We have Vaillant turbo max plus (which I have hated since the day it was installed, its very noisy changing from heating to hot water even after the diverter valve was replaced), It works ok ish for a while and then as soon as outside temperature drops it locks out. We have had engineers look at it but obviously by the time they get round we have managed to get it started again, by turning the heating up full blast then allowing the boiler to run for a while then we can attempt to put a hot tap on, so when the engineers look at it they can't find anything wrong, trouble is they don't want to come out at 5:30 in the morning when it has cut out, and we can't leave it off all day. any ideas anyone?