when my boiler is hot it fires up for about 3 to 5 secs goes of for about 20 to 30 secs. Timer not working as well when boiler is cold it runs ok . Hot water works ok.
unsure what the problem is but let me suggest boler is cycling on and off when all radiators are hot if this is the case high flame gas presues are set to high for system on heating mode this usualy happens when system curcuit small this can be corrected on pcb or gas valve
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