My new carrier infinity furnace was installed last month. There is a sweet smell from the vents and it is especially strong when the heat is high. We have has the ducts cleaned and technician has already checked the unit. Nothing was wrong with it.

Please help me figure this out.

With a central heat and air system the return air will pull air and smells from all parts of the house and push it out the supply ducts to all parts of the house. What I'm getting at is, you could have a smell in one room or parts of the house and when the system kicks on it will distribute the smell through out the whole house. In turn the smell may not be coming from the system but distributed through the hvac system.