The heater core is leaking coolant.
you have a blown head gasket
If that smoke is more like steam, you have a leaky heater core.
The smoke you see may be moisture/steam. The heater core is probably leaking engine coolant. If the "smoke" has a sweet smell to it, it's engine coolant. More info about the vehicle such as model and year would help.
Nope, that would be steam.
The heater core has failed.
Either you have a blockage present or your vents on your sewer lines have become blocked or you need to install more vents.
what kind of gasoline are you running? and what color is the smoke?
Blue smoke is oil burning, White is steam(water), Black smoke is unburnt fuel.
Warm(er) air rises - if vents were high up there would be little circulation without fans.
Is it smoke or is it atomized freon leaking from the AC system? If it fogs the windshield and smells funny, you may have a leak in the AC/Heater system.
You would type "/smoke" without the quotation marks.
Most of the time it is caused by a hole in the heater core, do you smell a sweet smell? The sweet smell would be the antifreeze.