Switch boiler to summer mode (heating off). Let everything go cold on boiler, (while you are waiting identify flow and return pipes for heating under boiler, usually 22mm). Turn on a hot tap and then "feel" the heating flow and return pipes. if either of these pipes start to get significantly warm then it is 90% sure your diverter valve is not working correctly, if they stay cool your secondary heat exchanger is not working correctly.
Dumb Plumber, April 2010
