When a home has interior doors that are not cut high enough above the carpet and the air flow is obviously restricted, what is the result or harm?
If there is returns in the rooms there is no harm.
If not, lack of heating and cooling efficiency is the main reason. I have seen numerous instances of visible mold in closets because of this also, especially in basements.
As mentioned if returns in room no harm.
However… the heating and cooling system, if properly designed will have the furnace blower pumping air out the supply ducts, and then drawing it back through the return registers.
The supply ducts will cause a positive pressure and the return a slight negative pressure or suction.
The pressure or difference between them should not exceed 2 or 3 pascals at any time. However, if the interior doors block the air flow, the pressure can rise to maybe 10 or even 15 pascals.
The higher pressure will cause excessive air leakage into the walls and ceilings, increasing heat loss and moisture flow into the building interiors.
On cooler days, this can then condense and collect as moisture. The other impact is on the return side, which is not increasing the negative pressure, which then will attempt to draw air from other sources which may be combustion appliances, etc.
To prevent this, the average room should have perhaps half an inch of air flow underneath, however depending upon the airflow delivered to the room this may be more… or as much as two inches to carry the air flow (you need around 1 sq inch per each cfm delivered to the room).
Always an informative, educated response.