I've just had a new Vaillant EcoTec 831 boiler fitted with a Salus St620 Programmable Thermostat. Read the instructions, programmed it as described to come on for an hour in the morning and a couple of hours in the evening - tootled off to bed. Woke up at 1am with the boiler cranking out heat like something possessed. Re programmed it, using 24 hour clock - same thing. Can't understand it. The instructions are simple enough. There's nothing obvious that's wrong.