Hi interference from either boiler ignition or more likely room thermostat, more usual if on indoor antenna as signal low and close by, try moving stat up/down to see if you can replicate prob. if so then maybe stat. contacts arcing due to slow action, made worse if no Neutral to stats. 'accelerator' resister. Only other [unlikely] cause is contact of motorised valve if you have one, this will happen a second or 2 after turning room stat up, regards SPARX
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