Complement gas heating with wood gasifiers, water-bearing fireplaces, and solar energy

Dear forum community,

I kindly need advice regarding the conversion of our heating system. I am quite skilled in some DIY areas but not when it comes to heating. Please forgive me if my questions seem amateurish. Ultimately, a professional will carry out the work, but I would like to have a better understanding and grasp of the topic to be able to assess everything better.

I am attaching sketches of the current and target states, as well as pictures of the buffer tank.

Currently, we heat our house (an old building from 1930 with an extension from the 70s) with a Viessmann Vitodens 300 condensing boiler connected to a simple 300 L hot water storage tank. Additionally, there is a wood stove with an external chimney.

The house is now undergoing energy-efficient renovation and, in order not to be dependent on gas, the primary energy source will henceforth be wood in combination with solar thermal energy.

That means:

- Installation of an Atmos wood gasifier with 25 kW
- Installation of two water-bearing stoves with 12 kW output each (7 kW room air, 5 kW water jacket)
- Installation of a 550 L buffer tank
- Installation of a 15 sqm solar thermal system

The two stoves will be regularly fired during the day and evening.

We practically have wood available free of charge.

So far so good, I still have the following issues / problems / questions:

- I do not want to keep refueling the wood gasifier permanently but rather bring the buffer to temperature once a day. Are 550L buffer tanks sufficient for 270 sqm living space? If not, how many liters would be advisable?

- Is the buffer tank suitable for the project with regard to the connections? Unfortunately, I have no data on this. It came along with one of the water-bearing stoves. The flange still has 3 inlet or outlet connections. For example, I also do not know whether it is suitable for drinking water production (of course it has an insulating jacket, which is not shown in the picture). Otherwise, I would also be grateful for alternative purchase recommendations.

- I think the current 300 L storage tank can be completely removed, it will be useless, right? Or will I need a storage tank for drinking water in the future and the buffer tank only for domestic hot water?!

- Is it advisable to place the buffer tank in the outbuilding or should it be better placed in the basement next to the gas heating system? Or does it not matter?

- How / by which device (control system?) can it be regulated so that the gas heating system starts when the temperature in the buffer tank becomes too low? The gas heating should remain in case for whatever reason the stoves, wood gasifier, and solar thermal system do not produce sufficient temperature and then activate.

- Approximately what share of the energy production will the solar thermal system contribute?

- Now a difficult question: How high might the costs be for the connection work / piping?

Many thanks and best regards,

Josef

Do you know how much heating capacity you will need at most after the energy retrofit? If a 22 kW condensing boiler is sufficient now, 25 kW will probably be oversized afterwards. Why 15 m² solar thermal instead of photovoltaics? Have you thought about a heat pump if you don’t like gas, supplemented by the fireplaces?

Best regards

Hello, thank you very much for your message. We received the [SOlarthermie] modules from the neighbor. We will also have photovoltaics in combination with a battery storage system.

Yes, maybe the heat pump would be something then!

Why even have such an installation if wood is free?

Why even install something like this if wood is free?

I brought the wood into the house but it stayed cold.

- I don't want to keep refueling the wood gasifier constantly but rather bring the buffer to temperature once a day. Are 550L buffer tanks sufficient for 270 sqm living space? If not, how many liters would be advisable?

It depends. Only buffer tanks and nothing else (no domestic hot water tank), then at least 1,500 liters. Alternatively, 800-1,000 liters of buffer tanks and 800 liters of hygienic storage for hot water (which also functions as a buffer tank).