I have a new lennox comfort system,it is chilly today and I wnt to the thermastat and turned it to heat and it will not let me turn the temp above 18,and it the furnace is blowing cool air?

It might be one of those deals that requires a 24 hour changeover period,
Just another one of the reasons that so many people don`t like lennox
But this is a guess ,only a guess,the names have been changed to protect the one that doesn`t own,(and glad of it) a lennox

Also it could also be that the installers set up the tstat to only reach a certain temp for heat and/ or cooling. I would call installing company and ask about wmroops qustion first. If its not that than have them come look at it.