Forced air heating is the second most popular means to heat a modern home. These types of systems consist of a furnace that heats air and a system of ducts that distribute the heated or conditioned air throughout the home. The system uses a large blower fan to push the air through the duct work and out through the registers throughout the home. Most forced hot air systems are installed with the original build of the home. Forced air is challenging to retrofit because of the size and the distribution of the duct work. Cool air returns through a system of return registers. These return registers are usually larger in size the the distribution registers. Sometimes these types of systems can be muti-zoned. more often additional furnaces are installed to handle additional areas of the home. The usual configuration is a furnace in the basement which heats the first floor and a second furnace in the attic that supplies heats to the second floor. A single furnace may be sufficient to heat a smaller home. If the system has zones dampers are used to control the air flow.