I came across this company whilst surfing for insert fires with a back boiler using German search engine,
www.normatherm.de. These boilers are fully controlled and will put out 15-45 kw depending on model, so plenty of heat for most houses. It sounds like you have some kind of buffer tank to store heat for distribution to rads, and so in the spring /winter you don't have to run the fire 24/7, nor do you have to feed it during the night. Also, it can be used in conjunction with solar and/or heat pump.