F20 is a flow sensor error. Can be due to a defective sensor/ not correctly fitted, or you may have poor circulation, a big difference in flow and return temperatures, can sometimes make the boiler think it has a defective sensor, check good circulation through system, or if you have thermostatic radiators valves on every single radiator, take 1 head off, as if you dont have a bypass fitted, it needs a radiator as a bypass circuit
kash, February 2013
