F28 means the boiler can't start, and occurred with my Vaillant boiler. In my case the cause was the outlet pipe draining the condensate away had frozen. This is a common problem in the current freezing weather. This results in a build-up of this water in the trap which is located within the boiler casing. When the trap becomes full the boiler cuts off. On my boiler the condensate pipe coming out below the trap is a push fit. With a container held underneath the boiler, simply pull off this pipe. The water which has built up should flow out. This will allow the boiler to re-fire. I've cut the condensate pipe a few inches under the boiler and am collecting the water in a container. It comes in short bursts because of a siphonic action out of the trap in the boiler, and should be a manageable amount which can be poured away each day. When the thaw comes, I will replace the output pipe aiming to get it better insulated particularly where it goes through the wall and outside. I hope this helps.
BH, January 2010
