically is not unusual for models from 2009. With 6.6 kWp (kilowatt peak) photovoltaic panels, you’re already in a range where self-consumption makes sense; feeding all power into the grid feels almost a bit outdated. Running the Vitocell solely with oil for domestic hot water is not very energy-efficient, but in realistic terms we’re talking about a few hundred liters per year, which quickly becomes less significant.

A heating rod plus storage tank sounds logical, but 10 kWh (kilowatt-hours) run out faster than you’d expect, especially when hot water is needed at 5 a.m. and the battery is mostly decorative in winter. The logic is handled by the inverter and home energy management system (HEMS)—priorities, permissions, lockout periods all work, but only if configured correctly, otherwise the boiler heats up again. For the inverters, I’d look less at brochures and more at service and update policies; SMA is reliable, Sungrow is technically strong, Growatt is cost-effective—do you want to tinker a bit or just have peace of mind? Regarding the old roof-mounted conduit, copper is copper; have the cross-section checked—it’s not a bad idea to keep it as a backup or for a future electric vehicle charging station.

A question for you: do you plan to add an electric car or heat pump in the future, or should the house’s energy system just be considered finished for now?

Thank you for your detailed explanation. The priority is to have peace and quiet. I will decide on a storage system and the feed-in model based on available incentives; if there are no incentives, then full feed-in will be used. An electric car and heat pump will definitely be added, but only once the existing diesel system and the oil condensing boiler are no longer operational.

However, the cable from the attic to the basement cannot be used for a wallbox, as the inverter is planned to be installed in the basement. Therefore, only the string from the panels needs to be routed to the basement; empty conduits are sufficiently available. Regarding the roof standpipe (renewed in 2009), I was rather thinking of connecting an air conditioning unit or something similar. Although a new cable with the appropriate cross-section can be installed quickly.

silverdok schrieb:

I base the battery storage and feed-in model on the subsidies available; no subsidy means full feed-in.

That doesn’t make sense. It primarily depends on self-consumption, then the feed-in tariff, and then future planning. But only on subsidies? No. That’s the wrong approach.

silverdok schrieb:

An electric car and heat pump will definitely come, but only once the existing diesel engine and oil condensing boiler no longer work.

You should also consider that an electric car with its own wallbox can be cheaper, sometimes significantly cheaper, than a combustion engine vehicle. We are now switching our second car from combustion to electric and are already saving a four-figure amount per year just on fuel. Lower taxes (actually none) and cheaper maintenance are not even included yet. But you always need to calculate this individually for yourself.

1800 kWh per year consumption, feed-in tariff 12.4 cents/kWh, 9000 km (5600 miles) annual mileage with fully paid-off car.