Abstract: In a heating system operation method, a heating circuit medium is conveyed through an overall heating circuit flow path and at least through one sub-flow path of the overall path. How much medium is respectively conveyed through the sub-flow paths is controlled, and the medium, in order to clean it of dirt, is routed through an overall path filter. At system start-up, all the medium is routed via the sub-flow path having optionally the at least variable flow resistances or a known and/or unchanging flow resistance, termed reference flow path, and a conveying power reference value is determined and saved. To check the filter fouling state, during system operation, all the medium is again routed through the reference flow path used on start-up, a conveying power value is determined and compared with the reference value and, beginning at a predetermined deviation, a filter fouling signal is generated.