The heater may not be on - but the fan still blows air through the vents when they're open !
the middle vents only supply air from the ac unit using a separate in dash fan. the floor and defrost vents come only from the heater
There are heater doors on the heat blower box that direct the path of the heat. Apparently the heater door to the floor vents is stuck closed.
I am sorry, I am not technical on this stuff, but we had the same problem on our 95 Mustang GT. We had to take it into the dealer and the problem was that one of the hoses had come loose, so it was not blowing enough air pressure to force the vents open.
I just had that problem with my 99 Monte Carlo. When I accelerated or went uphill No air would come out of the dash vents. Problem was a Vacuum hose. Cost about $40 to fix.
A duct has come apart under the home?
Yes, gnats can come out of dirty air vents. Gnats can actually come from anywhere. They are very pesky creatures.
If that smoke is more like steam, you have a leaky heater core.
Bad heater core.
The heater core has failed.
heater fan broken.
Because hot water is still in the pipes from when you turned on the hot water, and when you turned on the cold faucet it had to let out the hot water first. This means you have a single-pipe sink. Double-pipe sinks have a pipe for hot and a pipe for cold and it blends them as it turns on.