Pellet stove off - water cold - emergency system?

Hello everyone,

I'll try to keep it brief...

Starting point:
Semi-detached house built in 1965
Renovation to KFW 70
130 sqm + unheated cellar + unheated attic
Move-in 2019

Pellet stove with water jacket (Oranier Carus Aqua) approx. 1.1t pellets/winter
Solar thermal
800-liter buffer
Underfloor heating approx. 30% of the area, rest convection radiators

At the beginning of 2024, I’m installing 5.6 kWp solar with a 5 kWh storage.

Overall, we are very satisfied with our system... the possibility to heat the house in just a few minutes is a dream and the (existing) radiators are only used sporadically. The ratio of the pellet stove regarding hot water production to room heating fits pretty well (cold water and warm apartment does not happen).
The problem now is the following: When I call Oranier to order a spare part or have a service done, I always get to hear: “How many hours does it have? The stove is intended more as a supplement and not as a primary heating system”. Well, for us it’s not just the primary heating, but in winter the only way to generate heat, which applies to both heating and hot water. From the beginning it was clear that an emergency solution was needed. The plan was actually to install a “3 kW heating cartridge” in the 800 l tank (yes, 3 kW would be enough). But since the tank was set too close to the cellar wall, this is not possible and we are now in the fourth winter with the looming Damocles sword named “chill” hanging over our heads. My thought now was: “I already have 2 circuits bringing heat into the system, can I just simply put a small instantaneous water heater (a small one with about 5 kW) parallel into the circuit of the pellet stove (with two 3-way ball valves). Or even in series?? As said, ideally this thing will never be used...

Can, may, or should one do something like this? Thanks for your opinions and advice

Regards
Florian

There are also heating rods intended for heat pumps available as single elements for installation in the flow, e.g. "LG Elektroheizstab Therma V Monoblock 3kW". Normally, they are controlled via the heat pump's regulation, but if you have a universal controller for pellet and solar thermal, you might also be able to integrate them there as a heat source. Of course, these are also available from other manufacturers and with higher power. Alternatively, you can also connect an inexpensive Chinese heat pump or a Panasonic Geisha (5 kW, H) into the return flow and switch it separately in the emergency without the control of the rest of the system – it must then, of course, be installed outside as an external unit. For the backup emergency solution, however, the heating rod should be sufficient.