I experience intermittent interference on the upper HF spectrum including 18MHz, in the form of S5 signals spaced about 16kHz each with a comb of sidebands at n x 100Hz ish. I initially suspected television but soon confirmed that it is definitely my central heating system. With either hot water or central heating on this interference is present and it goes away when off. I have British Gas UP2 controller downstairs and Grunfos Super Selectric pump upstairs. I can "Sniff" with a Sony portable and the noise peaks to a similar level at both components but am unable to determine which end is the culprit. Has anyone else had experience either with these components or similar central heating EMC? Are central heating pumps powered by chopped AC?