The duct work that distributes the hot or the cold air to the individual rooms of the house connects to the plenum. As far as the supply plenum is concerned there is a blower in the HVAC system that forces the hot or the cold air to the plenum and from the plenum, the air is distributed to the individual rooms through the supply duct system. Generally, the air cleaning filters are located in the supply plenum. Now let us have a look at the return plenum. Forced air systems also have return ducts that connect to the HVAC equipment with the help of an air collection box which is known as the return plenum. When conditioned air enters the room the exhaust air from the individual rooms of the house gets sucked into the return ducts. The process that takes place is that the used air from the return plenum gets back to the HVAC system for another round of heating and cooling. Even return plenums have air filters.
Problem arises when plenums in an HVAC system has to be replaced. This is when they need replacement. Replacing a plenum is almost similar to the installation of a new one. This is because there is not much benefit by repairing a small part of the plenum sheet. The cost of the supply plenum largely depends on the size and the material of the plenum that is being bought. Homeowners should pay attention to the size of plenum that exactly attaches to their furnace. A plenum which is incorrectly sized will not bring about the desired results. However, in case a homeowner is not much confident about the size of the plenum that would be apt for an HVAC system, it is better to get in touch with an HVAC contractor. They will be able to determine the exact size. Generally, it is seen that the cost of plenum is something between $50 and $100 depending on the size and the material.