My central heat and ac unit fan is working but no heat when I turn the heat on vents are blowing cool air. Unit was recently checked out and all was fine and it has been working OK. Anyone know what the problem could be. Detailed answers appreciated.

(quote)Detailed answers appreciated.
Without being there to look your furnace over it will be only a guess,and lots of tech work to give a detailed answer
1st we don`t know ,what kind of furnace you have=gas electric,or oil
Brand name, if its gas do you have a pilot light? Electronic ignition?, there are so many items that need to be looked at, the more info you can give us the better we can help you