Oh my, that’s not much at all - DT50 means 50° above room temperature, and radiators output less at cooler temperatures. At a flow temperature of 50°C would emit only 4 kW of heat, and 60°C would emit 6kW.
This heating system is limited by the radiators not the heat pump. If the property has managed to maintain a steady internal temperature, then 6 kW might be all the heat you need at 0°C. The heat loss coefficient works out to be about ~300 W/K.
Here’s stats for my property for comparison:
- Heat loss at design temperature is 5.6 kW or 240 W/K
- Radiator output at DT50 is 19,000 W
- Heatpump is Ecodan R32 11.2 kW (oversized)
- Minimum flow temperature is 40° before it cycles
- Flow rate is at the low end, around 11 l/min
- Annual COP is around 3.6
So, bigger (or thicker) radiators will allow the heat pump to run cooler while still delivering the required amount of heat.