I have a combi boiler with a digital control unit in the hall and a dial thermostat next to that, I set the control unit to continuous and just use the thermostat to turn the heating on but the problem is the themostat wont click the heating on unless it's turned right round to almost as far as it will go! and sometimes it wont click on at all. Is there some adjustment to get the thermostat to click the heating on where it should be around 19-20c and not way off the scale? or do I need to replace?