Hello,
The starting point is a house from 1930, which may not be able to be upgraded energetically or only to a limited extent due to its design (possibly listed building protection à to be checked, other depreciation and funding options). Technologically and thus in terms of energy efficiency, the existing system is outdated. In contrast to the current situation, there will be a future hot water demand estimated at 200 to 300 liters per day for 5 people. It must be checked whether the specification of the final energy demand is based on a demand certificate or a consumption certificate; if it is a consumption certificate, the key figure is not suitable for further planning.
The heating load must be determined in a first step and the system dimensioned accordingly. If a gas connection is available, it makes sense to convert to an energy-efficient condensing boiler that reliably supplies the building. Thanks to modern technology and the use of the condensing effect, savings of 20 to 40% are realistic. In addition, the oil tank and the regular refueling and cleaning are no longer necessary. The hot water storage tank must be dimensioned according to the future demand.
The wood stoves should be integrated into the heating system via water jackets, as wood is available. This increases the economic efficiency of the heat supply. However, the wood stoves should not be considered a fixed part of the heating (What if the back hurts or there is bad air with the parents à then no wood or only through purchase). If applicable, the option of including solar support should be considered in the planning if the conditions are suitable.
For all this, there are KfW funds, either as a low-interest loan (program 151/152, approx. 1%, maximum €50,000 per residential unit) or as a grant (program 430, 10% on a maximum of €50,000 per residential unit).
The heating renovation/optimization is an individual measure that does not require other measures on the building (e.g., insulation) to be carried out.
Best regards, Erik