Hi
I've been keeping an eye on your question because we were having a similar problem, the hot water would get to 37 ish and then cut out and the boiler would fire up again until it got back up to 37 again, showers were not fun!!
We had the thermistor and the flow sensing switch replaced but it made no difference.
We've always had a cold radiator in the bathroom so we thought we'd drain the muck out and hope that would at least mean we were warm when the shower froze us!! After refilling the radiator, we had a F3 fault - heat exchanger blockage. The manual says to use a soft brush or vacuum cleaner to clean the heat exchanger. You need to isolate the electricity and gas supplies first. To access the heat exchanger, you take off the front cover of the boiler, then the second cover where the 2 clips are at the bottom. Then remove the 6 screws on the next cover and the heat exchanger is the compartment at the top. We used a vacuum with a soft brush end and gently cleaned out all the crap. We also gave the burners a quick clean too. Then put it all back together. We did this and turned the heating on. It came up as an F1 fault and there was a rapid clicking noise of the ignition. We turned the hot water on and it fired up and cleared the F1 fault and we've been in shower heaven ever since and the bathroom radiator is boiling!!!!
Hope this helps you to point the heating engineer in the right direction!!
KF
KF, February 2008
