The gas boiler has a small tank where the radiator circuit's water is heated. Warm water accumulator is analogous to making this tank larger. The radiator conne

The gas boiler has a small tank where the radiator circuit's water is heated. Warm water accumulator is analogous to making this tank larger. The radiator connections would be moved to the big tank and the boiler would be made to warm that tank. Perhaps typical combination boiler isn't optimal for this setup, so incorporating other heat sources might be best timed when the boiler is due for replacement anyway.