Hello , Sorry if this post is not in the correct section but here goes ... My heating will only come on when the Thermostat goes past 20 on the control unit which is in our hallway , boiler is upstairs in bedroom , if we turn it down to say 15 or lower the heating goes off , how comes people tell me they can have theirs on at 15 and it works yet mine has to be over 20 before it clicks in to life ?