Vitodens 100 - Brainstorming for replacing a defective heater

I would calculate the Viessmann heat exchanger replacement at around €2000 if you find someone and the material is available. Maybe much cheaper, 3-4 years ago it was more like €1000, but that doesn’t mean anything today.

If water only escapes during operation and the heating continues without errors and pressure loss, it is condensate water and the heat exchanger has rusted through somewhere at the mounting points due to microcracks or whatever. It definitely needs to be fixed, you can’t just let it run.

At the age of the heating system, you would normally install a new heating system. If you don’t live in BW or SH, I would quickly commission a new gas heating system with the mentioned buffer, and take advantage of the renewable ready funding from the BEG subsidy if it’s currently not enough for the complete implementation with solar.

We were faced with the same decision. Actually, the old gas heating was supposed to keep running until we had improved the house's energy efficiency enough to install a heat pump. Due to the control system failure and various unreliabilities, we bit the bullet and had a new condensing boiler installed. Now we can renovate completely at our own pace and see how low we can get the flow temperature. Based on that, we will eventually decide whether an air-to-water heat pump makes sense. At least, during the freezing cold week in December (-13° C), we managed with 55°C and consumed less than with the old heating at +10°C. We upgraded the windows, partially the radiators, and insulated the attic with 20 cm WLG 035. The next project is insulating the gable wall (90 sqm) with 14 cm WLG 035, currently the biggest weak spot.