help , thermostat advice please

probably not even spelt that right , anyhow last week i had a new boiler fitted and a new hard wired thermo , i have it set to come on at 16 which it does but then it goes up to 17 then 18(it may go higher but i dont let it as all the rads are hot and the house is warm by then) but donst seem to swich off automatically i have to got and turn the dial down abit manually is that right if the dial is at 16 , it is one of those basic ones no digital reading ,