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A Viessmann vitodens 100 boiler has begun to heat the radiators when on hot water only, causing it take along time,and alot of gas to heat the water to luke warm. I'm assuming It's the step motor cartridge that's at fault, the question is . How do you remove the step motor...

a 3 year old vitodens 100 suddenly stopped firing up on either water or heating, fault code F5 the book say's check fan & air pressure valve, the fan is working ok,the valve test's ok but is not operating can anyone give me a rough idea as to what the problem is .