I have a problem with a Halstead Ace high CH boiler which has beaten a Halsted engineer over five visits. The DHW usually fails to start unless the boiler is actually lit for central heating. If you turn on the water when the heating is on then it will switch to DHW faultlessly and does what its supposed to. As soon as the water is turned off the heating will fail to restart and will sit there for a considerable time before finally restarting, sometimes hours. When it does restart the heat output is low and intermittant. No fault lights illuminate. When starting on the timer from stone cold first thing in the morning the heating runs up to temperature and shuts down abruptly regardless of thermostat setting, goes through several short cycles before switching off for hours. At no time , other than as described does the DHW start on demand. Clearly the engineer is stumped and has reached the "not a clue stage". Advice is heading towards the random changing of bits until the problem disappears or change the boiler. I am not inclined to follow either suggestion without a proper diagnosis of the problem. The boiler has run faultlessly until now and is not scaled up or leaking. Its worrying when the engineer is stumped too!. Any ideas?