you have a leak in your heater core which is attached to your radiator and heats the air inside your car interior
Filth in air ducts can cause many diseases and problems. Mold, mildew, and fungus can cause serious respiratory problems including Legionnaire's Disease. Bacterial growth can cause all kinds of illnesses. Rat droppings likewise. The unclean air ducts would more likely cause allergies and asthma than the common cold.
Dirty filter, dirty coil, closed damper, or duct has come loose from main trunk line. Rats will sometimes nest in air ducts and may chew holes through the ducts, especially the insulated plastic ducts. That's the worst case.
Hot water tank needs bleed of air,
Return ducts are under less pressure, but they need to move the same amount of air, so they must be larger.
My opinion would be to reroute the air ducts through the attic space. Saves time and money.
AC flex ducts can fill with water due to condensation forming on the duct surface from temperature differences between the cold air inside the duct and the warmer air outside. Poor installation, insulation gaps, or clogs in the ductwork can exacerbate condensation issues. Additionally, high humidity levels or improper drainage can contribute to water accumulation in the ducts.
Ductless air conditioners are not better than ones with ducts. The reason why ductless air conditioners are not better than ones with ducts is that they don't remove the hot air as well.
Any break or leak on the suction side of your pump can cause air in the lines.
Airflow improperly set (too high) or the most common undersized ducts.
Air flows in a furnace from the return ducts into the furnace, where it is heated and then pushed out through the supply ducts to heat the home.
One can clean the air ducts by getting a bucket with warm water and wiping down the ducts individually. However, please note it may not produce the same results as having them cleaned professionally.
Insulation in air ducts is typically installed by professionals during the construction or renovation of a building. It is usually in the form of specialized insulation material wrapped around or lining the ducts to help maintain the desired temperature of the air flowing through them.